2.1 Properties and Characteristics
2.1.1 Forms of Uranium
Uranium, a radioactive element, is a silver-white metal in its pure form. It is a heavy metal nearly twice as dense as lead (19 grams per cubic centimeter [g/cm3] compared with 11.4 g/cm3).
Uranium occurs in nature in a wide variety of solid, liquid and gaseous compounds. It readily combines with other elements to form uranium oxides, silicates, carbonates and hydroxides. These compounds range from being highly mobile (soluble) to being relatively immobile (insoluble) in the environment and the human body (Erikson et al., 1993; Stokinger, 1981). Several conditions affect the formation of these compounds: the relative amounts of oxygen, moisture and acidity present; the presence of other metals alloyed with uranium; and the temperature history of the uranium solid (Erikson et al., 1993). The resultant uranium compound also depends on the original form of the uranium (alloy and mineral phase) and its interaction with environmental media (soil, air, surface and ground water, and biota). Uranium compounds dissolve and migrate at different rates.
Uranium metal alloys are readily machinable and have metallurgical properties similar to those of many high-strength steels (Magness, 1985; NMI, undated a). Small particles of uranium metal and some uranium alloys are pyrophoricůthey can ignite spontaneously in air, as a function of surface to volume ratio, and they burn rapidly at very high temperatures (Stokinger, 1981).
The earthŪs crust contains three naturally occurring uranium isotopes: uranium-234 (234U), uranium-235 (235U) and uranium-238 (238U). Each is radioactive. Isotopes of an element have essentially identical chemical and physical properties because they have the same number of protons in their atoms. Isotopes are differentiated by the number of neutrons they contain. Variation in the number of neutrons gives isotopes different radiological properties; uranium isotopes vary in their ability to undergo nuclear fission, interactions with nuclear particles, radioactive decay rates, and the types of radiation they emit upon radioactive decay. The radioactivity of isotopes can be compared using specific activity which is measured in nuclear transformations (disintegrations) per second per unit mass (e.g., in microcuries per gram ( Ci/g), where a microcurie is equal to 3.700 x 104 nuclear transformations per second). While 234U, 235U and 238U have essentially the same chemical and physical properties, the variation in the number of neutrons makes them radiologically different.
The 234U and 235U constitute less than 1 percent of naturally occurring uranium (see Table 2-1). The 234U is a major contributor to the radioactivity of naturally occurring uranium despite its small percentage by weight because its specific activity is so high in comparison with 235U and 238U. The 238U is the most abundant naturally occurring uranium isotope and the least radioactive. The specific activity of uranium with the natural isotopic mix is approximately 0.7› Ci/g. Each uranium isotope has a long half-life.› Each isotope decays to produce a series of radioactive daughter products, which in turn decay to subsequent daughter products. Each step in this process emits one or more forms of ionizing radiationůalpha ( ) particles, beta ( ) particles and gamma ( ) rays (Coleman et al., 1983; Cross, 1991; Piesch et al., 1986; Rohloff and Heinzelmann, 1986).
Table 2-1.› Components of Naturally Occurring Uranium
*› Exact weight percentages of uranium found in nature vary slightly with the source. Values shown here were reported by
? Half-life is the time required for 50 percent of an unstable material to decay.
An alpha particle is a positively charged (+2) ion composed of two protons and two neutrons. Isotopes that emit only alpha particles do not pose a health risk if they are outside the body because alpha particles cannot penetrate the skinŪs dead layers. However, internalized alpha particles do pose a health risk. A beta particle is an electron emitted during the radioactive decay of a neutron. Beta particles can penetrate the skin, so isotopes that emit beta particles pose health risks both externally and internally. A gamma ray is a discrete packet (quantum) of electromagnetic energy with no mass or charge. It is extremely penetrating and poses health hazards both externally and internally.
The serial decay chains presented in Figure 2-1 and Figure 2-2 show the breakdown of radioactive elements until stable isotopes of lead are reached. Stable isotopes do not decay and are, by definition, not radioactive. Since the formation of the earthŪs elements, sufficient time has elapsed for most natural deposits of uranium isotopes and their daughter products to be in equilibrium (i.e., their rates of formation to equal their rates of decay) (Eisenbud, 1987). The decay sequences of uranium isotopes produce several radioactive isotopes that are dangerous to plants and animals; these include radium, thorium and radon. Each daughter product migrates through the environment based on its chemical characteristics.
Isotopes with short half-lives will not migrate great distances, unless they are gaseous and near the ground surface because they decay rapidly to form other elements. The highest concentrations of daughter products generally occur in the soil, air and water where uranium concentrations are highest.
Figure 2-1.› The Uranium-238 Daughter Products
(Note: E means exponent of 10.› 5E8 = 5 X 108)
Figure 2-2.› The Uranium-235 Daughter Products
(Note: E means exponent of 10.› 5E8 = 5 X 108)
Each of the three uranium isotopes can also decay by spontaneous fission, which is so rare that it does not contribute to the daughter products formed or to the radiation emitted. The half-lives for the spontaneous fission of 238U, 235U and 234U are approximately 8 x 1015 years, 4 x 1017 years and 2 x 1016 years, respectively.
As shown in Figures 2-1 and 2-2 and in Tables 2-2 and 2-3, all uranium isotopes primarily decay by alpha emission. They also emit small amounts of gamma radiation during decay. The subsequent decay of radioactive daughter products emits additional radiation. Only the first two daughter products of 238U (thorium-234 and protactinium-234m) and the first daughter product from the decay of 235U (thorium-231) contribute to the radioactivity of DU. Other daughter products do not accumulate in significant concentrations because stopper isotopes preclude their formation. A stopper isotope is a decay product with such a long half-life that it blocks the significant production of the remainder of isotopes in the chain. 234U (half-life of 2.47 x 105 years) is the stopper isotope for 238U; protactinium-231 (half-life of 3.25 x 104 years) is the stopper for 235U; thorium-230 (half-life of 8.0 x 104 years) is the stopper for 234U. Thus, after stopper isotopes appear, daughter products from pure uranium will not begin to appear in quantities of environmental or human health concern for more than 10,000 years. The presence of these stopper isotopes blocks production of significant quantities of the radium and radon isotopes that contribute to the radiological hazards of naturally occurring uranium deposits.
Table 2-2.› The Uranium-238 Decay Series
HALF-LIFE RADIATION ENERGY (MEV) PERCENT YIELD
URANIUM-238 (238U) 4.5 x 109 years››› 4.2› 75.0
››› 4.15› 23.0
›››› 0.048› 23.0
THORIUM-234 (234Th) 24 days››› 0.192› 65.0
››› 0.100› 35.0
›››› 0.092› 4.0 (doublet)
PROTACTINIUM-234m (234mPa) 1.2 minutes››› 2.29› 98.0
››› 1.53› <1
››› 1.25› <1
››› (IT)› 0.39› 0.13
›››› 0.817› 4.0
PROTACTINIUM-234 (234Pa) 6.75 hours››› 0.53› 66.0
››› 1.13› 13.0
›››› 0.100› 50.0
››› 0.7› 24.0
››› 0.9› 70.0
URANIUM-234 (234U) 2.47 x 105 years››› 4.77› 72.0
››› 4.72› 28.0
›››› 0.093› 5.0
THORIUM-230 (230Th) 8.0 x 104 years››› 4.68› 76.0
››› 4.61› 24.0
››› 4.51› 0.35
›››› 0.068› 0.6
››› 0.253› 1.02
RADIUM-226 (226Ra) 1622 years››› 4.78› 94.3
››› 4.59› 5.7
›››› 0.186› 4.00
››› 0.26› 0.007
RADON-222 (222Rn) 3.8 days››› 5.48› 100.00
›››› 0.510› 0.007
POLONIUM-218 (218Po) 3.05 minutes››› 6.0› 99.98
›››› 0.33› 0.019
LEAD-214 (214Pb) 26.8 minutes››› 0.72› 100.0
›››› 0.053› 1.6
››› 0.242› 4.0
››› 0.295› 19.0
››› 0.352› 36.0
ASTATINE-218 (218At) 1.5 seconds››› 6.70› 94.0
››› 6.65› 6.0
BISMUTH-214 (214Bi) 19.7 minutes››› 3.26› 19.0
››› 1.51› 40.0
››› 1.00› 23.0
››› 1.88› 9.0
›››› 5.51› 0.008
››› 5.45› 0.012
››› 5.27› 0.001
›››› 0.609› 47.0
››› 1.12› 17.0
››› 1.764› 17.0
POLONIUM-214 (214Po) 1.64 x 10-4 sec››› 7.69› 100.0
›››› 0.799› 0.014
THALLIUM-210 (210TI) 1.3 minutes››› 1.9› 56.0
››› 1.3› 25.0
››› 2.3› 19.0
›››› 0.296› 80.0
››› 0.795› 100.0
››› 1.31› 21.0
LEAD-210 (210Pb) 22 years››› 0.015› 81.0
››› 0.061› 19.0
›››› 0.0465› 4.0
BISMUTH-210 (210Bi) 5.0 days››› 1.17› 99.99
›››› 5.0› 8 x 10-5
POLONIUM-210 (210Po) 138 days››› 5.3› 100.0
›››› 0.80› 0.0011
THALLIUM-206 (206Tl) 4.2 minutes››› 1.51› 100.0
LEAD-206 (206Pb) Stable --› --› --
Table 2-3.› The Uranium-235 Decay Series
ISOTOPE HALF-LIFE RADIATION ENERGY (MEV) PERCENT YIELD
URANIUM-235 (235U) 7.1 x 108 years››› 4.21› 5.7
››› 4.58› 8.0 (doublet)
››› 4.4› 57.0
››› 4.37› 18.0
›››› 0.110› 2.5
››› 0.143› 11.0
››› 0.163› 5.0
››› 0.185› 54.0
››› 0.205› 5.0
››› 0.302› 52.0
THORIUM-231 (231Th) 25.6 hours››› 0.218› 13.0
››› 0.140› 40.0
››› 0.026› 2.0
›››› 0.085› 10.0 (complex)
PROTACTINIUM-231 (231Pa) 3.25 x 104 years››› 5.00› 24.0
››› 4.94› 22.0
››› 5.02› 23.0
››› 5.05› 10.0
›››› 0.027› 6.0
››› 0.29› 6.0 (complex)
ACTINIUM-227 (227Ac) 21.6 years››› 0.046› 98.6
›››› 4.95› 1.4 (doublet)
›››› 0.070› 0.08
THORIUM-227 (227Th) 18.2 days››› 6.04› 23.0
››› 5.98› 24.0
››› 5.76› 21.0
›››› 5.72› 14.0 (doublet)
›››› 0.050› 8.0
››› 0.237› 15.0 (complex)
››› 0.31› 8.0 (complex)
FRANCIUM-223 (223Fr) 22 minutes››› 1.15› 99.99
›››› 5.35› 0.005
›››› 0.050› 40.0
››› 0.080› 13.0
››› 0.234› 4.0
RADIUM-223 (223Ra)› 11.4 days›››› 5.75› 9.0
›››› 5.71› 54.0
›››› 5.61› 26.0
›››› 5.54› 9.0
››››› 0.149› 10.0 (complex)
›››› 0.270› 10.0
›››› 0.33› 6.0
RADON-219 (219Rn) 4.0 seconds›››› 6.82› 81.0
›››› 6.55› 11.0
›››› 6.42› 8.0
››››› 0.272› 9.0
›››› 0.401› 5.0
POLONIUM-215 (215Po) 1.77 x 10-3 seconds›››› 7.38› 99.99
››››› --› 2.3 x 10-4
LEAD-211 (211Pb) 36.1 minutes›››› 1.36› 92.0
›››› 0.95› 1.4
›››› 0.53› 6.0
››››› 0.405› 3.4
›››› 0.427› 1.8
›››› 0.832› 3.4
ASTATINE-215 (215At) ~10-4 seconds›››› 8.00› 100.0
BISMUTH-211 (211Bi) 2.15 minutes›››› 6.62› 84.0
›››› 6.28› 16.0
››››› --› 0.27
››››› 0.35› 14.0
POLONIUM-211 (211Po) 0.52 seconds›››› 7.45› 99.0
›››› 6.89› 0.5
››››› 0.57› 0.5
›››› 0.90› 0.5
Thallium-207 (207Tl) 4.79 minutes›››› 1.44› 100.0
››››› 0.870› 0.16
LEAD-207 (207Pb) Stable› --› -- --
2.1.3 Chemical Behavior
Although the radiological properties of uranium isotopes differ considerably, their chemical behavior is essentially identical. Chemical behavior is determined by oxidation state, which is defined as the difference between the number of protons in the atom (each with a +1 charge) and the number of electrons (each with a -1 charge).
For example, U+6 has six more protons than electrons. The oxidation state of an element is commonly written as a parenthetical roman numeral following the symbol for the element. Thus, U(IV) and U(VI) signify uranium in its +4 and +6 oxidation states. These two states and the zero oxidation state (U0) are the most common oxidation states for uranium. The uranium metal used in penetrators and armor is in the zero oxidation state, which is thermodynamically unstable even at low temperatures. When exposed to the environment, metallic uranium will eventually oxidize (corrode) to U(IV). In the presence of oxygen, this oxidation is shown by the following reaction, where U is elemental uranium and UO2 is U(IV):
U + O2 --> UO2
Depending on environmental conditions, further oxidation may form U(VI), shown here as UO22+ :
UO2 + 2 H+ + 1/2 O2 --> UO22+ + H2O
Oxidation of uranium metal liberates a large amount of heat. When the uranium surface to volume ratio is high, the heat of oxidation can cause the metal to burn, hence the pyrophoric nature of uranium.
In the absence of oxygen, uranium can be oxidized by water, releasing hydrogen gas, as shown by the following reaction:
U + 2 H2O --> UO2 + 2 H2
These reactions are analogous to corrosion reactions for iron. Iron metal left in the soil will corrode to ferrous iron, Fe(II), and to ferric iron (rust), Fe(III).
As with iron, microbial action can speed the corrosion of uranium. The corrosion rate is controlled by several variables, including the oxygen content, presence of water, size of the metal particles, presence of protective coatings and the salinity of the water present. With respect to DoD applications, the principal factor controlling corrosion is the size of the particles. Small particles of uranium metal, produced by abrasion and fragmentation, corrode rapidly because they have relatively large surface areas with respect to volume. Large masses of uranium metal (such as ingots), that are protected from the elements, corrode very slowly (Abbott et al., 1983). The important point is that eventually all uranium metal, U0, will oxidize to U(IV) and U(VI).
Note that uncommon uranium oxides will form at high temperatures and pressures. These oxides are thermodynamically unstable and will quickly convert to more familiar low-temperature U(IV) and U(VI) oxides. Elder and Tinkle (1980) provide additional information on the high-temperature oxidation of uranium metal.
2.2 Life Cycle of Uranium and DU
Uranium ores are mined, milled and converted into metals and ceramics for nuclear reactors and nuclear weapons, the major uses of uranium. Figure 2-3 depicts the uranium life cycle from the mining of naturally occurring uranium to the ultimate storage and disposal of uranium and depleted uranium end products.
Historically, nearly all U.S. uranium has come from mines in New Mexico, Colorado, Wyoming, Utah and Arizona. The Bureau of Land Management (BLM), the U.S. Forest Service (USFS) and the Department of Energy (DOE) are the primary regulators for exploring and uranium mining on federal public lands. Uranium mining is not subject to the licensing requirements administered by the Nuclear Regulatory Commission (NRC). DOE regulates uranium mining leases on public lands designated for its use and on certain other lands under its control (10 CFR 760 et seq). Individuals may obtain mining rights on public lands through unpatented claims, patented claims and leases.
All mining operations on federal public lands are subject to the Federal Land Policy and Management Act (FLPMA) of 1976 (Public Law [PL] 94-579), which requires mining practices that lessen miningŪs adverse impact on the environment (43 USC 1712c). Mining operators and owners also are subject to liability under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) and the Resource Conservation and Recovery Act (RCRA). The Environmental Protection Agency (EPA) regulates waste discharges at uranium mining operations (40 CFR 440.30 et seq).
Most U.S. uranium ore deposits are sandstone formations where uranium
minerals have chemically precipitated onto the sand grains. The most common
minerals are oxides such as uraninite, (UO2), and silicates
such as coffinite, (USiO4 (Langmuir, 1978). Typical ore concentrations
range from about 0.05 percent to 1 percent or more by mass. A uranium concentration
of 0.05 percent weight/weight (w/w), or 500 milligrams per kilogram (mg/kg),
corresponds to about 2 pounds of uranium in a cubic yard of ore. The price
of uranium and the mining and milling costs determine the cutoff concentration
of ore that is mined. Uranium producers extract uranium from both open
pit and underground mines, depending on the oreŪs depth.
Figure 2-3.› Uranium/Depleted Uranium Life Cycle
As with any mining, these procedures often pose an environmental threat. The DOE has initiated a number of major remediation efforts to clean up surface and groundwater contamination created by uranium mine tailings.
After producers extract uranium ore, mills crush it and leach the uranium from the ore with sulfuric acid (acid leach process) or soda ash (alkaline leach process). Milling extracts more than 95 percent of the uranium from ore. Dissolved uranium is recovered from solution as a precipitate called žyellowcakeÓ (U3O8) by either solvent extraction or ion exchange (Merrit, 1971). The uranium in U3O8 has the same mix of uranium isotopes as natural uranium. U3O8 can be used directly in žheavy waterÓ nuclear reactors or can be processed into UF6 for subsequent enrichment. Heavy water nuclear reactors are designed to use natural uranium. In the United States, these reactors are only used in producing fissionable material for nuclear weapons.
Portions of washed or milled ore regarded as too poor to be processed further, called mill tailings, are pumped as a slurry onto large unlined piles. Mill tailings typically contain about 75 percent of the radioactivity of the original ore. This radioactivity comes from uranium daughter products such as radium, thorium and radon. Radon-222 is a gas and, as such, presents the greatest radiological hazard associated with mill tailings. The use of a strong acid or a strong base in the milling process also dissolves many other species present in the ore. Consequently, radioactive mill tailings frequently also have high concentrations of other potentially toxic substances such as arsenic, selenium, iron, nitrate and sulfate (Thomson and Heggen, 1983).
NRC issues licenses for refining, processing and milling uranium (10 CFR 40.1; 42 USC 2092). Congress has also written specific laws to address uranium mill tailings sites (42 USC 7901 et seq), and the EPA administrator has issued health and environmental standards for uranium mill tailings (42 USC 2022). NRC requires remedial actions for mill tailings to ensure that radioactivity does not exceed specified levels. Remedial actions apply to the control of residual radioactive material at designated processing or depository sites and to the restoration of such sites following any use of subsurface minerals. Remedial action must be effective for at least 200 years. DOE is charged with implementing these requirements. NRC and states that pay part of the remediation cost must concur with the DOE implementation plan (40 CFR 192.00-192.30).
2.2.2 EnrichmentŪs Byproduct:› Depleted Uranium
Enrichment is an industrial process that increases the percentage of 235U in the isotopic mix from approximately 0.7 percent found in nature to a content ranging from 2 percent to more than 90 percent. Various nuclear power and nuclear weapons applications require enriched uranium with a high concentration of 235U by weight, as it is the only natural uranium isotope that can sustain the nuclear chain reaction the applications require. Enrichment increases the 235U and 234U concentration in UF6 through a complex process based on slight differences in the atomic mass of uranium isotopes. The enriched UF6 is then converted either to the uraninite UO2 required for nuclear reactor applications or to the uranium metal U0 required for nuclear weapons.
Enrichment also produces a byproduct: UF6 that is depleted
of 235U and 234U. This depleted DUF6 is
used to manufacture DU metal for military and civilian applications. DUF6
is processed to produce depleted uranium tetrafluoride, DUF4,
also called žgreen salt.Ó DoD contractors use green salt to produce depleted
uranium metal, which is often referred to as žderby.Ó This metal is
heat-treated and alloyed to improve its physical properties for use in kinetic energy penetrators and other weapon systems.
2.2.3 Characteristics of DU Used by DoD
NRC defines DU as uranium in which the weight percentage of the 235U isotope is less than 0.711. This is slightly less than the concentration of 235U in uranium ore, which is approximately 0.72 percent (10 CFR 40.4). Military Specification MIL-U-70457 stipulates that DU used by DoD must have a 235U concentration of less than 0.3 percentůless than half of the fissionable 235U allowed by the NRC definition of DU. DoD actually uses DU containing approximately 0.2 percent 235U (Vumbaco, 1993b; Price 1980). As an artifact of the enrichment process, 234U is removed from the natural isotopic mix by approximately the same percentage as 235U.
Although the chemical and physical properties of natural uranium and DU are essentially identical, their radiological properties differ, as shown in Table 2-4. Three points should be noted here:
* The weight percentages quoted for naturally occurring uranium vary
slightly from source to source.
** Reported values for the radioactivity (specific activity) of depleted uranium vary depending primarily on the weight percentages of 234U and 235U (10 CFR 20). While the exact ratio will vary, the radioactivity of depleted uranium will always be less than that of naturally occurring uranium.
Uranium, a radioactive element, is very dense in its metallic form. Uranium ore is mined, milled and refined for use in nuclear reactors and nuclear weapons. To facilitate its use in reactors and weapons, uranium is enriched, a process that increases the weight percentage of the 235U isotope. The enrichment process also produces a byproduct, uranium depleted of 235U. Depleted uranium cannot sustain a nuclear reaction or be used as the fuel for nuclear weapons, but its high density› and metallurgical properties make it useful in kinetic energy weapons and armor systems. Depleted uranium is roughly 60 percent as radioactive as naturally occurring uranium. The NRC definition of DU is uranium with less than 0.711 weight percentage of the 235U isotope. DoD specifications require DU with a 235U concentration of less than 0.3 percent. Thus, the DU used in DoD weapon contains less than half of the 235U allowed by NRC.