Toxicological profiles - Uranium

TOXICOLOGICAL PROFILE FOR URANIUM

Prepared by: Research Triangle Institute Under Contract No. 205-93-0606 Prepared for:

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES Public Health Service Agency for Toxic Substances and Disease Registry

September 1999 CAS# 7440-61-1

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DISCLAIMER The use of company or product name(s) is for identification only and does not imply endorsement by the Agency for Toxic Substances and Disease Registry.

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UPDATE STATEMENT A Toxicology Profile for Uranium was released in September 1997. This edition supersedes any previously released draft or final profile. Toxicological profiles are revised and republished as necessary, but no less than once every three years. For information regarding the update status of previously released profiles, contact ATSDR at: Agency for Toxic Substances and Disease Registry Division of Toxicology/Toxicology Information Branch 1600 Clifton Road NE, E-29 Atlanta, Georgia 30333

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FOREWORD The Superfund Amendments and Reauthorization Act (SARA) of 1986 (Public Law 99-499) extended and amended the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA or Superfund). This public law directed the Agency for Toxic Substances and Disease Registry (ATSDR) to prepare toxicological profiles for hazardous substances which are most commonly found at facilities on the CERCLA National Priorities List and which pose the most significant potential threat to human health, as determined by ATSDR and the Environmental Protection Agency (EPA). The lists of the 250 most significant hazardous substances were published in the Federal Register on April 17, 1987, October 20, 1988, October 26, 1989, and on October 17, 1990. Section 104 (I) (3) of CERCLA, as amended, directs the Administrator of ATSDR to prepare a toxicological profile for each substance on the list. Each profile must include the following content: (A) An examination, summary, and interpretation of available toxicological information and epidemiological evaluations on the hazardous substance in order to ascertain the levels of significant human exposure for the substance and the associated acute, subacute, and chronic health effects, (B) A determination of whether adequate information on the health effects of each substance is available or in the process of development to determine levels of exposure which present a significant risk to human health of acute, subacute, and chronic health effects, and (C) Where appropriate, an identification of toxicological testing needed to identify the types or levels of exposure that may present significant risk of adverse health effects in humans. This toxicological profile is prepared in accordance with guidelines developed by ATSDR and EPA. The original guidelines were published in the Federal Register on April 17, 1987. Each profile will be revised and republished as necessary, but no less often than every three years, as required by SARA. The ATSDR toxicological profile is intended to characterize succinctly the toxicological and adverse health effects information for the hazardous substance being described. Each profile identifies and reviews the key literature (that has been peer-reviewed) that describes a hazardous substance's toxicological properties. Other pertinent literature is also presented but described in less detail than the key studies. The profile is not intended to be an exhaustive document; however, more comprehensive sources of specialty information are referenced. Each toxicological profile begins with a public health statement, which describes in nontechnical language a substance's relevant toxicological properties. Following the public health statement is information concerning levels of significant human exposure and, where known, significant health effects. The adequacy of information to determine a substance's health effects is described in a health effects summary. Data needs that are of significance to protection of public health will be identified by ATSDR, the National Toxicology Program (NTP) of the Public Health Service, and EPA. The focus of the profiles is on health and toxicological information; therefore, we have included this information in the beginning of the document.

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This profile reflects our assessment of all relevant toxicological testing and information that has been peer reviewed. It has been reviewed by scientists from ATSDR, the Centers for Disease Control and Prevention, the NTP, and other federal agencies. It has also been reviewed by a panel of nongovernment peer reviewers and is being made available for public review. Final responsibility for the contents and views expressed in this toxicological profile resides with ATSDR.

Jeffrey P. Koplan, M.D., M.P.H. Administrator Agency for Toxic Substances and Disease Registry

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QUICK REFERENCE FOR HEALTH CARE PROVIDERS Toxicological Profiles are a unique compilation of toxicological information on a given hazardous substance. Each profile reflects a comprehensive and extensive evaluation, summary, and interpretation of available toxicologic and epidemiologic information on a substance. Health care providers treating patients potentially exposed to hazardous substances will find the following information helpful for fast answers to often-asked questions.

Primary Chapters/Sections of Interest Chapter 1: Public Health Statement: The Public Health Statement can be a useful tool for educating patients about possible exposure to a hazardous substance. It explains a substance’s relevant toxicologic properties in a nontechnical, question-and-answer format, and it includes a review of the general health effects observed following exposure. Chapter 2: Health Effects: Specific health effects of a given hazardous compound are reported by route of exposure, by type of health effect (death, systemic, immunologic, reproductive), and by length of exposure (acute, intermediate, and chronic). In addition, both human and animal studies are reported in this section. NOTE: Not all health effects reported in this section are necessarily observed in the clinical setting. Please refer to the Public Health Statement to identify general health effects observed following exposure. Pediatrics: Four new sections have been added to each Toxicological Profile to address child health issues: Section 1.6 How Can (Chemical X) Affect Children? Section 1.7 How Can Families Reduce the Risk of Exposure to (Chemical X)? Section 2.6 Children’s Susceptibility Section 5.6 Exposures of Children Other Sections of Interest: Section 2.7 Biomarkers of Exposure and Effect Section 2.10 Methods for Reducing Toxic Effects

ATSDR Information Center Phone: 1-888-42-ATSDR or 404-639-6357 E-mail: [email protected]

Fax: 404-639-6359 Internet: http://atsdr1.atsdr.cdc.gov:8080

The following additional material can be ordered through the ATSDR Information Center: Case Studies in Environmental Medicine: Taking an Exposure History—The importance of taking an exposure history and how to conduct one are described, and an example of a thorough exposure history is provided. Other case studies of interest include Reproductive and Developmental Hazards; Skin Lesions and Environmental Exposures; Cholinesterase-Inhibiting Pesticide Toxicity; and numerous chemical-specific case studies.

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Managing Hazardous Materials Incidents is a three-volume set of recommendations for on-scene (prehospital) and hospital medical management of patients exposed during a hazardous materials incident. Volumes I and II are planning guides to assist first responders and hospital emergency department personnel in planning for incidents that involve hazardous materials. Volume III—Medical Management Guidelines for Acute Chemical Exposures—is a guide for health care professionals treating patients exposed to hazardous materials. Fact Sheets (ToxFAQs) provide answers to frequently asked questions about toxic substances.

Other Agencies and Organizations The National Center for Environmental Health (NCEH) focuses on preventing or controlling disease, injury, and disability related to the interactions between people and their environment outside the workplace. Contact: NCEH, Mailstop F-29, 4770 Buford Highway, NE, Atlanta, GA 303413724 • Phone: 770-488-7000 • FAX: 770-488-7015. The National Institute for Occupational Safety and Health (NIOSH) conducts research on occupational diseases and injuries, responds to requests for assistance by investigating problems of health and safety in the workplace, recommends standards to the Occupational Safety and Health Administration (OSHA) and the Mine Safety and Health Administration (MSHA), and trains professionals in occupational safety and health. Contact: NIOSH, 200 Independence Avenue, SW, Washington, DC 20201 • Phone: 800-356-4674 or NIOSH Technical Information Branch, Robert A. Taft Laboratory, Mailstop C-19, 4676 Columbia Parkway, Cincinnati, OH 45226-1998 • Phone: 800-35-NIOSH. The National Institute of Environmental Health Sciences (NIEHS) is the principal federal agency for biomedical research on the effects of chemical, physical, and biologic environmental agents on human health and well-being. Contact: NIEHS, PO Box 12233, 104 T.W. Alexander Drive, Research Triangle Park, NC 27709 • Phone: 919-541-3212.

Referrals The Association of Occupational and Environmental Clinics (AOEC) has developed a network of clinics in the United States to provide expertise in occupational and environmental issues. Contact: AOEC, 1010 Vermont Avenue, NW, #513, Washington, DC 20005 • Phone: 202-347-4976 • FAX: 202-347-4950 • e-mail: [email protected] • AOEC Clinic Director: http://occ-envmed.mc.duke.edu/oem/aoec.htm. The American College of Occupational and Environmental Medicine (ACOEM) is an association of physicians and other health care providers specializing in the field of occupational and environmental medicine. Contact: ACOEM, 55 West Seegers Road, Arlington Heights, IL 60005 • Phone: 847-228-6850 • FAX: 847-228-1856.

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CONTRIBUTORS

CHEMICAL MANAGER(S)/AUTHORS(S): Sam Keith, M.S., CHP ATSDR, Division of Toxicology, Atlanta, GA Wayne Spoo, DVM, DABT Research Triangle Institute, Research Triangle Park, NC James Corcoran, Ph.D. Research Triangle Institute, Research Triangle Park, NC

THE PROFILE HAS UNDERGONE THE FOLLOWING ATSDR INTERNAL REVIEWS: 1.

Health Effects Review. The Healths Effects Review Committee examines the health effects chapter of each profile for consistency and accuracy in interpreting health effects and classifying end points.

2.

Minimal Risk Level Review. The Minimal Risk Level Workgroup considers issues relevant to substance-specific minimal risk levels (MRLs), reviews the health effects database of each profile, and makes recommendations for derivation of MRLs.

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PEER REVIEW

A peer review panel was assembled for uranium. The panel consisted of the following members: 1.

Herman Cember, Ph.D., CHP, Lafayette, Indiana

2.

Ron Kathren, Ph.D., CHP, Professor, Richland, Washington.

3.

Paul Morrow, Ph.D., Professor Emeritus, Rochester, New York.

4.

Richard Leggett, Ph.D., Research Scientist, Oak Ridge, Tennessee.

5.

Darrell Fisher, Ph.D., Senior Scientist, Richland, Washington

These experts collectively have knowledge of the physical and chemical properties, toxicokinetics, key health end points, mechanisms of action, human and animal exposure, and quantification of risk to humans of uranium and uranium compounds. All reviewers were selected in conformity with the conditions for peer review specified in Section 104(i)(13) of the Comprehensive Environmental Response, Compensation, and Liability Act, as amended. Scientists from the Agency for Toxic Substances and Disease Registry (ATSDR) have reviewed the peer reviewers' comments and determined which comments will be included in the profile. A listing of the peer reviewers' comments not incorporated in the profile, with a brief explanation of the rationale for their exclusion, exists as part of the administrative record for this compound. A list of databases reviewed and a list of unpublished documents cited are also included in the administrative record. The citation of the peer review panel should not be understood to imply its approval of the profile's final content. The responsibility for the content of this profile lies with the ATSDR.

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CONTENTS FOREWORD QUICK REFERENCE FOR HEALTH CARE PROVIDERS CONTRIBUTORS PEER REVIEW LIST OF FIGURES LIST OF TABLES 1. PUBLIC HEALTH STATEMENT 1.1 WHAT IS URANIUM? 1.2 WHAT HAPPENS TO URANIUM WHEN IT ENTERS THE ENVIRONMENT? 1.3 HOW MIGHT I BE EXPOSED TO URANIUM? 1.4 HOW CAN URANIUM ENTER AND LEAVE MY BODY? 1.5 HOW CAN URANIUM AFFECT MY HEALTH? 1.6 HOW CAN URANIUM AFFECT CHILDREN? 1.7 HOW CAN FAMILIES REDUCE THE RISK OF EXPOSURE TO URANIUM? 1.8 IS THERE A MEDICAL TEST TO DETERMINE WHETHER I HAVE BEEN EXPOSED TO URANIUM? 1.9 WHAT RECOMMENDATIONS HAS THE FEDERAL GOVERNMENT MADE TO PROTECT HUMAN HEALTH? 1.10 WHERE CAN I GET MORE INFORMATION? 2. HEALTH EFFECTS 2.1 INTRODUCTION 2.2 DISCUSSION OF HEALTH EFFECTS BY ROUTE OF EXPOSURE 2.2.1 Inhalation Exposure 2.2.1.1 Death 2.2.1.2 Systemic Effects 2.2.1.3 Immunological and Lymphoreticular Effects 2.2.1.4 Neurological Effects 2.2.1.5 Reproductive Effects 2.2.1.6 Developmental Effects 2.2.1.7 Genotoxic Effects 2.2.1.8 Cancer 2.2.2 Oral Exposure 2.2.2.1 Death 2.2.2.2 Systemic Effects 2.2.2.3 Immunological and Lymphoreticular Effects 2.2.2.4 Neurological Effects 2.2.2.5 Reproductive Effects 2.2.2.6 Developmental Effects 2.2.2.7 Genotoxic Effects

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2.2.2.8 Cancer Dermal Exposure 2.2.3.1 Death 2.2.3.2 Systemic Effects 2.2.3.3 Immunological and Lymphoreticular Effects 2.2.3.4 Neurological Effects 2.2.3.5 Reproductive Effects 2.2.3.6 Developmental Effects 2.2.3.7 Genotoxic Effects 2.2.3.8 Cancer 2.3 TOXICOKINETICS 2.3.1 Absorption 2.3.1.1 Inhalation Exposure 2.3.1.2 Oral Exposure 2.3.1.3 Dermal Exposure 2.3.2 Distribution 2.3.2.1 Inhalation Exposure 2.3.2.2 Oral Exposure 2.3.2.3 Dermal Exposure 2.3.2.4 Other Routes of Exposure 2.3.3 Metabolism 2.3.4 Elimination and Excretion 2.3.4.1 Inhalation Exposure 2.3.4.2 Oral Exposure 2.3.4.3 Dermal Exposure 2.3.5 Physiologically Based Pharmacokinetic (PBPK)/Pharmacodynamic (PD) Models 2.4 MECHANISMS OF ACTION 2.4.1 Pharmacokinetic Mechanisms 2.4.2 Mechanisms of Toxicity 2.4.3 Animal-to-Human Extrapolations 2.5 RELEVANCE TO PUBLIC HEALTH 2.6 CHILDREN’S SUSCEPTIBILITY 2.7 BIOMARKERS OF EXPOSURE AND EFFECT 2.7.1 Biomarkers Used to Identify or Quantify Exposure to Uranium 2.7.2 Biomarkers Used to Characterize Effects Caused by Uranium 2.8 INTERACTIONS WITH OTHER CHEMICALS 2.9 POPULATIONS THAT ARE UNUSUALLY SUSCEPTIBLE 2.10 METHODS FOR REDUCING TOXIC EFFECTS 2.10.1 Reducing Peak Absorption Following Exposure 2.10.2 Reducing Body Burden 2.10.3 Interfering with the Mechanism of Action for Toxic Effects 2.11 ADEQUACY OF THE DATABASE 2.11.1 Existing Information on Health Effects of Uranium 2.11.2 Identification of Data Needs 2.11.3 Ongoing Studies 2.2.3

3. CHEMICAL AND PHYSICAL INFORMATION 3.1 CHEMICAL IDENTITY 3.2 PHYSICAL, CHEMICAL, AND RADIOLOGICAL PROPERTIES

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4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 4.1 PRODUCTION 4.2 IMPORT/EXPORT 4.3 USE 4.4 DISPOSAL 5. POTENTIAL FOR HUMAN EXPOSURE 5.1 OVERVIEW 5.2 RELEASES TO THE ENVIRONMENT 5.2.1 Air 5.2.2 Water 5.2.3 Soil 5.3 ENVIRONMENTAL FATE 5.3.1 Transport and Partitioning 5.3.2 Transformation and Degradation 5.3.2.1 Air 5.3.2.2 Water 5.3.2.3 Sediment and Soil 5.4 LEVELS MONITORED OR ESTIMATED IN THE ENVIRONMENT 5.4.1 Air 5.4.2 Water 5.4.3 Soil 5.4.4 Other Environmental Media 5.5 GENERAL POPULATION AND OCCUPATIONAL EXPOSURE 5.6 EXPOSURES OF CHILDREN 5.7 POPULATIONS WITH POTENTIALLY HIGH EXPOSURES 5.8 ADEQUACY OF THE DATABASE 5.8.1 Identification of Data Needs 5.8.2 Ongoing Studies 6. ANALYTICAL METHODS 6.1 BIOLOGICAL MATERIALS 6.1.1 Internal Uranium Measurements 6.1.2 In Vivo and In Vitro Uranium Measurements 6.2 ENVIRONMENTAL SAMPLES 6.2.1 Field Measurements of Uranium 6.2.2 Laboratory Analysis of Environmental Samples 6.3 ADEQUACY OF THE DATABASE 6.3.1 Identification of Data Needs 6.3.2 Ongoing Studies 7. REGULATIONS AND ADVISORIES 8. REFERENCES 9. GLOSSARY

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LIST OF APPENDICES A.

ATSDR MINIMAL RISK LEVELS AND WORKSHEETS

B.

USER'S GUIDE

C.

ACRONYMS, ABBREVIATIONS, AND SYMBOLS

D.

OVERVIEW OF BASIC RADIATION PHYSICS, CHEMISTRY AND BIOLOGY

I.

INDEX

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LIST OF FIGURES 2-1

Levels of Significant Exposure to Uranium, Inhalation—Chemical Toxicity

2-2

Levels of Significant Exposure to Uranium, Inhalation—Radiation Toxicity

2-3 Levels of Significant Exposure to Uranium, Oral—Chemical Toxicity 2-4

Conceptual Representation of a Physiologically Based Pharmacokinetic (PBPK) Model for a Hypothetical Chemical Substance

2-5

Respiratory Tract Compartments in Which Particles May Be Deposited

2-6

Compartment Model to Represent Time-Dependent Particle Transport in the Respiratory Tract

2-7

The Human Respiratory Tract Model: Absorption into Blood

2-8

Biokinetic Model for Uranium After Uptake to Blood

2-9

Multicompartmental Model

2-10 Existing Information on the Health Effects of Uranium 4-1

Flow Chart of Uranium Ore Processing

5-1

Frequency of NPL Sites with Uranium (U-238) Contamination

5-2

Major DOE Offices, Facilities, and Laboratories

5-3

Environmental Pathways for Potential Human Health Effects from Uranium

5-4 Average Uranium Concentrations in Drinking Water for States Where Concentration Exceeds 1 pCi/L 7-1

Nuclear Regulatory Commission Agreement States

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LIST OF TABLES 2-1

Levels of Significant Exposure to Uranium, Chemical Toxicity—Inhalation

2-2

Levels of Significant Exposure to Uranium, Radiation Toxicity—Inhalation

2-3

Levels of Significant Exposure to Uranium, Chemical Toxicity—Oral

2-4

Levels of Significant Exposure to Uranium, Chemical Toxicity—Dermal

2-5

Reference Respiratory Values for a General Caucasian Population at Different Levels of Activity

2-6

Reference Values of Parameters for the Compartment Model to Represent Time-dependent Particle Transport from the Human Respiratory Tract

2-7

Sensitivity and Calculated Transfer Coefficients (d-1)

2-8

Enriched, Natural, and Depleted Uranium Mass Equivalents for Radiological Effects

2-9

Genotoxicity of Uranium In Vivo

2-10 Genotoxicity of Uranium In Vitro 2-11 Ongoing Studies on Health Effects of Uranium 3-1

Chemical Identity of Uranium Metal

3-2

Physical and Chemical Properties of Selected Uranium Compounds

3-3

Percent Occurrence and Radioactive Properties of Naturally Occurring Isotopes of Uranium

3-4 Uranium Isotope Decay Series Showing the Decay Products of the Naturally Occurring Isotopes of Uranium 4-1

Uranium Ores

4-2

Uranium Production in the United States by Uranium Mills and Other Sources

4-3

Uranium Mining Production, 1985–1998

4-4

Import of Uranium and Compounds (in kg) into the United States

4-5

Export of Uranium and Compounds (in kg)

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5-1

Normalized Uranium Effluent Discharges from Uranium Mining, Milling, Conversion, Enrichment, and Fuel Fabrication

5-2

Uranium in Airborne Particles (Composites)

5-3

Uranium Analyses of Select Precipitation Composite Samples March–May 1993

5-4

Uranium in Rocks and Soils

5-5

Concentrations of Uranium in Some Foods

5-6

Ongoing Studies on Environmental Effects of Uranium

6-1

Analytical Methods for Determining Uranium in Biological Samples

6-2

Analytical Methods for Determining Uranium in Environmental Samples

6-3

Additional Analytical Methods for Determining Uranium in Environmental Samples

6-4

Ongoing Studies on Analytical Methods for Uranium

7-1

Regulations and Guidelines Applicable to Uranium

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1. PUBLIC HEALTH STATEMENT This public health statement tells you about uranium and the effects of exposure.

The Environmental Protection Agency (EPA) identifies the most serious hazardous waste sites in the nation. These sites make up the National Priorities List (NPL) and are the sites targeted for long-term federal cleanup activities. Elevated uranium levels have been found in at least 54 of the 1,517 current or former NPL sites. However, the total number of NPL sites evaluated for this substance is not known. As more sites are evaluated, the sites at which uranium is found may increase. This information is important because exposure to this substance may harm you and because these sites may be sources of exposure.

When a substance is released from a large area, such as an industrial plant, or from a container, such as a drum or bottle, it enters the environment. This release does not always lead to exposure. You are normally exposed to a substance only when you come in contact with it. You may be exposed by breathing, eating, or drinking the substance or by skin contact. However, since uranium is radioactive, you can also be exposed to its radiation if you are near it.

If you are exposed to uranium, many factors determine whether you'll be harmed. These factors include the dose (how much), the duration (how long), and how you come in contact with it. You must also consider the other chemicals you're exposed to and your age, sex, diet, family traits, lifestyle, and state of health.

1.1 WHAT IS URANIUM?

Uranium is a natural and commonly occurring radioactive element. It is found in very small amounts in nature in the form of minerals, but may be processed into a silver-colored metal. Rocks, soil, surface and underground water, air, and plants and animals all contain varying amounts of uranium. Typical concentrations in most materials are a few parts per million (ppm). This corresponds to around 4 tons of uranium in 1 square mile of soil 1 foot deep, or about half a

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teaspoon of uranium in a typical 8-cubic yard dump truck load of soil. Some rocks and soils may also contain greater amounts of uranium. If the amount is great enough, the uranium may be present in commercial quantities and can be mined. After the uranium is extracted, it is converted into uranium dioxide or other chemical forms by a series of chemical processes known as milling. The residue remaining after the uranium has been extracted is called mill tailings. Mill tailings contain a small amount of uranium, as well as other naturally radioactive waste products such as radium and thorium.

Natural uranium is a mixture of three types (or isotopes) of uranium, written as 234U, 235U, and 238

U, or as U-234, U-235, and U-238, and read as uranium two thirty-four, etc. All three isotopes

behave the same chemically, so any combination of the three would have the same chemical effect on your body. But they are different radioactive materials with different radioactive properties. That is why we must look at the actual percentages of the three isotopes in a sample of uranium to determine how radioactive the uranium is. For uranium that has been locked inside the earth for millions of years, we know the percentage of each isotope by weight and by radioactivity. By weight, natural uranium is about 0.01% 234U, 0.72% 235U, and 99.27% 238U. About 48.9% of the radioactivity is associated with 234U, 2.2% is associated with 235U, and 48.9% is associated with 238U.

The weight and radioactivity percentages are different because each isotope has a different physical half-life. Radioactive isotopes are constantly changing into different isotopes by giving off radiation. The half-life is the time it takes for half of that uranium isotope to give off its radiation and change into a different element. The half-lives of uranium isotopes are very long (244 thousand years for 234U, 710 million years for 235U, and 4½ billion years for 238U). The shorter half-life makes 234U the most radioactive, and the longer half-life makes 238U the least radioactive. If you have one gram of each isotope side by side, the 234U will be about 20 thousand times more radioactive and the 235U will be 6 times more radioactive than the 238U.

Uranium is measured in units of mass (grams) or radioactivity (curies or becquerels), depending on the type of equipment available or the level that needs to be measured. The becquerel (Bq) is

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a new international unit, and the curie (Ci) is a traditional unit; both are currently used. A Bq is the amount of radioactive material in which 1 billion atoms transform every second, and a Ci is the amount of radioactive material in which 37 billion atoms transform every second. The mass and activity ratios given in the previous paragraph are those found in rocks inside the earth’s crust, where 1.5 gram of uranium is equivalent to 1 millionth of a Ci (µCi). Although this ratio can vary in air, soil, and water, the conversions made in this profile use the 1.5-to-1 ratio unless the actual isotope ratios are known. When both mass and radioactivity units are shown, the first is normally the one reported in the literature. Some of the values may be rounded to make the text easier to read.

The uranium isotopes in the earth were present when the earth was formed. Both 235U and 238U have such long half-lives that part of the uranium originally on earth is still here, waiting to give off its radiation. The original 234U would have decayed away by now, but new 234U is constantly being made from the decay of 238U. When 238U gives off its radiation, it changes or decays through a series of different radioactive materials, including 234U. This series, or decay chain, ends when a stable, non-radioactive substance is made. This element is lead. This toxicological profile deals with the uranium isotopes and not with the other radioactive decay products, like radium, thorium, and radon. For uranium that has been in contact with water, the natural weight and radioactivity percentages can vary slightly from the percentages mentioned in the previous paragraphs. We don’t fully understand why that happens in nature, but measurements show us that it does. The processing of uranium for industrial and governmental use can also change the ratios. We give these ratios special names if they were changed by human activities. If the fraction of 235U is increased, we call it enriched uranium. However, if the portion of 235U is decreased, we call it depleted uranium. The differences between the weight and radioactivity ratios matter when we want to convert between radioactivity and mass, and when we talk about how toxic uranium might be. Depleted uranium is less radioactive than natural uranium, and enriched uranium is more radioactive than natural uranium. This profile focuses on natural and depleted uranium, which are more likely to be chemical hazards than radiation hazards. The profile also discusses enriched uranium, which can be both a chemical and a radiation hazard.

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The industrial process called enrichment is used to increase the amount of 234U and 235U and decrease the amount of 238U in natural uranium. The product of this process is enriched uranium, and the leftover is depleted uranium. Enriched uranium is more radioactive than natural uranium, and natural uranium is more radioactive than depleted uranium. When enriched uranium is 97.5% pure 235U, the same weight of enriched uranium has about 75 times the radioactivity as natural uranium. This is because enriched uranium also contains 234U, which is even more radioactive than 235U. The 235U is responsible for most of the radioactivity in enriched uranium. Natural uranium is typically about two times more radioactive than depleted uranium. Other isotopes of uranium called 232U and 233U are produced by industrial processes. These are also much more radioactive than natural uranium.

The total amount of natural uranium on earth stays almost the same because of the very long halflives of the uranium isotopes. The natural uranium can be moved from place to place by nature or by people, and some uranium is removed from the earth by mining. When rocks are broken up by water or wind, uranium becomes a part of the soil. When it rains, the soil containing uranium can be carried into rivers and lakes. Wind can blow dust that contains uranium into the air.

Natural uranium is radioactive but poses little radioactive danger because it gives off very small amounts of radiation. Uranium transforms into another element and gives off radiation. In this way uranium transforms into thorium and gives off a particle called an alpha particle or alpha radiation. Uranium is called the parent, and thorium is called the transformation product. When the transformation product is radioactive, it keeps transforming until a stable product is formed. During these decay processes, the parent uranium, its decay products, and their subsequent decay products each release radiation. Radon and radium are two of these products. Unlike other kinds of radiation, the alpha radiation ordinarily given off by uranium cannot pass through solid objects, such as paper or human skin. For more information on radiation, see Appendix D and the glossary at the end of this profile or the ASTDR Toxicological Profile for Ionizing Radiation.

The main civilian use of uranium is in nuclear power plants and on helicopters and airplanes. It is also used by the armed forces as shielding to protect Army tanks, parts of bullets and missiles

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to help them go through enemy armored vehicles, as a source of power, and in nuclear weapons. Very small amounts are used to make some ceramic ornament glazes, light bulbs, photographic chemicals, and household products. Some fertilizers contain slightly higher amounts of natural uranium. For more information about the properties and uses of uranium, see Chapters 3, 4, and 5.

1.2 WHAT HAPPENS TO URANIUM WHEN IT ENTERS THE ENVIRONMENT?

Uranium is a naturally occurring radioactive material that is present to some degree in almost everything in our environment, including soil, rocks, water, and air. It is a reactive metal, so it is not found as free uranium in the environment. In addition to the uranium naturally found in minerals, the uranium metal and compounds that are left after humans mine and process the minerals can also be released back to the environment in mill tailings. This uranium can combine with other chemicals in the environment to form other uranium compounds. Each of these uranium compounds dissolves to its own special extent in water, ranging from not soluble to very soluble. This helps determine how easily the compound can move through the environment, as well as how toxic it might be.

The amount of uranium that has been measured in air in different parts of the United States by EPA ranges from 0.011 to 0.3 femtocuries (0.00002 to 0.00045 micrograms) per cubic meter (m3). (One femtocurie is equal to 1 picocurie [pCi] divided by 1,000. A picocurie [pCi] is 1 onetrillionth of a curie and a microgram [µg] is one millionth of a gram Even at the higher concentration, there is so little uranium in a cubic meter of air that less than one atom transforms each day.

In the air, uranium exists as dust. Very small dust-like particles of uranium in the air fall out of the air onto surface water, plant surfaces, and soil either by themselves or when rain falls. These particles of uranium eventually end up back in the soil or in the bottoms of lakes, rivers, and ponds, where they stay and mix with the natural uranium that is already there.

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Uranium in water comes from different sources. Most of it comes from dissolving uranium out of rocks and soil that water runs over and through. Only a very small part is from the settling of uranium dust out of the air. Some of the uranium is simply suspended in water, like muddy water. The amount of uranium that has been measured in drinking water in different parts of the United States by EPA is generally less than 1.5 µg (1 pCi) for every liter of water. EPA has found that the levels of uranium in water in different parts of the United States are extremely low in most cases, and that water containing normal amounts of uranium is usually safe to drink. Because of the nature of uranium, not much of it gets into fish or vegetables, and most of that which gets into livestock is eliminated quickly in urine and feces.

Uranium is found naturally in soil in amounts that vary over a wide range, but the typical concentration is 3 µg (2 pCi) per gram of soil. Additional uranium can be added by industrial activities. Soluble uranium compounds can combine with other substances in the environment to form other uranium compounds. Uranium compounds may stay in the soil for thousands of years without moving downward into groundwater. When large amounts of natural uranium are found in soil, it is usually soil with phosphate deposits. The amount of uranium that has been measured in the phosphate-rich soils of north and central Florida ranges from 4.5 to 83.4 pCi of uranium in every gram of soil. In areas like New Mexico, where uranium is mined and processed, the amount of uranium per gram of soil ranges between 0.07 and 3.4 pCi (0.1–5.1 micrograms [µg]) of uranium per gram soil). The amount of uranium in soil near a uranium fuel fabrication facility in the state of Washington ranges from 0.51 to 3.1 pCi/gram (0.8–4.6 µg uranium/gram soil), with an average value of 1.2 pCi/gram (1.7 µg uranium/gram soil). These levels must be carefully compared with the levels in uncontaminated soil in that area, since they are within the normal ranges for uncontaminated soil.

Plants can absorb uranium from the soil onto their roots without absorbing it into the body of the plant. Therefore, root vegetables like potatoes and radishes that are grown in uraniumcontaminated soil may contain more uranium than if the soil contained levels of uranium that were natural for the area. Washing the vegetable or removing its skin often removes most or all of the uranium. For more information about what happens to uranium in the environment, see Chapter 5.

URANIUM 1. PUBLIC HEALTH STATEMENT

1.3 HOW MIGHT I BE EXPOSED TO URANIUM?

Since uranium is found everywhere in small amounts, you always take it into your body from the air, water, food, and soil. Food and water have small amounts of natural uranium in them. People eat about 1–2 micrograms (0.6–1.0 picocuries) of natural uranium every day with their food and take in about 1.5 micrograms (0.8 picocuries) of natural uranium for every liter of water they drink, but they breathe in much lower amounts. Root vegetables, such as beets and potatoes, tend to have a bit more uranium than other foods. In a few places, there tends to be more natural uranium in the water than in the food. People in these areas naturally take in more uranium from their drinking water than from their foods. It is possible that you may eat and drink more uranium if you live in an area with naturally higher amounts of uranium in the soil or water or if you live near a uranium-contaminated hazardous waste site. You can also take in (or ingest) more uranium if you eat food grown in contaminated soil, or drink water that has unusually high levels of uranium. Normally, very little of the uranium in lakes, rivers, or oceans gets into the fish or seafood we eat. The amounts in the air are usually so small that they can be safely ignored. People who are artists, art or craft teachers, ceramic hobbyists, or glass workers who still use certain banned uranium-containing glazes or enamels may also be near to higher levels of uranium, but they will not necessarily take any into their bodies. People who work at factories that process uranium, work with phosphate fertilizers, or live near uranium mines have a chance of taking in more uranium than most other people. People who work on gyroscopes, helicopter rotor counterbalances, or control surfaces of airplanes may work with painted uranium metal, but the coating normally will keep them from taking in any uranium. People who work with armor-piercing weapons that contain uranium will be exposed to low levels of radiation while close to these weapons, but are not likely to take in any uranium. Those who fire uranium weapons, work with weapons with damaged uranium, or on equipment which has been bombarded with these weapons can be exposed to uranium and may wear protective clothes and masks to limit their intake. Larger-than-normal amounts of uranium might also enter the

URANIUM 1. PUBLIC HEALTH STATEMENT

environment from erosion of tailings from mines and mills for uranium and other metals. Accidental discharges from uranium processing plants are possible, but these compounds spread out quickly into the air. For more information about how you may be exposed to uranium, see Chapter 5.

1.4 HOW CAN URANIUM ENTER AND LEAVE MY BODY?

We take uranium into our bodies in the food we eat, water we drink, and air we breathe. What we take in from industrial activities is in addition to what we take in from natural sources.

When you breathe uranium dust, some of it is exhaled and some stays in your lungs. The size of the uranium dust particles and how easily they dissolve determines where in the body the uranium goes and how it leaves your body. Uranium dust may consist of small, fine particles and coarse, big particles. The big particles are caught in the nose, sinuses, and upper part of your lungs where they are blown out or pushed to the throat and swallowed. The small particles are inhaled down to the lower part of your lungs. If they do not dissolve easily, they stay there for years and cause most of the radiation dose to the lungs from uranium. They may gradually dissolve and go into your blood. If the particles do dissolve easily, they go into your blood more quickly. A small part of the uranium you swallow will also go into your blood. The blood carries uranium throughout your body. Most of it leaves in your urine in a few days, but a little stays in your kidneys and bones.

When you eat foods and drink liquids containing uranium, most of it leaves within a few days in your feces and never enters your blood. A small portion will get into your blood and will leave your body through your urine within a few days. The rest can stay in your bones, kidneys, or other soft tissues. A small amount goes to your bones and may stay there for years. Most people have a very small amounts of uranium, about 1/5,000th of the weight of an aspirin tablet, in their bodies, mainly in their bones.

URANIUM 1. PUBLIC HEALTH STATEMENT

Although uranium is weakly radioactive, most of the radiation it gives off cannot travel far from its source. If the uranium is outside your body, such as in soil, most of its radiation cannot penetrate your skin and enter your body. To be exposed to radiation from uranium, you have to eat, drink, or breathe it, or get it on your skin. If uranium transformation products are also present, you can be exposed to their radiation at a distance. For more information about how uranium can leave your body, see Chapter 2.

1.5 HOW CAN URANIUM AFFECT MY HEALTH?

To protect the public from the harmful effects of toxic chemicals and to find ways to treat people who have been harmed, scientists must determine what the harmful effects are, how to test for them, how much of the chemical is required to produce each of the harmful effects, how we recognize an overexposure, and how to treat it.

One way to see if a chemical will hurt people is to learn how the chemical is absorbed, used, and released by the body; for some chemicals, animal testing may be necessary. Animal testing may also be used to identify health effects such as kidney or liver damage, cancer, or birth defects. Without laboratory animals, scientists would lose a basic method to get information needed to make wise decisions to protect public health. Scientists have the responsibility to treat research animals with care and compassion. Laws today protect the welfare of research animals, and scientists must comply with strict animal care guidelines.

Uranium is a chemical substance that is also radioactive. Scientists have never detected harmful radiation effects from low levels of natural uranium, although some may be possible. However, scientists have seen chemical effects. A few people have developed signs of kidney disease after intake of large amounts of uranium. Animals have also developed kidney disease after they have been treated with large amounts of uranium, so it is possible that intake of a large amount of uranium might damage your kidneys. There is also a chance of getting cancer from any radioactive material like uranium. Natural and depleted uranium are only weakly radioactive and are not likely to cause you to get cancer from their radiation. No human cancer of any type has ever

URANIUM 1. PUBLIC HEALTH STATEMENT

been seen as a result of exposure to natural or depleted uranium. Uranium can decay into other radionuclides, which can cause cancer if you are exposed to enough of them for a long enough period. Doctors that studied lung and other cancers in uranium miners did not think that uranium radiation caused these cancers. The miners smoked cigarettes and were exposed to other substances that we know cause cancer, and the observed lung cancers were attributed to large exposures to radon and its radioactive transformation products.

The chance of getting cancer is greater if you are exposed to enriched uranium, because it is more radioactive than natural uranium. Cancer may not become apparent until many years after a person is exposed to a radioactive material (from swallowing or breathing it). Just being near uranium is not dangerous to your health because uranium gives off very little of the penetrating gamma radiation. However, uranium is normally accompanied by the other transformation products in its decay chain, so you would be exposed to their radiation as well.

The Committee on the Biological Effects of Ionizing Radiation (BEIR IV) reported that eating food or drinking water that has normal amounts of uranium will most likely not cause cancer or other health problems in most people. The Committee used data from animal studies to estimate that a small number of people who steadily eat food or drink water that has larger-than-normal quantities of uranium in it could get a kind of bone cancer called a sarcoma. The Committee reported calculations showing that if people steadily eat food or drink water containing about 1 pCi of uranium every day of their lives, bone sarcomas would be expected to occur in about 1 to 2 of every million people after 70 years, based on the radiation dose alone. However, we do not know this for certain because people normally ingest only slightly more than this amount each day, and people who have been exposed to larger amounts have not been found to get cancer.

We do not know if exposure to uranium causes reproductive effects in people. Very high doses of uranium have caused reproductive problems (reduced sperm counts) in some experiments with laboratory animals. Most studies show no effects. For more information about how uranium can affect your health, see Chapter 2.

URANIUM 1. PUBLIC HEALTH STATEMENT

1.6 HOW CAN URANIUM AFFECT CHILDREN?

This section discusses potential health effects from exposures during the period from conception to maturity at 18 years of age in humans. Potential effects on children resulting from exposures of the parents are also considered.

Like adults, children are exposed to small amounts of uranium in air, food, and drinking water. However, no cases have been reported where exposure to uranium is known to have caused health effects in children. It is possible that if children were exposed to very high amounts of uranium they might have damage to their kidneys like that seen in adults. We do not know whether children differ from adults in their susceptibility to health effects from uranium exposure.

It is not known if exposure to uranium has effects on the development of the human fetus. Very high doses of uranium in drinking water can affect the development of the fetus in laboratory animals. One study reported birth defects and another reported an increase in fetal deaths. However, we do not believe that uranium can cause these problems in pregnant women who take in normal amounts of uranium from food and water, or who breathe the air around a hazardous waste site that contains uranium.

Very young animals absorb more uranium into their blood than adults when they are fed uranium. We do not know if this happens in children.

Measurements of uranium have not been made in pregnant women, so we do not know if uranium can cross the placenta and enter the fetus. In an experiment with pregnant animals, only a very small amount (0.03%) of the injected uranium reached the fetus. Even less uranium is likely to reach the fetus in mothers exposed by inhaling, swallowing, or touching uranium. No measurements have been made of uranium in breast milk. Because of the chemical properties of uranium, it is unlikely that it would concentrate in breast milk.

URANIUM 1. PUBLIC HEALTH STATEMENT

1.7 HOW CAN FAMILIES REDUCE THE RISK OF EXPOSURE TO URANIUM?

If your doctor finds that you have been exposed to significant amounts of uranium, ask whether your children might also be exposed. Your doctor might need to ask your state health department to investigate.

It is possible that higher-than-normal levels of uranium may be in the soil at a hazardous waste site. Some children eat a lot of dirt. You should prevent your children from eating dirt. Make sure they wash their hands frequently, and before eating. If you live near hazardous waste site, discourage your children from putting their hands in their mouths or from engaging in other hand-to-mouth activities.

1.8 IS THERE A MEDICAL TEST TO DETERMINE WHETHER I HAVE BEEN EXPOSED TO URANIUM?

Yes, there are medical tests that can determine whether you have been exposed by measuring the amount of uranium in your urine, blood, and hair. Urine analysis is the standard test. If you take into your body a larger-than-normal amount of uranium over a short period, the amount of uranium in your urine may be increased for a short time. Because most uranium leaves the body within a few days, normally the amount in the urine only shows whether you have been exposed to a larger-than-normal amount within the last week or so. If the intake is large or higher-thannormal levels are taken in over a long period, the urine levels may be high for a longer period of time. Many factors can affect the detection of uranium after exposure. These factors include the type of uranium you were exposed to, the amount you took into your body, and the sensitivity of the detection method. Also, the amount in your urine does not always accurately show how much uranium you have been exposed to. If you think you have been exposed to elevated levels of uranium and want to have your urine tested, you should do so promptly while the levels may still be high. In addition to uranium, the urine could be tested for evidence of kidney damage, by looking for protein, glucose, and nonprotein nitrogen, which are some of the chemicals that can appear in your urine because of kidney damage. Testing for these chemicals could determine

URANIUM 1. PUBLIC HEALTH STATEMENT

whether you have kidney damage. However, since kidney damage is also caused by several common diseases, such as diabetes, it would not tell you if the damage was caused by the presence of uranium in your body.

A radioactivity counter can tell if your skin is contaminated with uranium, because uranium is radioactive. If you inhale large amounts of uranium, it may be possible to measure the amount of radioactivity in your body with special radiation measurement instruments. See Chapters 2 and 6 for more information.

1.9 WHAT RECOMMENDATIONS HAS THE FEDERAL GOVERNMENT MADE TO PROTECT HUMAN HEALTH?

International and national organizations like the International Commission on Radiological Protection (ICRP) and the National Council on Radiation Protection and Measurements (NCRP) provide recommendations for protecting people from materials, like uranium, that give off ionizing radiation. The federal government considers these recommendations and develops regulations and guidelines to protect public health. Regulations can be enforced by law. Federal agencies that develop regulations for toxic substances include the EPA, the Nuclear Regulatory Commission (NRC), the Occupational Safety and Health Administration (OSHA), and the Food and Drug Administration (FDA). Recommendations provide valuable guidelines to protect public health but cannot be enforced by law. Federal organizations that develop recommendations for toxic substances include the Agency for Toxic Substances and Disease Registry (ATSDR) and the National Institute for Occupational Safety and Health (NIOSH).

Regulations and recommendations can be expressed as levels that are not to be exceeded in air, water, soil, or food that are usually based on levels that affect animals. Then they are adjusted with appropriate safety factors to help protect people. Sometimes these not-to-exceed levels differ among federal organizations because of different exposure times (an 8-hour workday or a 24-hour day), the use of different animal studies, or other factors.

URANIUM 1. PUBLIC HEALTH STATEMENT

Recommendations and regulations are also periodically updated as more information becomes available. For the most current information, check with the federal agency or organization that provides it. Some regulations and recommendations for uranium are discussed below.

EPA has not set a limit for uranium in air, but it has set a goal of no uranium in drinking water. EPA calls this the Maximum Contaminant Level Goal (MCLG), but recognizes that, currently, there is no practical way to meet this goal. Because of this, EPA proposed in 1991 to allow up to 20 µg of uranium per liter (20 µg/L) in drinking water, and states began regulating to achieve this level. EPA calls this the Maximum Contaminant Level (MCL). The MCL for uranium is based on calculations that if 150,000 people drink water that contains 20 µg/L of uranium for a lifetime, there is a chance that one of them may develop cancer from the uranium in the drinking water. In 1994, EPA considered changing the MCL to 80 µg per liter based on newer human intake and uptake values and the high cost of reducing uranium levels in drinking water supplies. In 1998, EPA temporarily dropped its 1991 limit, but is currently working to develop an appropriate limit based on a broader range of human and animal health studies. ATSDR, other federal agencies, Canada, and other professionals are advising EPA regarding a new MCL. Canada is currently developing its own national guideline value because that country has the richest known uranium ore deposits in the world and high uranium concentrations in some of its well water.

EPA has also decided that any accidental uranium waste containing 0.1 curies of radioactivity (150 kilograms) must be cleaned up. EPA calls this the Reportable Quantity Accidental Release. EPA also has established a standard for uranium mill tailings. In the workplace, NIOSH/OSHA has set a Recommended Exposure Limit (REL) and a Permissible Exposure Limit (PEL) of 0.05 mg/m3 (34 pCi/m3) for uranium dust, while the NRC has an occupational limit of 0.2 mg/m3 (130 pCi/m3). The NRC has set uranium release limits at 0.06 pCi/m3 (0.09 µg/m3) of air and 300 pCi/liter (450 µg/liter) of water. NRC and OSHA expect that the public will normally be exposed to much lower concentrations. For more information about recommendations the federal government has made to protect your health, see Chapter 7.

URANIUM 1. PUBLIC HEALTH STATEMENT

1.10 WHERE CAN I GET MORE INFORMATION?

If you have any more questions or concerns, please contact your community or state health or environmental quality department or Agency for Toxic Substances and Disease Registry Division of Toxicology 1600 Clifton Road NE, Mailstop E-29 Atlanta, GA 30333

* Information line and technical assistance Phone: 1-888-42-ATSDR (1-888-422-8737) Fax: (404) 639-6315 or 6324 ATSDR can also tell you the location of occupational and environmental health clinics. These clinics specialize in recognizing, evaluating, and treating illnesses resulting from exposure to hazardous substances. * To order toxicological profiles, contact National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Phone: (800) 553-6847 or (703) 605-6000

URANIUM

2. HEALTH EFFECTS 2.1 INTRODUCTION

The primary purpose of this chapter is to provide public health officials, physicians, toxicologists, and other interested individuals and groups with an overall perspective of the toxicology of uranium. It contains descriptions and evaluations of toxicological studies and epidemiologic investigations and provides conclusions, where possible, on the relevance of toxicity and toxicokinetic data to public health.

A glossary and list of acronyms, abbreviations, and symbols can be found at the end of this profile.

The health effects associated with oral or dermal exposure to natural and depleted uranium appear to be solely chemical in nature and not radiological, while those from inhalation exposure may also include a slight radiological component, especially if the exposure is protracted. A comprehensive review by the Committee on the Biological Effects of Ionizing Radiation (BEIR IV) concluded that ingesting food or water containing normal uranium concentrations will most likely not be carcinogenic or cause other health problems in most people. Inhaled uranium is associated with only a low cancer risk, with the main risk being associated with the co-inhalation of other toxic and/or carcinogenic agents, such as the radioactive transformation products of radon gas and cigarette smoke. Very high oral doses of uranium have caused renal damage in humans. Animal studies in a number of species and using a variety of compounds confirm that uranium is a nephrotoxin and that the most sensitive organ is the kidney. Hepatic and developmental effects have also been noted in some animal studies. This profile is primarily concerned with the effects of exposure to natural and depleted uranium, but does include limited discussion regarding enriched uranium, which is considered to be more of a radiological than a chemical hazard. Also, whenever the term “radiation” is used, it applies to ionizing radiation and not to nonionizing radiation.

Although natural and depleted uranium are primarily chemical hazards, the next several paragraphs describe the radiological nature of the toxicologically-important uranium isotopes, because individual isotopes are addressed in some of the health effects studies. Uranium is a naturally occurring radioactive element and a member of the actinide series. Radioactive elements are those that undergo spontaneous transformation (decay), in which energy is released (emitted) either in the form of particles, such as alpha or beta particles, or electromagnetic radiation with energies sufficient to cause ionization, such as gamma

URANIUM 2. HEALTH EFFECTS

or X-rays. This transformation or decay results in the formation of different elements, some of which may themselves be radioactive, in which case they will also decay. The process continues until a stable (nonradioactive) state is reached (see Appendix D for more information).

Uranium exists in several isotopic forms, all of which are radioactive. The most toxicologically important of the 22 currently recognized uranium isotopes are anthropogenic uranium-232 (232U) and uranium-233 (233U) and naturally occurring uranium-234 (234U), uranium-235 (235U), and uranium-238 (238U). When an atom of any of these five isotopes decays, it emits an alpha particle (the nucleus of a helium atom) and transforms into a radioactive isotope of another element. The process continues through a series of radionuclides until reaching a stable, non-radioactive isotope of lead. The radionuclides in these transformation series (such as radium and radon), emit alpha, beta, and gamma radiations with energies and intensities that are unique to the individual radionuclide. Natural uranium consists of isotopic mixtures of 234U, 235U, and 238U. There are three kinds of mixtures (based on the percentage of the composition of the three isotopes): natural uranium, enriched uranium, and depleted uranium. Natural uranium, including uranium ore, is comprised of 99.284% 238U, 0.711% 235

U, and 0.005% 234U by mass. Combining these mass percentages with the unique half-life of each

isotope converts mass into radioactivity units and shows that uranium ore contains 48.9% 234U, 2.25% 235

U, and 48.9% 238U by radioactivity, and has a very low specific activity of 0.68 µCi/g (Parrington et al.

1996). Enriched and depleted uranium are the products of a process which increases (or enriches) the percentages of 234U and 235U in one portion of a uranium sample and decreases (or depletes) their percentages in the remaining portion. Enriched uranium is quantified by its 235U percentage Uranium enrichment for nuclear energy produces uranium that typically contains 3% 235U. Uranium enrichment for a number of other purposes, including nuclear weapons, can produce uranium that contains as much as 97.3% 235U and has a higher specific activity (50 µCi/g). The residual uranium after the enrichment process is called “depleted uranium” (DU), which possesses even less radioactivity (0.36 µCi/g) than natural uranium. The Nuclear Regulatory Commission (NRC) considers the specific activity of depleted uranium to be 0.36 µCi/g (10 CFR 20), but more aggressive enrichment processes can drive this value slightly lower (0.33 µCi/g). In this profile, both natural and depleted uranium are referred to as "uranium." The higher specific-activity mixtures and isotopes are described in the profile as "enriched uranium," or as 232U, 233U, or 234U, as applicable, in the summary of the studies in which these mixtures and isotopes were used.

URANIUM 2. HEALTH EFFECTS

Because uranium is a predominantly alpha-emitting radionuclide, there is a concern for potential DNA damage and fragmentation if alpha particles reach cell nuclei. Attempts by cells to repair this fragmentation, if it occurs, may result in repair errors, producing gene mutations or chromosomal aberrations. These effects, when sufficiently severe, may be manifested as cancer and possibly as developmental malformations. However, the genetic effects of radiation have not been observed in humans with exposure to radiation, including that from uranium. Ionizing radiation may also promote carcinogenesis by an apoptotic mechanism by which radiation-induced cell death in tissues or organs elicits an increased cell proliferation response to replace the lost cells. Increased mitotic activity may afford cancer cells a preferential advantage in clonal expansion.

Although radiation exposure has been generally assumed to be carcinogenic at all dose levels, no correlation has been established at low doses such as occur from exposure to natural radiation background levels. This is largely attributable to two factors: (1) it is difficult to construct and obtain meaningful data from epidemiological studies where exposure is near background exposure levels, and (2) the data are not statistically significant enough to substantiate a detectable health impact. Recent risk assessment reviews of carcinogenicity and exposure to hazardous chemicals, including radiation, have been questioning the non-threshold assumption. With specific reference to radiation, there is increasing biological evidence that there is a threshold for radiation-induced carcinogenicity (Clark 1999).

The National Research Council Committee on the Biological Effects of Ionizing Radiation BEIR IV report stated that ingesting uranium in food and water at the naturally occurring levels will not cause cancer or other health problems in people. However, based on the zero-threshold linear dose-response model (a conservative model that is inherently unverifiable and is intended to be used as an aid to riskbenefit analysis and not for predicting cancer deaths), the BEIR IV committee calculated that the ingestion of an additional 1 pCi/day (0.0015 mg/day) of soluble natural uranium would lead to a fractional increase in the incidence rate of osteogenic sarcoma (bone cancer) of 0.0019. This means that over a period of 70 years (the nominal lifetime length), if everyone were exposed at that level, the number of bone cancer cases in a U.S. population of 250 million would increase from 183,750 to about 184,100. Currently, there are no unequivocal studies that show that intake of natural or depleted uranium can induce radiation effects in humans or animals. The available information on humans and animals suggests that intake of uranium at the low concentrations usually ingested by humans or at levels found at or near hazardous waste sites is not likely to cause cancer. The BEIR IV committee, therefore, concluded

URANIUM 2. HEALTH EFFECTS

that "...exposure to natural uranium is unlikely to be a significant health risk in the population and may well have no measurable effect."

Exposure to enriched uranium, used as uranium fuel in nuclear energy production, may present a radiological health hazard. Although uranium-associated cancers have not been identified in humans, even following exposure to highly enriched uranium, higher doses associated with highly enriched, high specific activity uranium may be able to produce bone sarcomas in humans. Evidence from animal studies suggests that high radiation doses associated with large intakes of 234U and 235U-enriched uranium compounds can be hazardous. Adverse effects reported from such exposures include damage to the interstitium of the lungs (fibrosis) and cardiovascular abnormalities (friable vessels). However, access to 235

U-enriched or other high specific-activity uranium is strictly regulated by the NRC and the U.S.

Department of Energy (DOE). Therefore, the potential for human exposure to this level of radioactivity is limited to rare accidental releases in the workplace.

The potential for adverse noncancerous radiological health effects from uranium is dependent on several factors, including physicochemical form and solubility, route of entry, distribution in the various body organs, the biological retention time in the various tissues, and the energy and intensity of the radiation. The potential for such effects is generally thought to be independent of the known chemical toxicity of uranium. While the chemical properties affect the distribution and biological half-life of a radionuclide, the damage from radiation is independent of the source of that radiation. In this profile, there is little, or equivocal, specific information regarding the influence of radiation from uranium on certain biological effect end points in humans, such as reproductive, developmental, or carcinogenic effects. There is evidence, however, from the large body of literature concerning radioactive substances that alpha radiation can affect these processes in humans (see Appendix D for additional information on the biological effects of radiation). However, because the specific activities of natural and depleted uranium are low, no radiological health hazard is expected from exposure to natural and depleted uranium. Since the radiological component of natural uranium has essentially been discounted as a significant source of health effects, this leaves only the chemical effects of uranium to contend with. The chemical (nonradiological) properties of natural uranium and depleted uranium are identical; therefore, the health effects exerted by each are expected to be the same. The results of the available studies in humans and animals are consistent with this conclusion. The potential health impacts of depleted uranium are specifically addressed in a recent Department of Energy publication (DOE 1999).

URANIUM 2. HEALTH EFFECTS

Uranium is a heavy metal that forms compounds and complexes of different varieties and solubilities. The chemical action of all isotopes and isotopic mixtures of uranium is identical, regardless of the specific activity (i.e., enrichment), because chemical action depends only on chemical properties. Thus, the chemical toxicity of a given amount or weight of natural, depleted, and enriched uranium is identical.

The toxicity of uranium varies according to its chemical form and route of exposure. On the basis of the toxicity of different uranium compounds in animals, it was concluded that the relatively more water-soluble compounds (uranyl nitrate hexahydrate, uranium hexafluoride, uranyl fluoride, uranium tetrachloride, uranium pentachloride) were the most potent renal toxicants. The less water-soluble compounds (sodium diuranate, ammonium diuranate) were of moderate-to-low renal toxicity, and the insoluble compounds (uranium tetrafluoride, uranium trioxide, uranium dioxide, uranium peroxide, triuranium octaoxide) had little potential to cause renal toxicity but could cause pulmonary toxicity when exposure was by inhalation. The terms soluble, moderately soluble, and insoluble are often used in this profile without relisting the specific compounds. Generally, hexavalent uranium, which tends to form soluble compounds, is more likely to be a systemic toxicant than tetravalent uranium, which forms insoluble compounds. Ingested uranium is less toxic than inhaled uranium, which may be partly attributable to the relatively low gastrointestinal absorption of uranium compounds. Only 100 µg/L is indicative of recent absorption, while a concentration of 90% enriched) is used in special research reactors (most of which have been removed from operation) (Weigel 1983), nuclear submarine reactor cores, and nuclear weapons. Depleted uranium metal (DU) is used as radiation shielding, missile projectiles, target elements in plutonium production reactors, a gyroscope component, and counterweights or stabilizers in aircraft.

Uranium continuously undergoes transformation through the decay process whereby it releases energy to ultimately become a stable or nonradioactive element. For the uranium isotopes, this is a complex process involving the serial production of a chain of decay products, called progeny, until a final stable element is formed. The decay products of the uranium isotopes, which are also radioactive, are shown in Table 3-4.

238

U is the parent isotope of the uranium series (234U is a decay product of 238U), while 235U is

the parent isotope of the actinide series. All natural uranium isotopes and some of their progeny decay by emission of alpha particles; the other members of both series decay by emission of beta particles and gamma rays (Cowart and Burnett 1994). Both the uranium and the actinide decay series have three features in common. Each series begins with a long-lived parent, 235U or 238U, each series contains an isotope of the noble gas radon, and each series ends with a stable isotope of lead, 207Pb or 206Pb.

The amount of time required for one-half of the atoms of a radionuclide to transform is called its radioactive half-life. The rate of decay, and thus the half-life, for each radionuclide is unique. The half-life of 238

U is very long, 4.5×109 years; the half-lives of 235U and 234U are orders of magnitude lower,

7.1×108 years and 2.5×105 years, respectively. Since the activity of a given mass of uranium depends on the mass and half-life of each isotope present, the greater the relative abundance of the more rapidly decaying 234U and 235U, the higher the activity will be (EPA 1991). Thus, depleted uranium is less radioactive than natural uranium and enriched uranium is more radioactive.

Uranium is unusual among the elements because it is both a chemical and a radioactive material. The hazards associated with uranium are dependent upon uranium's chemical and physical form, route of intake, and level of enrichment. The chemical form of uranium determines its solubility and, thus,

URANIUM 3. CHEMICAL AND PHYSICAL INFORMATION

transportability in body fluids as well as retention in the body and various organs. Uranium’s chemical toxicity is the principal health concern, because soluble uranium compounds cause heavy metal damage to renal tissue. The radiological hazards of uranium may be a primary concern when inhaled, enriched (>90%) and insoluble uranium compounds are retained long-term in the lungs and associated lymphatics.

URANIUM

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL 4.1 PRODUCTION

Uranium is present in the earth's crust at approximately 3 ppm (2 pCi/g) (du Preez 1989). Although there are more than 100 uranium ores, carnotite, pitchblende, coffinite, uraninite, tobernite, autunite, tyuyamunite, and a few others are the main ores of commercial interest. The main ores are described in Table 4-1. The most economically attractive uranium ores have uranium concentrations in excess of 1,000 ppm (700 pCi/g) (Stokinger 1981; Weigel 1983). In the United States, the major ore deposits are located in Colorado, Utah, Arizona, New Mexico, Wyoming, Nebraska, and Texas (EPA 1985a). The steps necessary to produce uranium for its various uses include mining, milling, conversion to uranium hexafluoride (UF6), enrichment, reduction to metal or oxidation to uranium oxide, and fabrication into the desired shape. The steps for preparing commercial reactor grade, submarine reactor grade, or weapons-grade uranium are the same, except the last two require a more aggressive enrichment process. Depleted uranium metal is produced by reducing the depleted uranium hexafluoride byproduct. Conventional fabrication methods are used to configure the uranium for specific uses, such as rectangular solid blocks for helicopter rotor counterbalances and parabolic or cylindrical solids for military depleted uranium projectiles.

Mining.

Open-pit mining, in situ leaching, and underground mining are three techniques that have

been used for mining uranium-containing ores (EPA 1985a). Uranium is found in all soil and rock, but the higher concentrations found in phosphate rock, lignite, and monazite sands are sufficient in some areas for commercial extraction (Lide 1994). The two most commonly used mining methods are open-pit and underground mining. The choice of method is influenced by factors such as the size, shape, grade, depth, and thickness of the ore deposits (Grey 1993). In situ leaching involves leaching (or dissolving) uranium from the host rock with liquids without removing the rock from the ground and can only be carried out on unconsolidated sandstone uranium deposits located below the water table in a confined aquifer. A leaching solution is introduced into or below the deposit and pumped to the surface, where the uranium-pregnant liquor is processed in a conventional mill to precipitate the uranium as yellowcake (U3O8 and other oxides) (DOE 1995b; Grey 1993).

URANIUM 4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

Table 4-1. Uranium Ores Ore

Composition

Description

Uraninite

UO2 + UO3

A major ore of uranium and radium; can dissolve in acids

Pitchblende

UO2 + UO3

Essentially the same as uraninite

Carnotite

K2O • 2U2O3 • V2O5 • 3 H2O (uranium potassium vanadate)

Autunite

Ca (UO2)2 (PO4)2 •10 H2O

Can lose water to form meta-autunite; dissolves in acids

Torbernite

Cu (UO2)2 (PO4)2 •10 H2O

Loses water easily in air forming metatorbernite; dissolves in acids

Coffinite

U(SiO4)1-x (OH)4x (uranium silicate)

Tyuyamunite

Ca(UO2)2 (VO4)2 • 5–8 H2O (uranium calcium vanadate)

Closely related to carnotite

Source: Amethyst Galleries 1995; Stockinger 1981; Uranium Institute 1996

URANIUM 4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

Milling.

Ore mined in an open-pit or underground mine is crushed and leached in a uranium mill. The

initial step in conventional milling involves crushing, grinding, and wet and/or dry classification of the crude ore to produce uniformly sized particles that are similar in size to beach sand. A slurry generated in the grinding circuit is transferred to a series of tanks for leaching by either an alkaline or acid process. Generally, leaching is a simple process whereby uranyl ions are extracted by a solvent. Uranyl ions are stripped from the extraction solvent and precipitated as yellowcake, predominantly U3O8 (EPA 1995d). Yellowcake is pressed, dried, banded, and shipped for refinement and enrichment. Some of the process streams can also be used to extract other oxides, such as vanadium pentoxide. The byproduct of this process is the leftover sand, known as tailings. Thus, tailings are the original sand minus much of the uranium plus residual process chemicals and tailings are less radioactive than the original ore. (Uranium metal production in a conversion facility is done post-enrichment.) Generalized flow charts for the alkaline and acid leaching processes for ore concentration and uranium production are shown in Figure 4-1.

Enrichment.

Next, the U3O8 is chemically converted to UF6. The enrichment process increases, or

enriches, the percentage of the fissionable 235U isotope, as well as 234U. In the United States, the process used for enrichment is gaseous diffusion. The mechanism for enrichment is based on the fact that a UF6 molecule containing 235U or 234U is lighter and smaller, and has, therefore, a slightly higher thermal velocity than a UF6 molecule containing 238U. As the UF6 passes through the series of diffusion stages, the 234UF6 and 235UF6 molecules gradually become more concentrated downstream and less concentrated upstream, while the 238UF6 concentrates conversely. The lead portion of the stream is collected and recycled to reach the desired enrichment. The tail portion containing a reduced 235UF6 content called depleted UF6 can be stored in the vicinity of the gaseous diffusion plant sites (DOE 1994b). There are an estimated 560,000 metric tons of depleted uranium currently in storage as UF6. A second enrichment technology, gas centrifuge separation, has been used in Europe. A third technology, laser separation, is currently under development (DOE 1995b). A fourth technology, thermal separation, is inefficient and no longer used.

Fuel fabrication.

The enriched UF6 is either reduced to metallic uranium and machined to the

appropriate shape, or oxidized to uranium dioxide and formed into pellets of ceramic uranium dioxide (UO2). The pellets are then stacked and sealed inside metal tubes that are mounted into special fuel assemblies ready for use in a nuclear reactor (DOE 1995b; Uranium Institute 1996).

URANIUM 4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

Figure 4-1. Flow Chart of Uranium Ore Processing

Open Pit Mining

Crushing and Grinding

Underground Mining

Leaching

Tailings Disposal

Separate Solids

Extract and Concentrate U in Liquor

Precipitate Uranium

Wash Solids

Barren Rinsate

Drying, Pressing and Heating

Yellow Cake Uranium Ore Concentrate

Source: ATSDR 1997; Uranium Institute 1996

Barren Liquor

URANIUM 4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

Product fabrication.

Uranium metal has commercial and industrial uses due to its great density and

strength. It is alloyed with a range of metals to meet other commercial and industrial needs. As with steel, uranium can be formed and fashioned by drop forging, dye casting, and machining and is often painted to minimize oxidation. Some well known uses for these products are gyroscopic wheels in guidance systems, helicopter rotor blade counterbalances, weights in airplane control surfaces, and radiation shields for high radioactivity sources (e.g. industrial radiography).

Production.

Uranium production from 1975 to 1996 is shown in Table 4-2. Peak production of

uranium occurred in 1980 at 21,852 short tons (1.98x107 kg) and decreased until 1993. This was the same period when the planning and construction of new nuclear power plants ceased in the United States. Production of U3O8 had decreased to 4,443 short tons (4.03x106 kg) in 1990 and to 1,534 short tons (1.39x106 kg) in 1993, a 65% reduction (ABMS 1994; EPA 1985a). In 1996, U.S. uranium production was 3,160 (2.87x106 kg) short tons, an increase of 5% from the 1995 level and the highest level since 1991 (DOE 1996a). Underground and open-pit mining have been the two most commonly used methods of mining uranium ores. However, by 1994, uranium was produced primarily by in situ leaching methods. A summary of U.S. mine production from 1985 through 1996 (see Table 4-3) illustrates the shift from underground and open-pit mining to in situ leaching.

Leached uranium concentrate was produced in 1996 in Wyoming, Louisiana, Nebraska, New Mexico, and Texas. At the end of 1996, two phosphate by-product plants and five in situ leaching plants were in operation. In addition, seven phosphate by-product and in situ leaching plants were inactive, and seven conventional uranium mills were being maintained in stand-by mode (DOE 1996b).

4.2 IMPORT/EXPORT

The importation of uranium increased significantly in the 1980s (EPA 1985a). In 1983, 3,960 short tons of U3O8 equivalent were imported into the United States (USDOC 1984), which was about 37% of the domestic production. In 1987, the amount of U3O8 equivalent imported into the United States was 5,630 short tons (USDOC 1988). The amounts of uranium and uranium compounds imported into the United States during the period 1989–1997 are presented in Table 4-4 (USDOC 1995, 1997). The importation of uranium and uranium compounds peaked in 1990 at about 23 million kg (about 1 million tons) and has remained approximately the same, with some fluctuation, since that time.

URANIUM 4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

Table 4-2. Uranium Production in the United States by Uranium Mills and Other Sources Year

Short tons of U3O8

1975

11600

1.05x107

1976

12747

1.16x107

1977

14939

1.35x107

1978

18486

1.68x107

1979

18736

1.70x107

1980

21852

1.97x107

1981

19237

1.74x107

1982

13434

1.22x107

1983

10579

9.60x106

1989

6919

6.28x106

1990

4443

4.03x106

1991

3976

3.61x106

1992

2823

2.56x106

1993

1534

1.39x106

1994

1676

1.52x106

1995

3022

2.74x106

1996

3160

2.87x106

1997

2820

2.56x106

1998

2550

2.13x106

Source: ABMS 1994; DOE 1996a, 1996b, 1999a; EPA 1985a a

Kilograms of U3O8

Short ton = 2,000 pounds = 907 kilograms

URANIUM

Table 4-3. Uranium Mining Production, 1985–1998 Percentage of total 1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

Underground

52.3

77.8

81.7

56.8

54.4

Wa

W

W

0

0

0

W

W

W

Open-pit

23.3

W

W

W

W

32.0

48.8

W

0

0

0

0

0

0

In situ leaching

No data

No data

W

W

W

W

W

W

W

96.9

95.6

93.1

86.7

77.8

Other

24.4b

22.2c

18.3c

43.2c

45.6c

68.0d

51.2d

100e

100f

3.1g

4.4g

6.9h

13.3

22.2

a

Withheld; data included with "Other" In situ leach, mine water, water-treatment plant solutions c Open-pit, in situ leach, heap leach, mine water, water-treatment plant solutions d Underground, in situ leach, heap leach (1990), restoration e Underground, open-pit, and in situ leach mines, uranium-bearing water from mine workings, tailing ponds, restoration f In situ leach mines, uranium-bearing water from mine workings and restoration g Production from uranium-bearing water from mine workings and restoration h Production from an underground mine and uranium-bearing water from mine workings and restoration b

Source: DOE 1995, 1996a, 1996b, 1999b

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

Mining method

URANIUM

Table 4-4. Import of Uranium and Compounds (in kg) into the United States Year Substance U-metal (depleted)

1991

1992

1993

1994

1995

1996

1997

1998

18,343

9,673

9,008

4,458

1,735

792

1572

36,359

508

5

2,444

9

25

No data

No data

309,681

10

2

7,925,762 9,713,406

8,992,532

8,880,669

9,259,002

No data

U-alloys, dispersions and ceramic materials U-oxide (natural)

8,459,924

12,630,433 10,043,472

U-oxide (enriched)

204,592

200,733

63,875

35,779

14,214

97,976

57,241

158,082

36,121

U-oxide (depleted)

886,853

19,410

608,472

495

0

0

11,253

0

0

U-fluoride (natural)

9,432,470

8,109,402

5,844,985

U-fluoride (enriched)

598,763

874,251

875,831

U-fluoride (depleted)

479,601

4,523

125,466

0

U-compounds (NOS)

42,277

191,221

847,425

U-compounds (NOS) (enriched)

159,220

28,950

U-compounds (NOS) (depleted)

4,731

Ores and concentrates

934,046

1,024,148

858,807

1,252,438

0

58,000

0

0

147,691

1,275,137

121,439

86,935

324

446,812

253,211

6

6

69,063

0

6,800

29,682

No data

294

1,666

107

0

122

245

100

248

2,763,185

1,344,927

0

0

No data

No data

0

212,434

0

0

0

0

4,431

No data

No data

No data

No data

No data

16,401

5,033

0

115

45

0

23

306

No data

Mixture (depleted) Spent fuel

10,827,786 9,583,669 11,140,026 10,936,114 12,210,369 12,965,093

NOS = Not otherwise stated Source: USDOC 1995, 1997, 1999

868,652 1,000,950

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

1990

URANIUM 4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

The amount of uranium and uranium compounds exported from the United States during 1989–1993 is shown in Table 4-5. The total volume of uranium and uranium compounds exported during 1989–1993 was two orders of magnitude lower than the quantities imported during this same time period. Exports in 1996 were 5.2 million kg. Most of the foreign sales (Canada, France, Germany, Japan, South Korea, United Kingdom) occurred after the uranium entered the U.S. market as imports (DOE 1999b).

4.3 USE

Uranium has been produced for use in the commercial nuclear power industry as low-enriched metal or ceramic UO2 fuel pellets; smaller quantities of high-enriched fuel are produced for U.S. Navy ships and for weapons manufacture (EPA 1985b; Stokinger 1981). Uranium fuel lasts months to years before refueling is needed, and then only a small fraction of the uranium has actually been fissioned, making fuel reprocessing an option used in other countries. One pound of completely fissioned uranium produces the same amount of energy as 1,500 tons of coal (Lide 1994). Depleted uranium is used in the manufacture of armor-piercing ammunition for the military, in inertial guidance devices and gyro compasses, as counterbalances for helicopter rotors, as counterweights for aircraft control surfaces, as radiation shielding material, and as x ray targets (EPA 1985b; USDI 1980). Uranium dioxide is used to extend the lives of filaments in large incandescent lamps used in photography and motion picture projectors. Uranium compounds are used in photography for toning, in the leather and wood industries for stains and dyes, and in the silk and wood industries as mordants. Ammonium diuranate is used to produce colored glazes in ceramics. Uranium carbide is a good catalyst for the production of synthetic ammonia (Hawley 1981). Additionally, uranium was used in dental porcelains for many years, but this practice has been discontinued (Thompson 1976). According to the USDI (1980), the major uses of depleted uranium in 1978 were military ammunition, 71.8%; counterweights, 11.4%; radiation shielding, 13.6%; and chemical catalysts, 3.2% although this ratio may shift to support war efforts.

4.4 DISPOSAL

Radioactive waste containing uranium is usually grouped into three categories: uranium mill tailings, low-level waste, and, in the case of spent reactor fuel, high-level waste.

URANIUM

Table 4-5. Export of Uranium and Compounds (in kg) Year Substance

1990

1991

1992

1993

1994

1995

1996

1997

1998

6,302

318

0

96,748

6,196

690,449

351,169

192,296

250,443

0

U-oxide (enriched)

0

0

85

26,596

64

418,873

299,175

323,990

903,810

646,984

U-fluoride (natural)

85

0

20,175

186,530

4,231

No data

No data

0

688,873

53,800

U-fluoride (enriched)

15,698

39,262

0

175,445

90,459

No data

No data

No data

No data

No data

U-compounds (NOS)

0

9,801

12,596

8,019

0

No data

No data

No data

No data

No data

U-compounds (enriched)

28,221

0

6,609

3

0

0

66,893

418,447

10,506

99,456

U-compounds (depleted)

0

90

160

0

0

246,765

379,530

406,079

30,426

41,674

No data

No data

No data

No data

No data

270

496

299,117

3,159

0

No data

No data

No data

No data

No data

59,461

0

0

0

0

No data

No data

No data

No data

No data

74,712

45,424

29,759

152,920

0

0

21,576

0

0

0

No data

No data

No data

No data

No data

U-oxide (natural)

U-metal U-ores and concentrates Alloys, Dispersions, Ceramics Spent fuel

a

NOS = Not otherwise stated a

Alloys, Dispersions (Including Cermets), Ceramic Products And Mixtures Containing Natural Uranium Compounds, Nesoi (SIC2819)

Source: USDOC 1995, 1999

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

1989

URANIUM 4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

Uranium mill tailings are the residual sand and trace chemicals from the processing of uranium ore. About 150 tons of enriched uranium are required per year to fuel a 1,000-megawatt electric nuclear power reactor, and about 500 times that amount of ore is required to obtain the uranium. The total accumulation of uranium mill tailings in the United States is approximately 140 million tons (Murray 1994). Tailings from mines and mills that process other metals should also be expected to contain elevated concentrations of uranium and its progeny, although this may not be readily recognized.

Disposal methods for processed uranium tailings have been discussed by Bearman (1979). In the late 1940s, mainly unconfined disposal systems were used. Untreated solid wastes were stored as open piles and, in some cases, were spread in urban areas where they were used as fill and as the sand in concrete used to build roads, walks, drives, and concrete block, and in brick mortar. As a result of the Animas River Survey in the United States, tailing control programs were instituted in 1959 to prevent airborne and waterborne dispersal of the wastes. Confined disposal methods were devised to reduce the exposure and dispersion of wastes and to reduce seepage of toxic materials into groundwater to the maximum extent reasonably achievable. Under the Uranium Mill Tailings Radiation Control Act (UMTRCA) of 1978, the U.S. Department of Energy (DOE) designated 24 inactive tailings piles for cleanup. These 24 sites contained a total of about 28 million tons of tailings and covered a total of approximately 1,000 acres (EPA 1985b). Cleanup has been completed at some sites.

In 1977, the EPA issued Environmental Radiation Protection Standards to limit the total individual radiation dose due to emissions from uranium fuel cycle facilities, including licensed uranium mills. This standard specified that the "annual dose equivalent does not exceed 25 millirems (0.25 mSv) to the whole body, 75 millirems (0.75 mSv) to the thyroid, and 25 millirems (0.25 mSv) to any other organ of any member of the public as the result of exposures of planned discharges of radioactive materials...to the general environment" (40 CFR 190). The EPA also established environmental standards for cleanup of open lands and buildings contaminated with residual radioactivity from inactive uranium processing sites (40 CFR 192).

Low-level radioactive waste (LLRW), which may contain uranium, is disposed of at DOE facilities and at commercial disposal facilities. Since 1963, six commercial LLRW facilities have operated, but only two were in operation in 1995. A 1992 report listed the total volume of LLRW buried at all 6 sites to be approximately 50 million cubic feet (Murray 1994). Only a small fraction of the LLRW contains uranium. The method of disposal for commercial and DOE LLRW has been shallow land burial, in

URANIUM 4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

which the waste is disposed of in large trenches and covered. This method of disposal relies upon natural features to isolate the waste. Although U.S. Nuclear Regulatory Commission (USNRC) regulations for LLRW disposal (10 CFR 61) permit shallow land burial, many states have enacted more stringent regulations that require artificial containment of the waste in addition to natural containment (Murray 1994).

The EPA has proposed regulations for LLRW disposal that would apply to DOE facilities (EPA

1998b).

High-level radioactive waste (HLRW) includes spent fuel, which is the uranium fuel rods that have been used in a nuclear reactor. When the fuel rods are removed from the reactor for refueling, they still contain most of the original unfissioned uranium. However, the hazard from the large activity of fission products and plutonium that have been produced in the fuel rods overshadows that of uranium. Approximately 30,000 metric tons of spent fuel have been removed from U.S. power reactors through 1994 (Murray 1994). There is currently no permanent disposal facility for HLRW in the United States; these wastes are being stored at commercial nuclear power plants and DOE facilities where they were produced. The NRC has issued standards for the disposal of HLRW (10 CFR 60), and the DOE is pursuing the establishment of an HLRW facility. Efforts to establish an HLRW facility, which began over two decades ago, have experienced many delays. A facility for the permanent disposal of HLRW is not projected to be in operation before 2010 (Murray 1994).

URANIUM

5. POTENTIAL FOR HUMAN EXPOSURE 5.1 OVERVIEW

Elevated levels of uranium have been identified in at least 54 of the 1,517 current or former EPA National Priorities List (NPL) hazardous waste sites (HazDat 1999). However, the number of sites evaluated for uranium is not known. The distribution of these sites within the United States is shown in Figure 5-1.

Uranium is a naturally occurring radioactive element that is present in nearly all rocks and soils; it has an average concentration in U.S. soils of about 2 pCi/g (3 ppm) (du Preez 1989; NCRP 1984a). Some parts of the United States, particularly the western portion, exhibit higher than average uranium levels due to natural geological formations. Most uranium ores contain between 0.05 and 0.2% uranium, up to 1,000 times the levels normally found in soil (Uranium Institute 1996).

Uranium can undergo oxidation-reduction reactions in the environment or microbial reactions to form complexes with organic matter (Premuzie et al. 1995). The only mechanism for decreasing the radioactivity of uranium is radioactive decay. Since all three of the naturally occurring uranium isotopes have very long half-lives (234U, 2.4x105 years; 235U, 7.0x108 years; and 238U, 4.5x109 years), the rate at which the radioactivity diminishes is very slow (NCRP 1984a). Therefore, the activity of uranium remains essentially unchanged over periods of thousands of years.

Uranium may be redistributed in the environment by both anthropogenic and natural processes. The three primary industrial processes that cause this redistribution are operations associated with the nuclear fuel cycle that include the mining, milling, and processing of uranium ores or uranium end products; the production of phosphate fertilizers for which the phosphorus is extracted from phosphate rocks containing uranium; and the improper disposal of uranium mine tailings (Cottrell et al. 1981; Hart et al. 1986; NCRP 1984a; Yang and Edwards 1984). Essentially no uranium is released from nuclear power plants because of the fuel assembly design and the chemical and physical nature of the uranium oxide fuel. Examples of uranium redistribution by natural processes include activities and processes that move soil and rock, such as resuspension of soils containing uranium through wind and water erosion, volcanic eruptions (Kuroda et al. 1984), operation of coal-burning power plants (coal containing significant quantities of uranium), and construction of roads and dams.

URANIUM

Figure 5-1. Frequency of NPL Sites with Uranium (U-238) Contamination

5. POTENTIAL FOR HUMAN EXPOSURE

Derived from HazDat 1998

URANIUM 5. POTENTIAL FOR HUMAN EXPOSURE

Uranium becomes airborne due to direct releases into the air from these processes. Deposition of atmospheric uranium may occur by wet (rain, sleet, or snow) or dry (gravitation or wind turbulence) processes. The rate of uranium deposition is dependent upon such factors as particle size, particle density, particle concentration, wind turbulence, and chemical form. Data are lacking on residence times of particulate uranium in the atmosphere, although UNSCEAR (1988) assumed that it behaves like atmospheric dust, for which meteorological models exist.

Uranium deposited by wet or dry precipitation will be deposited on land or in surface waters. If land deposition occurs, the uranium can be reincorporated into soil, resuspended in the atmosphere (typically factors are around 10-6), washed from the land into surface water, incorporated into groundwater, or deposited on or adsorbed onto plant roots (little or none enters the plant through leaves or roots). Conditions that increase the rate of formation of soluble complexes and decrease the rate of sorption of labile uranium in soil and sediment enhance the mobility of uranium. Significant reactions of uranium in soil are formation of complexes with anions and ligands (e.g., CO3-2, OH-1) or humic acid, and reduction of U+6 to U+4. Other factors that control the mobility of uranium in soil are the oxidation-reduction potential, the pH, and the sorbing characteristics of the sediments and soils (Allard et al. 1979, 1982; Brunskill and Wilkinson 1987; Herczeg et al. 1988; Premuzie et al. 1995).

Uranium in surface water can disperse over large distances to ponds, rivers, and oceans. The transport and dispersion of uranium in surface water and groundwater are affected by adsorption and desorption of uranium on aquatic sediments. As with soil, factors that control mobility of uranium in water include oxidation-reduction potential, pH, and sorbing characteristics of sediments and the suspended solids in the water (Brunskill and Wilkinson 1987; Swanson 1985). In one study of a stream with low concentrations of inorganics, low pH, and high concentrations of dissolved organic matter, the concentration of uranium in sediments and suspended solids was several orders of magnitude higher than in the surrounding water because the uranium was adsorbed onto the surface of the sediments and suspended particles (Brunskill and Wilkinson 1987).

The levels of uranium in aquatic organisms decline with each successive trophic level because of very low assimilation efficiencies in higher trophic animals. Bioconcentration factors measured in fish were low (Mahon 1982; Poston 1982; Waite et al. 1988) and were thought to arise from the extraction of uranium from the water or simply from the accumulation of uranium on gill surfaces (Ahsanullah and Williams 1989). In plants, uptake of uranium may be restricted to the root system and may actually

URANIUM 5. POTENTIAL FOR HUMAN EXPOSURE

represent adsorption to the outer root membrane rather than incorporation into the interior of the root system (Sheppard et al. 1983). Most of this uranium may be removed by washing the vegetable surfaces; cutting away the outer membrane will essentially result in complete removal. No significant translocation of uranium from soil to the aboveground parts of plants has been observed (Van Netten and Morley 1983).

The EPA has established a nationwide network called the Environmental Radiation Ambient Monitoring System (ERAMS) for obtaining data concerning radionuclides, including natural uranium isotopes, in environmental media. Sampling locations for ERAMS were selected to provide optimal population coverage (i.e., located near population centers). Airborne uranium concentrations and precipitation levels of uranium were quite low, in the attocurie/m3 (10-3 nanoBq/m3) and 0.006–0.098 picocurie/L (0.0002–0.004 Bq/L/m3) ranges, respectively (EPA 1994). However, both air samples and water samples taken near facilities producing uranium ore or processing uranium were found to be higher, in the pCi/L range (Eadie et al. 1979; Lapham et al. 1989; Laul 1994; NCRP 1984a; Tracy and Meyerhof 1987). The ERAMS reports document 234U to 238U concentration ratios in drinking water which deviate from the equilibrium value of unity found in undisturbed crustal rock. Theories proposed to account for this natural phenomenon involve water contact with soil and permeable rock containing uranium. The 238U atoms transform through 234Th to 234U, and any process which removes either of these radionuclides from the solid changes the 234U to 238U ratio.

238

U atoms at the solid-liquid interface which emit decay alpha

particles inward may experience a kinetic energy recoil sufficient to either tear the 234Th progeny from the solid or fracture the surficial solid layer making the 234Th more accessible for the enhanced dissolution that thorium typically experiences relative to uranium in mineral matrices. Either process can enhance the relative 234U content of the liquid. Should that liquid stabilize in another location and evaporate, the localized solids could show an enhanced 234U ratio.

A large drinking water study was performed in which data from the National Uranium Resource Evaluation (NURE) program plus data prepared for the EPA (Drury 1981) were compiled for a total of over 90,000 water samples. Domestic water supplies were represented by 28,000 samples and averaged 1.7 pCi/L (2.5 µg/L) uranium, with a population weighted mean value for finished waters, based on 100 measurements, of 0.8 pCi/L (1.2 µg/L). Other studies show the population weighted average concentration of uranium in U.S. community drinking water to range from 0.3 to 2.0 pCi/L (0.4–3.0 µg/L) (Ohanian 1989), while concentrations of uranium from selected drinking water supplies analyzed by EPA laboratories were generally 50%) so that wastes from uranium milling contain only low levels of uranium; however, the levels of uranium progeny (e.g., radium) remain essentially unchanged. Uncontrolled erosion of these wastes from open tailings piles not protected from the weather occurred at a Shiprock, New Mexico, uranium mill site, resulting in contamination of the surrounding area (Hans et al. 1979). Uncontrolled erosion also occurred in storage areas such as the St. Louis Airport Storage Site in Missouri (Seelley and Kelmers 1985). Increased levels of uranium, radium, radon, and other decay products of uranium have also been measured around these sites, particularly in the soil. A number of controlled disposal locations on government-owned mill sites exist, but the ones identified involved uncontrolled disposal.

At various facilities that process uranium for defense programs, uranium is released to the atmosphere under controlled conditions, resulting in deposition on the soil and surface waters. Monitoring data from the area surrounding the Fernald Environmental Management Project (formerly the Fernald Feed Materials Production Center) showed that soil contained uranium released from the facility (Stevenson and Hardy 1993).

The uranium content of phosphate rock, a source of phosphorus for fertilizers and phosphoric acid for the chemical industry, ranges from several pCi/g to 130 pCi/g (several ~200 µg/g) (Boothe 1977; UNSCEAR 1977, 1982). During milling, much of the uranium content becomes concentrated in slag by-products (Melville et al. 1981). The slag by-products are often used for bedrock in the paving of roads, thus transferring the uranium-rich slag to the soil (Melville et al. 1981; Williams and Berven 1986). Because of the large amounts of phosphate fertilizer produced annually (12–15 million tons), trace amounts of uranium progeny remaining in the fertilizer result in the distribution of about 120 Ci (180 metric tons) per year over U.S. agricultural lands (Kathren 1984).

URANIUM 5. POTENTIAL FOR HUMAN EXPOSURE

Combustion of coal is a significant source of enhanced natural radioactivity (especially combustion of coal from the western United States, which contains significantly more uranium than coal from the eastern United States). When coal is burned, some of the radioactivity is released directly to the atmosphere, but a significant fraction is retained in the bottom ash. Enhanced concentrations of uranium have been found on the ground around coal-fired power plants (UNSCEAR 1982).

Unauthorized landfill disposal of uranium processing wastes (e.g., Shpack Landfill in Norton, Massachusetts, and the Middlesex Municipal Landfill in Middlesex, New Jersey) has resulted in soil contamination (Bechtel National 1984; Cottrell et al. 1981). Also, elevated uranium concentrations have been measured in soil samples collected at 30 of 51 hazardous waste sites and in sediment samples at 16 of 51 hazardous waste sites (HazDat 1998). The HazDat data includes both Superfund and NPL sites. Elevated concentrations of uranium have been detected in soil, in surface water, in groundwater, or in all three of these environmental media from these sites. In several cases, the uranium concentrations in soils were significantly elevated. For example, uranium concentrations from the Shpack/ALI site were found to be 16,460 pCi/g (24,000 µg/g). At the United States Radium Corporation site (New Jersey), uranium concentrations ranged from 90 to 12,000 pCi/g (130–18,000 µg/g); for the Monticello site (Utah), uranium levels were reported to range from 1 to 24,000 pCi/g (1.5–36,000 µg/g) (HazDat 1998).

5.3 ENVIRONMENTAL FATE

5.3.1 Transport and Partitioning

The components of an ecosystem can be divided into several major compartments (Figure 5-3) (NCRP 1984b). None of the environmental compartments exist as separate entities; they have functional connections or interchanges between them. Figure 5-3 also shows the transport pathways between the released uranium and the environmental compartments as well as the mechanisms that lead to intakes by the population. Initial uranium deposition in a compartment, as well as exchanges between compartments (mobility), are dependent upon numerous factors such as chemical and physical form of the uranium, environmental media, organic material present, oxidation-reduction potential, nature of sorbing materials, and size and composition of sorbing particles.

URANIUM 5. POTENTIAL FOR HUMAN EXPOSURE

; ;; ; ;

Figure 5-3. Environmental Pathways for Potential Human Health Effects from Uranium Source Term

(Erosion directly by Stream and Rivers)

Atmospheric Transport

Surface Water Transport

Groundwater Transport

(Irrigation)

Depostion on Terrestrial Surfaces and in Sediment (Uptake by Plants and Animals

Bioaccumulation in Food Products (Food Ingestion)

(Inhalation)

Usage Rates for Individuals

(Water Ingestion)

; ; ;;;;; ;;

Uptake Rates for Individuals

Dose Rate Factors

Health Effects

Source: NCRP 1984b

URANIUM 5. POTENTIAL FOR HUMAN EXPOSURE

Natural processes of wind and water erosion, dissolution, precipitation, and volcanic action acting on natural uranium in rock and soil redistribute far more uranium in the total environment than the industries in the nuclear fuel cycle. However, those industries may release large quantities of uranium in specific locations, mainly in the form of solids placed on tailings piles, followed by liquids released to tailings ponds and then airborne releases, both directly from the facilities and by wind erosion of the tailings piles. Although solid releases represent the largest quantity of uranium redistribution, they remain on the facility grounds and are normally inaccessible to the public. It is the airborne (direct and wind erosion on tailings piles) and liquid releases (tailings pond runoff and water erosion of tailings) which most likely represent the important pathways for public exposure (i.e., inhalation and ingestion) if pathways can be completed.

While entrained in the air, particulate uranium represents an inhalation source for humans, the extent of which is dependent upon concentration and particle size. For particulate uranium to be an inhalation hazard to humans, the particulates must be in the size range of 1–10 µm (Bigu and Duport 1992; ICRP 1979). In some cases, the solid tailings have been removed from the site for use as fill or construction material, which can lead to external radiation exposures primarily from the uranium progeny.

Deposition of the atmospheric uranium can occur by dry deposition or wet deposition (Essien et al. 1985). Dry deposition results from gravitational settling and impaction on surfaces exposed to turbulent atmospheric flow. The rate of dry deposition is dependent upon particle size distribution, chemical form, particle density, and degree of air turbulence. Few experimental data on the particle size and residence time of uranium and uranium compounds present in ambient atmospheres are available; however, uranium particles are expected to behave like other particles for which data are available, which show that smaller uranium particles ( > = < < % α β γ δ µm µg q1* – + (+) (–)

Nuclear Regulatory Commission uncertainty factor Volatile Organic Compound World Health Organization week Working Level Working Level Month year greater than greater than or equal to equal to less than less than or equal to percent alpha beta gamma delta micrometer microgram cancer slope factor negative positive weakly positive result weakly negative result

URANIUM

APPENDIX D OVERVIEW OF BASIC RADIATION PHYSICS, CHEMISTRY AND BIOLOGY Understanding the basic concepts in radiation physics, chemistry, and biology is important to the evaluation and interpretation of radiation-induced adverse health effects and to the derivation of radiation protection principles. This appendix presents a brief overview of the areas of radiation physics, chemistry, and biology and is based to a large extent on the reviews of Mettler and Moseley (1985), Hobbs and McClellan (1986), Eichholz (1982), Hendee (1973), Cember (1996), and Early et al. (1979).

D.1 RADIONUCLIDES AND RADIOACTIVITY The substances we call elements are composed of atoms. Atoms in turn are made up of neutrons, protons and electrons: neutrons and protons in the nucleus and electrons in a cloud of orbits around the nucleus. Nuclide is the general term referring to any nucleus along with its orbital electrons. The nuclide is characterized by the composition of its nucleus and hence by the number of protons and neutrons in the nucleus. All atoms of an element have the same number of protons (this is given by the atomic number) but may have different numbers of neutrons (this is reflected by the atomic mass numbers or atomic weight of the element). Atoms with different atomic mass but the same atomic numbers are referred to as isotopes of an element. The numerical combination of protons and neutrons in most nuclides is such that the nucleus is quantum mechanically stable and the atom is said to be stable, i.e., not radioactive; however, if there are too few or too many neutrons, the nucleus is unstable and the atom is said to be radioactive. Unstable nuclides undergo radioactive transformation, a process in which a neutron or proton converts into the other and a beta particle is emitted, or else an alpha particle is emitted. Each type of decay is typically accompanied by the emission of gamma rays. These unstable atoms are called radionuclides; their emissions are called ionizing radiation; and the whole property is called radioactivity. Transformation or decay results in the formation of new nuclides some of which may themselves be radionuclides, while others are stable nuclides. This series of transformations is called the decay chain of the radionuclide. The first radionuclide in the chain is called the parent; the subsequent products of the transformation are called progeny, daughters, or decay products. In general there are two classifications of radioactivity and radionuclides: natural and artificial (manmade). Naturally-occurring radioactive material (NORM) exists in nature and no additional energy is necessary to place them in an unstable state. Natural radioactivity is the property of some naturally occurring, usually heavy elements, that are heavier than lead. Radionuclides, such as radium and uranium, primarily emit alpha particles. Some lighter elements such as carbon-14 and tritium (hydrogen3) primarily emit beta particles as they transform to a more stable atom. Natural radioactive atoms heavier than lead cannot attain a stable nucleus heavier than lead. Everyone is exposed to background radiation from naturally-occurring radionuclides throughout life. This background radiation is the major source of radiation exposure to man and arises from several sources. The natural background exposures are frequently used as a standard of comparison for exposures to various artificial sources of ionizing radiation. Artificial radioactive atoms are produced either as a by-product of fission of uranium or plutonium atoms in a nuclear reactor or by bombarding stable atoms with particles, such as neutrons or protons, directed at the stable atoms with high velocity. These artificially produced radioactive elements usually decay by

URANIUM APPENDIX D

emission of particles, such as positive or negative beta particles and one or more high energy photons (gamma rays). Unstable (radioactive) atoms of any element can be produced. Both naturally occurring and artificial radioisotopes find application in medicine, industrial products, and consumer products. Some specific radioisotopes, called fall-out, are still found in the environment as a result of nuclear weapons use or testing.

D.2 RADIOACTIVE DECAY D.2.1 Principles of Radioactive Decay The stability of an atom is the result of the balance of the forces of the various components of the nucleus. An atom that is unstable (radionuclide) will release energy (decay) in various ways and transform to stable atoms or to other radioactive species called daughters, often with the release of ionizing radiation. If there are either too many or too few neutrons for a given number of protons, the resulting nucleus may undergo transformation. For some elements, a chain of daughter decay products may be produced until stable atoms are formed. Radionuclides can be characterized by the type and energy of the radiation emitted, the rate of decay, and the mode of decay. The mode of decay indicates how a parent compound undergoes transformation. Radiations considered here are primarily of nuclear origin, i.e., they arise from nuclear excitation, usually caused by the capture of charged or uncharged nucleons by a nucleus, or by the radioactive decay or transformation of an unstable nuclide. The type of radiation may be categorized as charged or uncharged particles, protons, and fission products) or electromagnetic radiation (gamma rays and x rays). Table D-1 summarizes the basic characteristics of the more common types of radiation encountered.

D.2.2 Half-Life and Activity For any given radionuclide, the rate of decay is a first-order process that is constant, regardless of the radioactive atoms present and is characteristic for each radionuclide. The process of decay is a series of random events; temperature, pressure, or chemical combinations do not effect the rate of decay. While it may not be possible to predict exactly which atom is going to undergo transformation at any given time, it is possible to predict, on average, the fraction of the radioactive atoms that will transform during any interval of time. The activity is a measure of the quantity of radioactive material. For these radioactive materials it is customary to describe the activity as the number of disintegrations (transformations) per unit time. The unit of activity is the curie (Ci), which was originally related to the activity of one gram of radium, but is now defined as that quantity of radioactive material in which there are: 1 curie (Ci) = 3.7x1010 disintegrations (transformations)/second (dps) or 2.22x1012 disintegrations (transformations)/minute (dpm). The SI unit of activity is the becquerel (Bq); 1 Bq = that quantity of radioactive material in which there is 1 transformation/second. Since activity is proportional to the number of atoms of the radioactive material, the quantity of any radioactive material is usually expressed in curies, regardless of its purity or concentration. The transformation of radioactive nuclei is a random process, and the number of transformation is directly proportional to the number of radioactive atoms present. For any pure radioactive substance, the rate of decay is usually described by its radiological half-life, TR, i.e., the time it takes for a specified source material to decay to half its initial activity. The specific activity is an

URANIUM APPENDIX D

indirect measure of the rate of decay, and is defined as the activity per unit mass or per unit volume. The higher the specific activity oa a radioisotope, the faster it is decaying. The activity of a radionuclide at time t may be calculated by: A = Aoe-0.693t/Trad where A is the activity in dps or curies or becquerels, Ao is the activity at time zero, t is the time at which measured, and Trad is the radiological half-life of the radionuclide (Trad and t must be in the same units of time). The time when the activity of a sample of radioactivity becomes one-half its original value is the radioactive half-life and is expressed in any suitable unit of time. Table D-1. Characteristics of Nuclear Radiations

Radiation Alpha (α)

Rest massa 4.00 amu

Charge +2

Negatron (β–)

5.48x10-4 amu; 0.51 MeV 5.48x10-4 amu; 0.51 Mev

–1

0–4 Mev

Path lengthb Air Solid Comments 5–10 cm 25–80 µm Identical to ionized He nucleus 0–10 m 0–1 cm Identical to electron

+1

0-4 Mev

0–10 m

0–1 cm

1.0086 amu; 939.55 MeV –

0

0–15 MeV

b

b

0

5 keV–100 keV

b

b

–

0

10 keV–3 MeV

b

b

Positron (β+)

Neutron X ray (e.m.

photon)

Gamma (γ)

Typical energy range 4–10 MeV

(e.m. photon)

a b

Identical to electron except for sign of charge Free half-life: 16 min Photon from transition of an electron between atomic orbits Photon from nuclear transformation

The rest mass (in amu) has an energy equivalent in MeV that is obtained using the equation E=mc2, where 1 amu = 932 MeV. Path lengths are not applicable to x- and gamma rays since their intensities decrease exponentially; path lengths in solid tissue are

variable, depending on particle energy, electron density of material, and other factors. amu = atomic mass unit; e.m. = electromagnetic; MeV = MegaElectron Volts

The specific activity is a measure of activity, and is fined as the activity per unit mass or per unit volume. This activity is usually expressed in curies per gram and may be calculated by curies/gram =

1.3x108 / (Trad) (atomic weight)

or

[3.577 x 105 x mass(g)] / [Trad x atomic weight] where Trad is the radiological half-life in days. In the case of radioactive materials contained in living organisms, an additional consideration is made for the reduction in observed activity due to regular processes of elimination of the respective chemical or

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biochemical substance from the organism. This introduces a rate constant called the biological half-life (Tbiol) which is the time required for biological processes to eliminate one-half of the activity. This time is virtually the same for both stable and radioactive isotopes of any given element. Under such conditions the time required for a radioactive element to be halved as a result of the combined action of radioactive decay and biological elimination is the effective clearance half-time: Teff = (Tbiol x Trad) / (Tbiol + Trad). Table D-2 presents representative effective half-lives of particular interest.

Table D-2. Half-Lives of Some Radionuclides in Adult Body Organs Half-lifea Radionuclide

Critical organ

Physical

Biological

Effective

Uranium-238

Kidney

4,460,000,000 y

4d

4d

Hydrogen-3 (Tritium)

Whole body

12.3 y

10 d

10 d

Iodine-131

Thyroid

8d

80 d

7.3 d

Strontium-90

Bone

28 y

50 y

18 y

Plutonium-239

Bone surface

24,400 y

50 y

50 y

Lung

24,400 y

500 d

500 d

Cobalt-60

Whole body

5.3 y

99.5 d

95 d

Iron-55

Spleen

2.7 y

600 d

388 d

Iron-59

Spleen

45.1 d

600 d

42 d

Manganese-54

Liver

303 d

25 d

23 d

Cesium-137

Whole body

30 y

70 d

70 d

b

a

d = days, y = years Mixed in body water as tritiated water

b

D.2.3 Interaction of Radiation with Matter Both ionizing and nonionizing radiation will interact with materials; that is, radiation will lose kinetic energy to any solid, liquid or gas through which it passes by a variety of mechanisms. The transfer of energy to a medium by either electromagnetic or particulate radiation may be sufficient to cause formation of ions. This process is called ionization. Compared to other types of radiation that may be absorbed, such as ultraviolet radiation, ionizing radiation deposits a relatively large amount of energy into a small volume. The method by which incident radiation interacts with the medium to cause ionization may be direct or indirect. Electromagnetic radiations (x rays and gamma photons) are indirectly ionizing; that is, they give up their energy in various interactions with cellular molecules, and the energy is then utilized to

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produce a fast-moving charged particle such as an electron. It is the electron that then may react with a target molecule. This particle is called a “primary ionizing particle. Charged particles, in contrast, strike the tissue or medium and directly react with target molecules, such as oxygen or water. These particulate radiations are directly ionizing radiations. Examples of directly ionizing particles include alpha and beta particles. Indirectly ionizing radiations are always more penetrating than directly ionizing particulate radiations. Mass, charge, and velocity of a particle all affect the rate at which ionization occurs. The higher the charge of the particle and the lower the velocity, the greater the propensity to cause ionization. Heavy, highly charged particles, such as alpha particles, lose energy rapidly with distance and, therefore, do not penetrate deeply. The result of these interaction processes is a gradual slowing down of any incident particle until it is brought to rest or "stopped" at the end of its range.

D.2.4 Characteristics of Emitted Radiation D.2.4.1 Alpha Emission. In alpha emission, an alpha particle consisting of two protons and two neutrons is emitted with a resulting decrease in the atomic mass number by four and reduction of the atomic number of two, thereby changing the parent to a different element. The alpha particle is identical to a helium nucleus consisting of two neutrons and two protons. It results from the radioactive decay of some heavy elements such as uranium, plutonium, radium, thorium, and radon. All alpha particles emitted by a given radioisotope have the same energy. Most of the alpha particles that are likely to be found have energies in the range of about 4 to 8 MeV, depending on the isotope from which they came. The alpha particle has an electrical charge of +2. Because of this double positive charge and their size, alpha particles have great ionizing power and, thus, lose their kinetic energy quickly. This results in very little penetrating power. In fact, an alpha particle cannot penetrate a sheet of paper. The range of an alpha particle (the distance the charged particle travels from the point of origin to its resting point) is about 4 cm in air, which decreases considerably to a few micrometers in tissue. These properties cause alpha emitters to be hazardous only if there is internal contamination (i.e., if the radionuclide is inside the body). D.2.4.2 Beta Emission. A beta particle (β ) is a high-velocity electron ejected from a disintegrating nucleus. The particle may be either a negatively charged electron, termed a negatron (β -) or a positively charged electron, termed a positron (β +). Although the precise definition of "beta emission" refers to both β - and β +, common usage of the term generally applies only to the negative particle, as distinguished from the positron emission, which refers to the β + particle. D.2.4.2.1 Beta Negative Emission. Beta particle (β -) emission is another process by which a radionuclide, with a neutron excess achieves stability. Beta particle emission decreases the number of neutrons by one and increases the number of protons by one, while the atomic mass number remains unchanged.4 This transformation results in the formation of a different element. The energy spectrum of beta particle emission ranges from a certain maximum down to zero with the mean energy of the spectrum being about one-third of the maximum. The range in tissue is much less. Beta negative emitting radionuclides can cause injury to the skin and superficial body tissues, but mostly present an internal contamination hazard.

4

Neutrinos also accompany negative beta particles and positron emissions.

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D.2.4.2.2 Positron Emission. In cases in which there are too many protons in the nucleus, positron emission may occur. In this case a proton may be thought of as being converted into a neutron, and a positron (β +) is emitted.4 This increases the number of neutrons by one, decreases the number of protons by one, and again leaves the atomic mass number unchanged. The gamma radiation resulting from the annihilation (see glossary) of the positron makes all positron emitting isotopes more of an external radiation hazard than pure β emitters of equal energy. D.2.4.2.3 Gamma Emission. Radioactive decay by alpha, beta, or positron emission, or electron capture often leaves some of the energy resulting from these changes in the nucleus. As a result, the nucleus is raised to an excited level. None of these excited nuclei can remain in this high-energy state. Nuclei release this energy returning to ground state or to the lowest possible stable energy level. The energy released is in the form of gamma radiation (high energy photons) and has an energy equal to the change in the energy state of the nucleus. Gamma and x rays behave similarly but differ in their origin; gamma emissions originate in the nucleus while x rays originate in the orbital electron structure or from rapidly changing the velocity of an electron (e.g., as occurs when shielding high energy beta particles or stopping the electron beam in an x ray tube).

D.3 ESTIMATION OF ENERGY DEPOSITION IN HUMAN TISSUES Two forms of potential radiation exposures can result: internal and external. The term exposure denotes physical interaction of the radiation emitted from the radioactive material with cells and tissues of the human body. An exposure can be "acute" or "chronic" depending on how long an individual or organ is exposed to the radiation. Internal exposures occur when radionuclides, which have entered the body (e.g., through the inhalation, ingestion, or dermal pathways), undergo radioactive decay resulting in the deposition of energy to internal organs. External exposures occur when radiation enters the body directly from sources located outside the body, such as radiation emitters from radionuclides on ground surfaces, dissolved in water, or dispersed in the air. In general, external exposures are from material emitting gamma radiation, which readily penetrate the skin and internal organs. Beta and alpha radiation from external sources are far less penetrating and deposit their energy primarily on the skin's outer layer. Consequently, their contribution to the absorbed dose of the total body dose, compared to that deposited by gamma rays, may be negligible. Characterizing the radiation dose to persons as a result of exposure to radiation is a complex issue. It is difficult to: (1) measure internally the amount of energy actually transferred to an organic material and to correlate any observed effects with this energy deposition; and (2) account for and predict secondary processes, such as collision effects or biologically triggered effects, that are an indirect consequence of the primary interaction event.

D.3.1 Dose/Exposure Units D.3.1.1 Roentgen. The roentgen (R) is a unit of x or gamma-ray exposure and is a measured by the amount of ionization caused in air by gamma or x radiation. One roentgen produces 2.58x10-4 coulomb per kilogram of air. In the case of gamma radiation, over the commonly encountered range of photon energy, the energy deposition in tissue for a dose of 1 R is about 0.0096 joules (J) /kg of tissue. D.3.1.2 Absorbed Dose and Absorbed Dose Rate. The absorbed dose is defined as the energy imparted by the incident radiation to a unit mass of the tissue or organ. The unit of absorbed dose is the rad; 1 rad = 100 erg/gram = 0.01 J/kg in any medium. An exposure of 1 R results in a dose to soft tissue of approximately 0.01 J/kg. The SI unit is the gray which is equivalent to 100 rad or 1 J/kg. Internal and

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external exposures from radiation sources are not usually instantaneous but are distributed over extended periods of time. The resulting rate of change of the absorbed dose to a small volume of mass is referred to as the absorbed dose rate in units of rad/unit time. D.3.1.3 Working Levels and Working Level Months. Working level (WL) is a measure of the atmospheric concentration of radon and its short-lived progeny. One WL is defined as any combination of short-lived radon daughters (through polonium-214), per liter of air, that will result in the emission of 1.3x105 MeV of alpha energy. An activity concentration of 100 pCi radon-222/L of air, in equilibrium with its daughters, corresponds approximately to a potential alpha-energy concentration of 1 WL. The WL unit can also be used for thoron daughters. In this case, 1.3x105 MeV of alpha energy (1 WL) is released by the thoron daughters in equilibrium with 7.5 pCi thoron/L. The potential alpha energy exposure of miners is commonly expressed in the unit Working Level Month (WLM). One WLM corresponds to exposure to a concentration of 1 WL for the reference period of 170 hours, or more generally WLM = concentration (WL) x exposure time (months) (one “month” = 170 working hours).

D.3.2 Dosimetry Models Dosimetry models are used to estimate the dose from internally deposited to radioactive substances. The models for internal dosimetry consider the amount of radionuclides entering the body, the factors affecting their movement or transport through the body, distribution and retention of radionuclides in the body, and the energy deposited in organs and tissues from the radiation that is emitted during spontaneous decay processes. The dose pattern for radioactive materials in the body may be strongly influenced by the route of entry of the material. For industrial workers, inhalation of radioactive particles with pulmonary deposition and puncture wounds with subcutaneous deposition have been the most frequent. The general population has been exposed via ingestion and inhalation of low levels of naturally occurring radionuclides as well as radionuclides from nuclear weapons testing. The models for external dosimetry consider only the photon doses to organs of individuals who are immersed in air or are exposed to a contaminated object. D.3.2.1 Ingestion. Ingestion of radioactive materials is most likely to occur from contaminated foodstuffs or water or eventual ingestion of inhaled compounds initially deposited in the lung. Ingestion of radioactive material may result in toxic effects as a result of either absorption of the radionuclide or irradiation of the gastrointestinal tract during passage through the tract, or a combination of both. The fraction of a radioactive material absorbed from the gastrointestinal tract is variable, depending on the specific element, the physical and chemical form of the material ingested, and the diet, as well as some other metabolic and physiological factors. The absorption of some elements is influenced by age, usually with higher absorption in the very young. D.3.2.2 Inhalation. The inhalation route of exposure has long been recognized as being a major portal of entry for both nonradioactive and radioactive materials. The deposition of particles within the lung is largely dependent upon the size of the particles being inhaled. After the particle is deposited, the retention will depend upon the physical and chemical properties of the dust and the physiological status of the lung. The retention of the particle in the lung depends on the location of deposition, in addition to the physical and chemical properties of the particles. The converse of pulmonary retention is pulmonary clearance. There are three distinct mechanisms of clearance which operate simultaneously. Ciliary clearance acts only in the upper respiratory tract. The second and third mechanisms act mainly in the

URANIUM APPENDIX D

deep respiratory tract. These are phagocytosis and absorption. Phagocytosis is the engulfing of foreign bodies by alveolar macrophages and their subsequent removal either up the ciliary "escalator" or by entrance into the lymphatic system. Some inhaled soluble particles are absorbed into the blood and translocated to other organs and tissues.

D.3.3 Internal Emitters An internal emitter is a radionuclide that is inside the body. The absorbed dose from internally deposited radioisotopes depends on the energy absorbed per unit tissue by the irradiated tissue. For a radioisotope distributed uniformly throughout an infinitely large medium, the concentration of absorbed energy must be equal to the concentration of energy emitted by the isotope. An infinitely large medium may be approximated by a tissue mass whose dimensions exceed the range of the particle. All alpha and most beta radiation will be absorbed in the organ (or tissue) of reference. Gamma-emitting isotope emissions are penetrating radiation, and a substantial fraction of gamma energy may be absorbed in tissue. The dose to an organ or tissue is a function of the effective retention half-time, the energy released in the tissue, the amount of radioactivity initially introduced, and the mass of the organ or tissue.

D.4 BIOLOGICAL EFFECTS OF RADIATION When biological material is exposed to ionizing radiation, a chain of cellular events occurs as the ionizing particle passes through the biological material. A number of theories have been proposed to describe the interaction of radiation with biologically important molecules in cells and to explain the resulting damage to biological systems from those interactions. Many factors may modify the response of a living organism to a given dose of radiation. Factors related to the exposure include the dose rate, the energy of the radiation, and the temporal pattern of the exposure. Biological considerations include factors such as species, age, sex, and the portion of the body exposed. Several excellent reviews of the biological effects of radiation have been published, and the reader is referred to these for a more in-depth discussion (Brodsky 1996; Hobbs and McClellan 1986; ICRP 1984; Mettler and Moseley 1985; Rubin and Casarett 1968).

D.4.1 Radiation Effects at the Cellular Level According to Mettler and Moseley (1985), at acute doses up to 10 rad (100 mGy), single strand breaks in DNA may be produced. These single strand breaks may be repaired rapidly. With doses in the range of 50–500 rad (0.5–5 Gy), irreparable double-stranded DNA breaks are likely, resulting in cellular reproductive death after one or more divisions of the irradiated parent cell. At large doses of radiation, usually greater than 500 rad (5 Gy), direct cell death before division (interphase death) may occur from the direct interaction of free-radicals with essentially cellular macromolecules. Morphological changes at the cellular level, the severity of which are dose-dependent, may also be observed. The sensitivity of various cell types varies. According to the Bergonie-Tribondeau law, the sensitivity of cell lines is directly proportional to their mitotic rate and inversely proportional to the degree of differentiation (Mettler and Moseley 1985). Rubin and Casarett (1968) devised a classification system that categorized cells according to type, function, and mitotic activity. The categories range from the most sensitive type, "vegetative intermitotic cells, " found in the stem cells of the bone marrow and the gastrointestinal tract, to the least sensitive cell type, "fixed postmitotic cells," found in striated muscles or long-lived neural tissues.

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Cellular changes may result in cell death, which if extensive, may produce irreversible damage to an organ or tissue or may result in the death of the individual. If the cell recovers, altered metabolism and function may still occur, which may be repaired or may result in the manifestation of clinical symptoms. These changes may also be expressed at a later time as tumors or cellular mutations, which may result in abnormal tissue.

D.4.2 Radiation Effects at the Organ Level In most organs and tissues the injury and the underlying mechanism for that injury are complex and may involve a combination of events. The extent and severity of this tissue injury are dependent upon the radiosensitivity of the various cell types in that organ system. Rubin and Casarett (1968) describe and schematically display the events following radiation in several organ system types. These include: a rapid renewal system, such as the gastrointestinal mucosa; a slow renewal system, such as the pulmonary epithelium; and a nonrenewal system, such as neural or muscle tissue. In the rapid renewal system, organ injury results from the direct destruction of highly radiosensitive cells, such as the stem cells in the bone marrow. Injury may also result from constriction of the microcirculation and from edema and inflammation of the basement membrane, designated as the histohematic barrier (HHB), which may progress to fibrosis. In slow renewal and nonrenewal systems, the radiation may have little effect on the parenchymal cells, but ultimate parenchymal atrophy and death over several months result from HHB fibrosis and occlusion of the microcirculation.

D.4.2 Low Level Radiation Effects Cancer is the major latent harmful effect produced by ionizing radiation and the one that most people exposed to radiation are concerned about. The ability of alpha, beta, and gamma radiation to produce cancer in virtually every tissue and organ in laboratory animals has been well-demonstrated. The development of cancer is not an immediate effect. Radiation-induced leukemia has the shortest latent period at 2 years, while other radiation induced cancers have latent periods >20 years. The mechanism by which cancer is induced in living cells is complex and is a topic of intense study. Exposure to ionizing radiation can produce cancer at any site within the body; however, some sites appear to be more common than others, such as the breast, lung, stomach, and thyroid. DNA is a major target molecule during exposure to ionizing radiation. Other macromolecules, such as lipids and proteins, are also at risk of damage when exposed to ionizing radiation. The genotoxicity of ionizing radiation is an area of intense study, as damage to the DNA is ultimately responsible for many of the adverse toxicological effects ascribed to ionizing radiation, including cancer. Damage to genetic material is basic to developmental or teratogenic effects, as well. However, for effects other than cancer, there is little evidence of human effects at low levels of exposure.

D.5 UNITS IN RADIATION PROTECTION AND REGULATION D.5.1 Dose Equivalent and Dose Equivalent Rate. Dose equivalent or rem is a special radiation protection quantity that is used, for administrative and radiation safety purposes only, to express the absorbed dose in a manner which considers the difference in biological effectiveness of various kinds of ionizing radiation. The ICRU has defined the dose equivalent, H, as the product of the absorbed dose, D, and the quality factor, Q, at the point of interest in biological tissue. This relationship is expressed as H = D x Q. The dose equivalent concept is applicable only to doses that are not great enough to produce biomedical effects.

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The quality factor is a dimensionless quantity that depends in part on the stopping power for charged particles, and it accounts for the differences in biological effectiveness found among the types of radiation. Originally relative biological effectiveness (RBE) was used rather than Q to define the quantity, rem, which was of use in risk assessment. The generally accepted values for quality factors for various radiation types are provided in Table D-3. The dose equivalent rate is the time rate of change of the dose equivalent to organs and tissues and is expressed as rem/unit time or sievert/unit time.

Table D-3. Quality Factors (Q) and Absorbed Dose Equivalencies

Quality factor (Q)

Absorbed dose equal to a unit dose equivalent*

X, gamma, or beta radiation

1

1

Alpha particles, multiple-charged particles, fission fragments and heavy particles of unknown charge

20

0.05

Neutrons of unknown energy

10

0.1

High-energy protons

10

0.1

Type of radiation

* Absorbed dose in rad equal to 1 rem or the absorbed dose in gray equal to 1 sievert. Source: USNRC. 1999. Standards for the protection against radiation, table 1004(b).1. 10 CFR 20.1004. U.S. Nuclear Regulatory Commission, Washington, D.C.

D.5.2 Relative Biological Effectiveness. RBE is used to denote the experimentally determined ratio of the absorbed dose from one radiation type to the absorbed dose of a reference radiation required to produce an identical biologic effect under the same conditions. Gamma rays from cobalt-60 and 200–250 keV x-rays have been used as reference standards. The term RBE has been widely used in experimental radiobiology, and the term quality factor used in calculations of dose equivalents for radiation safety purposes (ICRP 1977; NCRP 1971; UNSCEAR 1982). RBE applies only to a specific biological end point, in a specific exposure, under specific conditions to a specific species. There are no generally accepted values of RBE.

D.5.3 Effective Dose Equivalent and Effective Dose Equivalent Rate. The absorbed dose is usually defined as the mean absorbed dose within an organ or tissue. This represents a simplification of the actual problem. Normally when an individual ingests or inhales a radionuclide or is exposed to external radiation that enters the body (gamma), the dose is not uniform throughout the whole body. The simplifying assumption is that the detriment will be the same whether the body is uniformly or non-uniformly irradiated. In an attempt to compare detriment from absorbed dose of a limited portion of the body with the detriment from total body dose, the ICRP (1977) has derived a concept of effective dose equivalent. The effective dose equivalent, HE, is HE = (the sum of) Wt Ht

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where Ht is the dose equivalent in the tissue, Wt is the weighting factor, which represents the estimated proportion of the stochastic risk resulting from tissue, T, to the stochastic risk when the whole body is uniformly irradiated for occupational exposures under certain conditions (ICRP 1977). Weighting factors for selected tissues are listed in Table D-4. The ICRU (1980), ICRP (1984), and NCRP (1985) now recommend that the rad, roentgen, curie, and rem be replaced by the SI units: gray (Gy), Coulomb per kilogram (C/kg), Becquerel (Bq), and sievert (Sv), respectively. The relationship between the customary units and the international system of units (SI) for radiological quantities is shown in Table D-5.

Table D-4. Weighting Factors for Calculating Effective Dose Equivalent for Selected Tissues Weighting factor Tissue

ICRP60

NCRP115/ ICRP60

NRC

Bladder

0.040

0.05

–

Bone marrow

0.143

0.12

0.12

Bone surface

0.009

0.01

0.03

Breast

0.050

0.05

0.15

Colon

0.141

0.12

–

Liver

0.022

0.05

–

Lung

0.111

0.12

0.12

Esophagus

0.034

0.05

–

Ovary

0.020

0.05

–

Skin

0.006

0.01

–

Stomach

0.139

0.12

–

Thyroid

0.021

0.05

0.03

Gonads

0.183

0.20

0.25

subtotal

0.969

1

0.70

Remainder

0.031

0.05

0.30

ICRP60 = International Commission on Radiological Protection, 1990 Recommendations of the ICRP; NCRP115 = National Council on Radiation Protection and Measurements. 1993. Risk Estimates for Radiation Protection, Report 115. Bethesda, Maryland; NRC = Nuclear Regulatory Commission. NRC = Nuclear Regulatory Commission, Title 10, Code of Federal Regulations, Part 20

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Table D-5. Comparison of Common and SI Units for Radiation Quantities

Quantity

Customary units

Definition

SI units

Definition

Activity (A)

curie (Ci)

3.7x1010 transformations s-1

becquerel (Bq)

Absorbed dose (D)

rad (rad)

10-2 Jkg-1

gray (Gy) Jkg-1

Absorbed dose rate v (D )

rad per second (rad s-1)

10-2 Jkg-1s-1

gray per second (Gy s-1)

Dose equivalent (H)

rem (rem)

10-2 Jkg-1

sievert (Sv)

Dose equivalent ^ rate (H )

rem per second (rem s-1)

10-2 Jkg-1s-1

sievert per second (Sv s-1)

Jkg-1 s-1

Linear energy transfer (LET)

kiloelectron volts per micrometer (keV µm-1)

1.602x10-10 Jm-1

kiloelectron volts per micrometer (keV µm-1)

1.602x10-10 Jm-1

Jkg-1 = Joules per kilogram; Jkg-1s-1 = Joules per kilogram per second; Jm-1 = Joules per meter; s-1 = per second

s-1

Jkg-1 s-1 Jkg-1

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REFERENCES FOR APPENDIX D ATSDR. 1990a. Toxicological profile for thorium. U.S. Department of Health and Human Services. Public Health Service. Agency for Toxic Substances and Disease Registry. Atlanta, GA. ATSDR. 1990b. Toxicological profile for radium. U.S. Department of Health and Human Services. Public Health Service. Agency for Toxic Substances and Disease Registry. Atlanta, GA. ATSDR. 1990c. Toxicological profile for radon. U.S. Department of Health and Human Services. Public Health Service. Agency for Toxic Substances and Disease Registry. Atlanta, GA. ATSDR. 1999. Toxicological profile for uranium. U.S. Department of Health and Human Services. Public Health Service. Agency for Toxic Substances and Disease Registry. Atlanta, GA. BEIR III. 1980. The effects on populations of exposure to low levels of ionizing radiation. Committee on the Biological Effects of Ionizing Radiations, National Research Council. Washington, DC: National Academy Press. BEIR IV. 1988. Health risks of radon and other internally deposited alpha emitters. Committee on the Biological Effects of Ionizing Radiations, National Research Council. Washington, DC: National Academy Press. BEIR V. 1988. Health effects of exposure to low levels of ionizing radiation. Committee on the Biological Effects of Ionizing Radiations, National Research Council. Washington, DC: National Academy Press. Brodsky A. 1996. Review of radiation risks and uranium toxicity with application to decisions associated with decommissioning clean-up criteria. Hebron,Connecticut: RSA Publications. Cember H. 1996. Introduction to health physics. New York., NY: McGraw Hill. Early P, Razzak M, Sodee D. 1979. Nuclear medicine technology. 2nd ed. St. Louis: C.V. Mosby Company. Eichholz G. 1982. Environmental aspects of nuclear power. Ann Arbor, MI: Ann Arbor Science. Hendee W. 1973. Radioactive isotopes in biological research. New York, NY: John Wiley and Sons. Hobbs C, McClellan R. 1986. Radiation and radioactive materials. In: Doull J, et al., eds. Casarett and Doull's Toxicology. 3rd ed. New York, NY: Macmillan Publishing Co., Inc., 497-530. ICRP. 1977. International Commission on Radiological Protection. Recommendations of the International Commission on Radiological Protection. ICRP Publication 26. Vol 1. No. 3. Oxford: Pergamon Press. ICRP. 1979. International Commission on Radiological Protection. Limits for intakes of radionuclides by workers. ICRP Publication 20. Vol. 3. No. 1-4. Oxford: Pergamon Press.

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ICRP. 1979. Limits for Intakes of Radionuclides by Workers. Publication 30. International Commission on Radiological Protection. Pergamon Press. ICRP. 1984. International Commission on Radiological Protection. A compilation of the major concepts and quantities in use by ICRP. ICRP Publication 42. Oxford: Pergamon Press. ICRP. 1990. International Commission on Radiological Protection 1990 Recommendations of the ICRP ICRU. 1980. International Commission on Radiation Units and Measurements. ICRU Report No. 33. Washington, DC. James A. 1987. A reconsideration of cells at risk and other key factors in radon daughter dosimetry. In: Hopke P, ed. Radon and its decay products: Occurrence, properties and health effects. ACS Symposium Series 331. Washington, DC: American Chemical Society, 400-418. James A, Roy M. 1987. Dosimetric lung models. In: Gerber G, et al., ed. Age-related factors in radionuclide metabolism and dosimetry. Boston: Martinus Nijhoff Publishers, 95-108. Kondo S. 1993. Health effects of low-level radiation. Kinki University Press, Osaka, Japan (available from Medical Physics Publishing, Madison, Wisconsin). Kato H, Schull W. 1982. Studies of the mortality of A-bomb survivors. Report 7 Part 8, Cancer mortality among atomic bomb survivors, 1950-78. Radiat Res 90;395-432. Mettler F, Moseley R. 1985. Medical effects of ionizing radiation. New York: Grune and Stratton. NCRP. 1971. Basic radiation protection criteria. National Council on Radiation Protection and Measurements. Report No. 39. Washington, DC. NCRP. 1985. A handbook of radioactivity measurements procedures. 2nd ed. National Council on Radiation Protection and Measurements. Report No. 58. Bethesda, MD: NCRP. 1993. Risk estimates for radiation protection. National Council on Radiation Protection and Measurements. Report 115. Bethesda, Maryland Otake M, Schull W. 1984. Mental retardation in children exposed in utero to the atomic bombs: A reassessment. Technical Report RERF TR 1-83, Radiation Effects Research Foundation, Japan. Rubin P, Casarett G. 1968. Clinical radiation pathology. Philadelphia: W.B. Sanders Company, 33. UNSCEAR. 1977. United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and effects of ionizing radiation. New York: United Nations. UNSCEAR. 1982. United Nations Scientific Committee on the Effects of Atomic Radiation. Ionizing radiation: Sources and biological effects. New York: United Nations. UNSCEAR. 1986. United Nations Scientific Committee on the Effects of Atomic Radiation. Genetic and somatic effects of ionizing radiation. New York: United Nations.

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UNSCEAR. 1988. United Nations Scientific Committee on the Effects of Atomic Radiation. Sources, effects and risks of ionization radiation. New York: United Nations. UNSCEAR. 1993. United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and effects of ionizing radiation. New York: United Nations. USNRC. 1999. Standards for the protection against radiation, table 1004(b).1. 10 CFR 20.1004. U.S. Nuclear Regulatory Commission, Washington, D.C.