Recent and not-so-recent developments in uranium deposits and implications for exploration

261 Pages·2009·15.37 MB·English

by  Michel Cuney, T. K. Kyser, Mineralogical Association of Canada, Society for Geology Applied to Mineral Deposits

CHAPTER 1: INTRODUCTION

Kurt Kyser
Department of Geological Sciences and Geological Engineering,
Queen’s University,
Kingston, Ontario, K7L 3N6, Canada
K. KYSER & M. CUNEY
TABLE 1-1. FINAL DECAY PRODUCTS OF U AND TH 2008). About 25% of these reactors are in North
ISOTOPES America and 37% are in Europe (Fig. 1-2). Given
238 206 9
U → Pb t½ = 4.469 x 10 all of the existing reactors and the 27 reactors
235 207 9
U → Pb t½ = 0.7038 x 10 currently under construction and those anticipated,
232 208 9 the need for U will increase by about 30% to
Th → Pb t½ = 14.01 x 10
82,000 to 100,000 t U (212 to 259 million lbs
238 235
( U/ U)now is 137.88
U3O8) by the year 2025 (EIA 2007). The balance of
204
Pb is the only non-radiogenic isotope of Pb
need and supply is discussed more in detail in
Chapter 2.
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 The Nuclear Fuel Cycle
238 235 235
sciences. Also indicated are the normal U/ U ratio The isotope U is important for both nuclear
and the only non-radiogenic isotope of Pb. reactors and nuclear weapons because it is the only
isotope existing in nature to any appreciable extent
isotopes in the decay schemes. Disequilibria in the
that is fissile. There are two types of reactors
radioactivity of isotopes in the decay schemes are
235
currently in use, those that use U enriched in U
used to determine the timing at which recent events
and normal water around the fuel rods (light water
have produced open systems in U ores. In deposit
235 238 reactors) and those that use U without any
studies, the decay schemes of U and U along
enrichment and heavy water to moderate the fission
with the isotopes of Pb are used to determine the
reactions (heavy water reactors). Enrichment of
age of U minerals, as discussed in Chapter 3.
natural, mined U through isotope separation to

235
concentrate the fissionable U is needed for use in
NUCLEAR POWER
nuclear weapons and light water reactors as shown
The current major use of U is the generation
by the U cycle in Figure 1-3.
of electricity. Globally, there are 440 nuclear
235
Only 0.7204% of natural U is U. This is too
power reactors in 30 countries and 270 research
low a concentration to sustain a nuclear chain
reactors that required nearly 70,000 t (181
reaction without the help of a moderator that can
million pounds U3O8) of U in 2007 (OECD
238 234 235
U U U U
4.5 * 109 y b 2.3 * 106 y 7.5 * 108 y
a 234 a a 231
Pa 4.196 Pa 4.776 4.395 Pa
Mev 6.7 h Mev Mev b 3.2 * 104 y
234 230 231 a 227 232 228
Th
25Tdh 8.3 *T10h4 y 25Thh 5M.0e1v3 1.T9 dh 1.4 * 1T01h0 y b 1.9Tyh
Ac 4a.688 227Ac b 6a.038 4.a010 228Ac 5a.423
Mev 21 y Mev Mev 6.1 h Mev
226 223 228 224
Ra Ra Ra Ra b Ra
1.6 * 103 y 11 d 6.7 y 3.7 y
a a a
Fr 4.784 5.176 5.688
Mev Mev Mev
222 219 218
Rn Rn Rn Rn
3.8 d 3.9 s 55 s
a a a
At 5.490 6.819 6.288
Mev Mev Mev
218 214 210 215 216 212
Po Po Po Po Po Po
Po -4
3.1 m b 1.5 * 10 y b 140 d 1.8 0.16 s b 0.3 ms
a 214 a 210 a a 211 a 212 a
Bi 6M.0e0v3
20Bmi 7M.6e0v7 5 dBi 5M.3e0v4 7M.3e1v6 b 2.2Bmi 7M.3e8v6 b 61Bmi 8M.7e8v4
Pb 21247Pmb 21202Pyb 20
s6taPblbe 21316Pmb 6M.a6e2v3 20s7taPblbe 21121Phb 6M.a0e5v1 20s8tPablbe
Tl 207Tl b 208Tl b
4.8 m 3.1 m

235 238 232 234 238
FIG. 1-1. Decay schemes of U, U and Th showing the position of U in the decay chain of U. 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).
slow down a neutron without absorbing it. Reactors (around 0.001%) which makes its radioactivity
using natural U can be made using graphite or slightly more than half that of natural U.
238
heavy water as a moderator whereas normal water The most abundant isotope of U, U, can be
239
can be used as a moderator in reactors that use converted by irradiation with neutrons into Pu, a
235
enriched U with greater than 3% U. This fissionable material that can also be used as a fuel
235
enriched U typically has a U concentration of in nuclear reactors as MOX, mixed oxides of U and
between 3 and 5%. The process produces huge Pu (Fig. 1-3). Although it does not occur naturally,
235 233
quantities of depleted U with 0.2% to 0.3% U U is also a fissionable material that can be used
234
(Fig. 1-3). Enriched U contains more U than as a fuel in nuclear reactors. Thorium, which
234 232
natural U, and depleted U contains much less U naturally is 100% Th, is exposed to neutrons to
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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).
233
produce U, an isotope that produces more 1-3). The “yellow cake” is sent to a processing
neutrons per captured neutron at thermal energies plant and either converted to UO2 for use in heavy
235 239 235
than either U or Pu. Thus, thorium can be water reactors, or to UF6 and then enriched in U
partially substituted for U in reactors with the and fabricated into UO2 fuel rods for use in light
advantage of reducing the amount of long-lived water reactors. These are used by the reactors,
isotopes of Np, Am, Cu and Pu in the spent fuel resulting in spent fuel rods that are reprocessed and
and producing high-level nuclear waste that is converted into additional fuel (or Pu for weapons),
“safer” (MacFarlane & Miller 2007). If completely or stored (Fig. 1-3).
233
fissioned, one pound (0.45 kg) of U will provide
the same amount of energy as burning 1,350 tonnes Concerns about nuclear power
of coal. The use of U as a fuel for nuclear reactors is
The U cycle shown in Figure 1-3 is often not without its caveats. Constraints on the use of
referred to by others as the nuclear fuel cycle. In nuclear energy include the cost, the waste
most renditions of the nuclear fuel cycle, the very produced, safety and resources (MacFarlane &
first step involving exploration is overlooked. This Miller 2007). Although the focus of this volume is
is the first critical step and probably the least trivial the latter, those working in U exploration must be
in the nuclear fuel cycle as it cannot be engineered. aware of the strongest and weakest links in the
Once an ore deposit is discovered, a mine is industry and the complexities that affect the need
developed and U extracted by a variety of mining for U, both perceived and real.
techniques, depending on the type of deposit. The The operational, maintenance and fuel costs
resulting ore is treated to extract the U, which is for nuclear power are only half of that for fossil
normally converted to ammonium diuranate, which fuels, but nuclear power plants cost 4 times as
is yellow in color, and tailings are produced (Fig much and require much longer times to build than
4
INTRODUCTION
plants that use fossil fuels (Ansolabehere et al. always persist. Projections by the Energy
2003). Securing licenses for nuclear power plants Information Agency (EIA 2007) indicate that all
is an arduous and expensive task, as is getting forms of energy will increase by 32% over the next
acceptance by the local community. Some of these 20 years (Fig. 1-4), primarily in response to
higher costs for nuclear power plants may be offset demand from India and China. Nuclear energy and
by carbon taxes in the future, but the lead time for renewable sources of energy will remain equal, but
building a nuclear power plant is lengthy, at least both pale in proportion to the energy supplied by
10 years (Ansolabehere et al. 2003). The amount of fossil fuels. The current known reserves of U might
spent fuel produced each year is about 12,000 be sufficient to supply the increased demand for
tonnes (World Nuclear Association 2008). The nuclear reactors for the next century, but this
238
composition of spent enriched fuel is 95% U, 1% depends greatly on the economic landscape that
235 239
U and 1% Pu, so that most of the potential develops. The development of the fourth
energy in the fuel is not used. However, the spent generation of nuclear reactors in the foreseeable
fuel also contains fission products such as future could theoretically increase the usefulness of
131 90 137
bioactive I, Sr and Cs with relatively short U resources by a factor of a hundred.
half-lives, and many longer lived, intensely
99 140 239
radioactive isotopes such as Tc, Ce and Pu MAJOR DEPOSIT TYPES
that render it dangerous, thus requiring There are many different ways of classifying
reprocessing, disposal or storage for hundreds to U deposits depending on the tendency of the
thousands of years. classifier to put deposits into more generic classes,
Spent fuel is currently stored in cooling pools or divide deposits according to their style of
at reactor sites or centralized facilities as no occurrence. Most classifications by the IAEA and
country has established a permanent solution to major U mining companies are similar and involve
disposing of high-level waste, although most are between 12 and 18 deposit types. One of the
examining sites based on knowledge of the problems with subdividing deposits is that a
geochemical behavior of U in natural geological genetic connotation can be implied, but one of the
environments. The method of disposal has been advantages is that the model of formation or
debated, not only on the grounds of whether it location that is implied can be used to refine
should be permanent or temporary so that the exploration strategies. This can lead to the discov-
unused portion of the fuel can be reclaimed with ery of new deposits if the model is sophisticated
technological advances, but also whether the enough to identify prospective areas from barren
current practice of storing the fuel in densely areas. One widely used classification (OECD 2000,
packed configurations in cooling pools proximal to IAEA 2004, 2006) groups deposits into 15
reactors is a target for terrorists. The disposal issue categories based on their geologic setting, with the
is charged with political and social concerns that 11 major types shown in Table 1-2. The following
are unlikely to be solved, or to remain static, in the is a brief description of each, outlining the salient
near future. features as summarized in Table 1-2 and Figure
Uranium used in nuclear reactors has a 1-5.
significant risk in the proliferation of nuclear
weapons. Plants constructed to enrich natural U (1) Unconformity-related deposits occur close to
with a centrifuge process to produce fuels for light major unconformities between Archean–
water reactors also can be converted to produce Paleoproterozoic metasedimentary rocks and
233
weapons-grade U which has > 90% U. In overlying Paleo–Mesoproterozoic sandstone units
addition, spent fuel can be reprocessed using a in large marginal or intracratonic basins (Fig. 1-5).
closed fuel cycle to extract the Pu, which is used The deposits occur within the basement or
by France, UK, Russia, India and Japan to reuse Pu sandstone, but within a few hundred metres of the
in mixed oxide fuels. In the past, this Pu was unconformity. The deposits are hosted by faults
diverted to make nuclear weapons, and the fear is and are commonly associated with brecciation.
that this could be done again, even under the They involve formation from brines at 150–250°C.
auspices of strict regulations. Regardless of how Those discovered so far are restricted to the
secure the process is by which fuels are made and Proterozoic and most formed shortly after the
reused, the legacy of U as the world’s most assembly of large continents (Table 1-2).
dangerous waste and its use in weapons will
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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 Basin in Canada and the Alligator River region of
about 12% of the total U resources globally, but Australia and these deposits average 0.8% U, with
33% of the current global production and include the largest deposits up to 23% U (Jefferson et al.
some of the largest and richest U deposits (e.g. 2007). Recent discoveries have indicated
McMurray 2006). Uraninite is the major mineral in unconformity-related deposits occur in the Karelia
these deposits, although many of them have been region of Russia and Finland (Velichkin et al.
affected by later fluid events that have altered the 2005), the Paterson Basin in Western Australia
uraninite. Major deposits occur in the Athabasca (McKay & Miezitis 2001) and the Thelon Basin in
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INTRODUCTION

7
TABLE 1-2. GENERAL CHARACTERISTICS OF URANIUM DEPOSITS LISTED IN ORDER OF ECONOMIC SIGNIFICANCE.
DEPOSIT GEOLOGY GRADE GLOBAL GLOBAL TEMP. TYPES OF FLUIDS MAIN AGE EXAMPLES
TYPE (% U) RESOURCE PRODUCTION FORMED
2007 (tU) 2007 (tU) (°C)
Unconform- Intracratonic/ 1.0 - 649700 15600 200-300 Basinal & 1500-1900 Rabbit Lake, McArthur River
ity-related marginal basins 25.0 basement brines Ma (Athabasca Basin); Narbarlek, Ranger
(NT, Australia); Karku (Russia)
Sandstone Extensional rift 0.05 - 1524000 10000 25-100 Groundwater Phanerozoic Mokum, Inkay (Kazakhstan), Colorado
hosted basins 0.5 to Tertiary Plateau (USA), Beverley (Australia),
Akouta (Niger)
Vein Post orogenic 0.1 - 324500 4220 200-500 Metamorphic & Proterozoic Pribram , Jachymov (Czech Republic),
deposits extensional basins 2.0 hydrothermal to Mesozoic Shinkolobwe (DR Congo), Massif
Central (France), Iberia
(Spain/Portugal), Dhada (Congo),
Schwartzwalder (USA)
Metasomatic Rift/back arc 0.05 - 673700 3800 300-700 Metamorphic & Proterozoic Pervomayskoye (Ukraine), Lagoa Real
basins 0.2 magmatic (Brazil), Valhalla (Australia)
IOCG/ Arc/Marginal 0.03 - 901000 2000 200-500 Magmatic & later Proterozoic Olympic Dam (Australia)
breccia Basin 0.05 hydrothermal
Intrusive Alkaline 0.01 - 287900 1000 300-700 Magmatic & All Rössing (Namibia), Ilimaussaq
complexes 0.5 hydrothermal (Greenland), Palabora (S. Africa)
Volcanic Rift/back arc 0.03 - 211300 1000 200-500 Basinal brines & All Streltsovsk caldera (Russia), Dornot
associated basins 0.3 magmatic fluids (Mongolia), Nopal (Mexico),
McDermitt (USA), Xiangshan (China)
Quartz- Intracratonic & 300000 600 50-300 Groundwater & 2200-2700 Witswatersrand (S. Africa), Blind
pebble con- marginal basins later Ma River/ Elliot Lake (Canada)
glomerate hydrothermal
Surficial Drainage basin 0.03 - 100000 0 25-50 Groundwater Tertiary Yeelirrie (Australia), Langer Heinrich
0.1 (Namibia)
Collapse Intracratonic & 0.4 - 50000 0 200-300 Basinal brines Tertiary Arizona Strip (USA)
breccia pipes extensional basins 1.5
Phosphorite Continental 0.005 - 447000 0 20-50 Marine & Phanerozoic Uncle Sam (USA), Gantour,
shelf/marine 0.05 brackish fluids (Morocco), Al-Abiad (Jordan),
Randstat (Sweden)
Data sources: Dahlkamp (1993), Plant et al. (1999), OCED (2008), McMurray (2006), Shatalov et al. (2006). Resources are for <US$ 130/kg. and total 5,469,000.