CHAPTER 1: INTRODUCTION Kurt Kyser Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario, K7L 3N6, Canada
[email protected] and Michel Cuney G2R, Nancy-Université, CNRS, CREGU, B.P. 239, F-54506 Vandoeuvre lés Nancy, France
[email protected] the pitchblende led to the discovery of Po and Ra shortly thereafter by the Curies (Curie & Curie 1898). Much later, a team led by Enrico Fermi in 1934 observed that bombarding U with neutrons produces the emission of beta rays, and lead to the discovery of fission of U. On December 2, 1942, a team led by Enrico Fermi initiated a controlled nuclear chain reaction. Uranium is a ubiquitous element that occurs everywhere; the concentration of U is normally low, averaging only 3 ppm in the upper continental crust (1.7 ppm in the entire crust) and 0.0032 mg/L in seawater (Clark et al. 1966). Uranium is a member of the actinide series of elements, as are thorium and plutonium. Uranium has an atomic number of 92 and has 3 naturally occurring radioactive isotopes, 234 U, 235U and 238U. The most abundant isotopes of U, 235U and 238U, along with 232Th are commonly used in geological sciences as geochronometers because of their long half-lives (the time required for half the number of atoms of an isotope to decay) and because they each decay to an isotope of Pb (Table 1-1). The isotope 234U, which occurs as the decay product of 238U and is used as a geochronometer as well, makes up only 0.0055% of natural U because its half-life of 246,000 years is only about 0.000055 as long as the half-life of 238U. The decays of 235U and 238U are complex, with many intermediate daughter products. These products decay to daughters along the chain until 207 Pb is reached in the 235U chain or 206Pb is produced in the 238U decay scheme (Fig. 1-1). Because of differences in the geochemical properties of the daughter elements produced, these can be differentially dispersed into the environment resulting in disequilibrium in the radioactivity of
Uranium is one of the most important energyrelated materials, with current use almost entirely for generating electricity and a small proportion for producing medical isotopes. About 17% of the world’s electricity is generated from 440 nuclear reactors spread across 30 countries, and 8% of the total energy consumed globally comes from nuclear power (EIA 2007). Energy generated from U has a minimal “carbon footprint” and substitution of nuclear generated electricity for coal has been proposed to offset the additional emissions expected from the increase in energy anticipated in the future (Pacala & Socolow 2004). To meet the current and projected needs of the uranium industry, discovery of new deposits and development of new technologies for both exploration and processing are critical. The discovery of uranium is attributed to Klaproth, a German chemist who, in 1789, precipitated a yellow compound by dissolving pitchblende in nitric acid and neutralizing the solution with sodium hydroxide. He heated it with charcoal to obtain a black powder, which was a uranium oxide. He named the newly discovered element after the planet Uranus, which had been discovered eight years earlier by William Herschel. In 1841, Péligot, a French chemist working at the Baccarat crystal factory in Lorraine, isolated the first sample of U metal by heating U tetrachloride with K. Uranium was not seen as being particularly dangerous during much of the 19th century, leading to use of the element in coloring pottery and glass. Radioactivity was discovered by Becquerel in 1896 when he accidentally exposed a photographic plate to U (Becquerel 1896). Realization that U isolated from pitchblende was less radioactive than
Short course co-sponsored by the the SGA and MAC, Québec City, May 2008, p. 1–14
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K. KYSER & M. CUNEY
2008). About 25% of these reactors are in North America and 37% are in Europe (Fig. 1-2). Given all of the existing reactors and the 27 reactors currently under construction and those anticipated, the need for U will increase by about 30% to 82,000 to 100,000 t U (212 to 259 million lbs U3O8) by the year 2025 (EIA 2007). The balance of need and supply is discussed more in detail in Chapter 2.
TABLE 1-1. FINAL DECAY PRODUCTS OF U AND TH ISOTOPES 238 U → 206Pb t½ = 4.469 x 109 235 U → 207Pb t½ = 0.7038 x 109 232 Th → 208Pb t½ = 14.01 x 109 204
(238U/235U)now is 137.88 Pb is the only non-radiogenic isotope of Pb
Final decay products of naturally occurring U and Th isotopes to isotopes of Pb along with their half-lives (t½) that are used for geochronometers in the earth sciences. Also indicated are the normal 238U/235U ratio and the only non-radiogenic isotope of Pb.
The Nuclear Fuel Cycle The isotope 235U is important for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile. There are two types of reactors currently in use, those that use U enriched in 235U and normal water around the fuel rods (light water reactors) and those that use U without any enrichment and heavy water to moderate the fission reactions (heavy water reactors). Enrichment of natural, mined U through isotope separation to concentrate the fissionable 235U is needed for use in nuclear weapons and light water reactors as shown by the U cycle in Figure 1-3. Only 0.7204% of natural U is 235U. This is too low a concentration to sustain a nuclear chain reaction without the help of a moderator that can
isotopes in the decay schemes. Disequilibria in the radioactivity of isotopes in the decay schemes are used to determine the timing at which recent events have produced open systems in U ores. In deposit studies, the decay schemes of 235U and 238U along with the isotopes of Pb are used to determine the age of U minerals, as discussed in Chapter 3. NUCLEAR POWER The current major use of U is the generation of electricity. Globally, there are 440 nuclear power reactors in 30 countries and 270 research reactors that required nearly 70,000 t (181 million pounds U3O8) of U in 2007 (OECD U
238
U 9
4.5 * 10 y
Pa Th
a 4.196
Mev 234
234
6
Th
230
Ra
8
a 4.395
231
Pa
b 3.2 * 10 y 231 Th a 227 Th 4
Mev
Th 4
25 h
8.3 * 10 y
a
Ac
U
7.5 * 10 y
Mev
6.7 h
25 d
235
U
b 2.3 * 10 y a 234 Pa 4.776
5.013 Mev
Ra 3
222
219
Rn
3.8 d
Po Bi
Po
3.1 m
a
6.003 Mev 214
Pb
Pb
224
214
210
Po
b 1.5 * 10 y b 214 a 210 Bi Bi 7.607 -4
Mev
210
Pb
22 y
215
Po
a
218
Rn
a
206
7.316 Mev 211
Pb
216
211
b
a
Bi
7.386 Mev
2.2 m
a Pb 6.623 Mev
207
4.8 m
Pb
stable
Tl b
207
Tl
212
Po
b
0.16 s
36 m
stable
a
6.288 Mev
Po
a
5.304 Mev
5d
Rn
55 s
1.8
140 d
Ra
3.7 y
5.688 Mev
a
20 m
27 m
5.423 Mev
6.819 Mev
5.490 Mev 218
Ra b
3.9 s
a
At
a
Ac
6.1 h
5.176 Mev
4.784 Mev
Th
1.9 y
6.7 y
a
a
228
228
Ra
11 d
1.6 * 10 y
Rn
a
4.010 Mev
Mev 223
226
b
10
a Ac b 6.038
21 y
228
Th
1.4 * 10 y
227
4.688 Mev
Fr
232
1.9 d
212
b
a
Bi
8.784 Mev
61 m
a Pb 6.051
212
11 h
Po
0.3 ms
Mev 208
208
Pb
stable
Tl b
3.1 m
FIG. 1-1. Decay schemes of 235U, 238U and 232Th showing the position of 234U in the decay chain of 238U. The geochemical properties of the decay products are quite distinct, including production of radon gas, and these will tend to separate from the ores in open systems.
2
INTRODUCTION
FIG. 1-2. Number of reactors (top) and % electricity generated by nuclear power (bottom) as a function of country. Data from OECD (2006) and OECD (2008).
(around 0.001%) which makes its radioactivity slightly more than half that of natural U. The most abundant isotope of U, 238U, can be converted by irradiation with neutrons into 239Pu, a fissionable material that can also be used as a fuel in nuclear reactors as MOX, mixed oxides of U and Pu (Fig. 1-3). Although it does not occur naturally, 233 U is also a fissionable material that can be used as a fuel in nuclear reactors. Thorium, which naturally is 100% 232Th, is exposed to neutrons to
slow down a neutron without absorbing it. Reactors using natural U can be made using graphite or heavy water as a moderator whereas normal water can be used as a moderator in reactors that use enriched U with greater than 3% 235U. This enriched U typically has a 235U concentration of between 3 and 5%. The process produces huge quantities of depleted U with 0.2% to 0.3% 235U (Fig. 1-3). Enriched U contains more 234U than natural U, and depleted U contains much less 234U
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K. KYSER & M. CUNEY
FIG. 1-3. Condensed version of the U fuel cycle, of which exploration and finding deposits is the first and most critical step. Modified from World Nuclear Association (2008).
produce 233U, an isotope that produces more neutrons per captured neutron at thermal energies than either 235U or 239Pu. Thus, thorium can be partially substituted for U in reactors with the advantage of reducing the amount of long-lived isotopes of Np, Am, Cu and Pu in the spent fuel and producing high-level nuclear waste that is “safer” (MacFarlane & Miller 2007). If completely fissioned, one pound (0.45 kg) of 233U will provide the same amount of energy as burning 1,350 tonnes of coal. The U cycle shown in Figure 1-3 is often referred to by others as the nuclear fuel cycle. In most renditions of the nuclear fuel cycle, the very first step involving exploration is overlooked. This is the first critical step and probably the least trivial in the nuclear fuel cycle as it cannot be engineered. Once an ore deposit is discovered, a mine is developed and U extracted by a variety of mining techniques, depending on the type of deposit. The resulting ore is treated to extract the U, which is normally converted to ammonium diuranate, which is yellow in color, and tailings are produced (Fig
1-3). The “yellow cake” is sent to a processing plant and either converted to UO2 for use in heavy water reactors, or to UF6 and then enriched in 235U and fabricated into UO2 fuel rods for use in light water reactors. These are used by the reactors, resulting in spent fuel rods that are reprocessed and converted into additional fuel (or Pu for weapons), or stored (Fig. 1-3). Concerns about nuclear power The use of U as a fuel for nuclear reactors is not without its caveats. Constraints on the use of nuclear energy include the cost, the waste produced, safety and resources (MacFarlane & Miller 2007). Although the focus of this volume is the latter, those working in U exploration must be aware of the strongest and weakest links in the industry and the complexities that affect the need for U, both perceived and real. The operational, maintenance and fuel costs for nuclear power are only half of that for fossil fuels, but nuclear power plants cost 4 times as much and require much longer times to build than
4
INTRODUCTION
always persist. Projections by the Energy Information Agency (EIA 2007) indicate that all forms of energy will increase by 32% over the next 20 years (Fig. 1-4), primarily in response to demand from India and China. Nuclear energy and renewable sources of energy will remain equal, but both pale in proportion to the energy supplied by fossil fuels. The current known reserves of U might be sufficient to supply the increased demand for nuclear reactors for the next century, but this depends greatly on the economic landscape that develops. The development of the fourth generation of nuclear reactors in the foreseeable future could theoretically increase the usefulness of U resources by a factor of a hundred.
plants that use fossil fuels (Ansolabehere et al. 2003). Securing licenses for nuclear power plants is an arduous and expensive task, as is getting acceptance by the local community. Some of these higher costs for nuclear power plants may be offset by carbon taxes in the future, but the lead time for building a nuclear power plant is lengthy, at least 10 years (Ansolabehere et al. 2003). The amount of spent fuel produced each year is about 12,000 tonnes (World Nuclear Association 2008). The composition of spent enriched fuel is 95% 238U, 1% 235 U and 1% 239Pu, so that most of the potential energy in the fuel is not used. However, the spent fuel also contains fission products such as bioactive 131I, 90Sr and 137Cs with relatively short half-lives, and many longer lived, intensely radioactive isotopes such as 99Tc, 140Ce and 239Pu that render it dangerous, thus requiring reprocessing, disposal or storage for hundreds to thousands of years. Spent fuel is currently stored in cooling pools at reactor sites or centralized facilities as no country has established a permanent solution to disposing of high-level waste, although most are examining sites based on knowledge of the geochemical behavior of U in natural geological environments. The method of disposal has been debated, not only on the grounds of whether it should be permanent or temporary so that the unused portion of the fuel can be reclaimed with technological advances, but also whether the current practice of storing the fuel in densely packed configurations in cooling pools proximal to reactors is a target for terrorists. The disposal issue is charged with political and social concerns that are unlikely to be solved, or to remain static, in the near future. Uranium used in nuclear reactors has a significant risk in the proliferation of nuclear weapons. Plants constructed to enrich natural U with a centrifuge process to produce fuels for light water reactors also can be converted to produce weapons-grade U which has > 90% 233U. In addition, spent fuel can be reprocessed using a closed fuel cycle to extract the Pu, which is used by France, UK, Russia, India and Japan to reuse Pu in mixed oxide fuels. In the past, this Pu was diverted to make nuclear weapons, and the fear is that this could be done again, even under the auspices of strict regulations. Regardless of how secure the process is by which fuels are made and reused, the legacy of U as the world’s most dangerous waste and its use in weapons will
MAJOR DEPOSIT TYPES There are many different ways of classifying U deposits depending on the tendency of the classifier to put deposits into more generic classes, or divide deposits according to their style of occurrence. Most classifications by the IAEA and major U mining companies are similar and involve between 12 and 18 deposit types. One of the problems with subdividing deposits is that a genetic connotation can be implied, but one of the advantages is that the model of formation or location that is implied can be used to refine exploration strategies. This can lead to the discovery of new deposits if the model is sophisticated enough to identify prospective areas from barren areas. One widely used classification (OECD 2000, IAEA 2004, 2006) groups deposits into 15 categories based on their geologic setting, with the 11 major types shown in Table 1-2. The following is a brief description of each, outlining the salient features as summarized in Table 1-2 and Figure 1-5. (1) Unconformity-related deposits occur close to major unconformities between Archean– Paleoproterozoic metasedimentary rocks and overlying Paleo–Mesoproterozoic sandstone units in large marginal or intracratonic basins (Fig. 1-5). The deposits occur within the basement or sandstone, but within a few hundred metres of the unconformity. The deposits are hosted by faults and are commonly associated with brecciation. They involve formation from brines at 150–250°C. Those discovered so far are restricted to the Proterozoic and most formed shortly after the assembly of large continents (Table 1-2).
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K. KYSER & M. CUNEY
FIG. 1-4. World Marketed Energy Use by Fuel Type for 1980-2030. Historical data from before 2005 and other data from projections by EIA (2007).
FIG. 1-5. Schematic representation of the location of various types of U deposits. See Table 1-2 and text for explanation.
Unconformity-related deposits constitute about 12% of the total U resources globally, but 33% of the current global production and include some of the largest and richest U deposits (e.g. McMurray 2006). Uraninite is the major mineral in these deposits, although many of them have been affected by later fluid events that have altered the uraninite. Major deposits occur in the Athabasca
Basin in Canada and the Alligator River region of Australia and these deposits average 0.8% U, with the largest deposits up to 23% U (Jefferson et al. 2007). Recent discoveries have indicated unconformity-related deposits occur in the Karelia region of Russia and Finland (Velichkin et al. 2005), the Paterson Basin in Western Australia (McKay & Miezitis 2001) and the Thelon Basin in
6
300000
100000
Post orogenic extensional basins
Rift/back arc basins
Arc/Marginal Basin
Alkaline complexes
Rift/back arc basins
Intracratonic & marginal basins
Drainage basin
Intracratonic & extensional basins
Continental shelf/marine
Vein deposits
Metasomatic
IOCG/ breccia
Intrusive
7
Volcanic associated
Quartzpebble conglomerate
Surficial
Collapse breccia pipes
Phosphorite
447000
50000
211300
287900
901000
673700
324500
0
0
0
600
1000
1000
2000
3800
4220
10000
GLOBAL PRODUCTION 2007 (tU) 15600
20-50
200-300
25-50
50-300
200-500
300-700
200-500
300-700
200-500
25-100
TEMP. FORMED (°C) 200-300
Marine & brackish fluids
Basinal brines
Groundwater
Groundwater & later hydrothermal
Basinal brines & magmatic fluids
Magmatic & hydrothermal
Magmatic & later hydrothermal
Metamorphic & magmatic
Metamorphic & hydrothermal
Groundwater
Basinal & basement brines
TYPES OF FLUIDS
Phanerozoic
Tertiary
Tertiary
2200-2700 Ma
All
All
Proterozoic
Proterozoic
Proterozoic to Mesozoic
Phanerozoic to Tertiary
1500-1900 Ma
MAIN AGE
Uncle Sam (USA), Gantour, (Morocco), Al-Abiad (Jordan), Randstat (Sweden)
Arizona Strip (USA)
Yeelirrie (Australia), Langer Heinrich (Namibia)
Witswatersrand (S. Africa), Blind River/ Elliot Lake (Canada)
Streltsovsk caldera (Russia), Dornot (Mongolia), Nopal (Mexico), McDermitt (USA), Xiangshan (China)
Rössing (Namibia), Ilimaussaq (Greenland), Palabora (S. Africa)
Olympic Dam (Australia)
Pervomayskoye (Ukraine), Lagoa Real (Brazil), Valhalla (Australia)
Pribram , Jachymov (Czech Republic), Shinkolobwe (DR Congo), Massif Central (France), Iberia (Spain/Portugal), Dhada (Congo), Schwartzwalder (USA)
Mokum, Inkay (Kazakhstan), Colorado Plateau (USA), Beverley (Australia), Akouta (Niger)
Rabbit Lake, McArthur River (Athabasca Basin); Narbarlek, Ranger (NT, Australia); Karku (Russia)
EXAMPLES
Data sources: Dahlkamp (1993), Plant et al. (1999), OCED (2008), McMurray (2006), Shatalov et al. (2006). Resources are for
