TAB C - Properties and Characteristics of DU
Natural uranium (extracted from uranium ore) is processed to form enriched uranium for nuclear power. Depleted uranium (DU) is the by-product of this uranium enrichment process. Natural uranium is composed of three isotopes, uranium-238 (238U), uranium-235 (235U) and uranium-234 (234U). Although the exact percentages vary slightly, natural uranium typically is composed of approximately 99.28% 238U, 0.71% 235U, and 0.0055% 234U (See Figure 10). Isotopes of an element have essentially the same chemical and physical properties because they have the same number of protons (92) in their atoms. They differ only in the number of neutrons per atom. For example, 234U, 235U, and 238U have 142, 143, 146 neutrons in each atom, respectively. It is this variation in the number of neutrons that gives the different isotopes their radiological properties. Isotopes differ in the types of radiation emitted during the nuclear decay process, decay rate, interactions with nuclear particles, and ability to undergo nuclear fission.
The relative radioactivity of isotopes is measured by their specific activity, which is defined as the number of transformations or disintegrations per second per unit of mass. The unit of measurement of specific activity is microcuries per gram with a microcurie equal to 3.700 x 104 disintegrations per second. Although by weight 234U is only 0.005% of the natural uranium, it accounts for 48.9% of the radioactivity of uranium. 235U and 238U account for the remaining 2.3% and 48.8% of the radioactivity of uranium, respectively.
Figure 10. Content by Mass of Uranium Forms
To be used as nuclear fuel or weapons grade uranium, natural uranium must be enriched through a process that increases the 235U content to approximately 3% for power reactor fuel, or over 90% for weapons grade uranium. This decreases the 238U content to 97% or less than 10%, respectively, leaving "depleted uranium" with approximately 0.2% 235U and 99.8% 238U. 234U is generally ignored because it is present in such small quantities. In the gaseous diffusion process a gaseous compound of uranium and fluorine, UF6, is separated into two fractions one enriched in 235U and one depleted in 235U. The depleted fraction is then chemically transformed into a uranium metal derby. This is the first stage at which the depleted material is in the state necessary for further processing by ammunition manufacturers.
The Nuclear Regulatory Commission (NRC) defines "depleted uranium" as uranium in which the weight percentage of the 235U isotope is less than 0.711%. Military specifications mandated by the Department of Defense (DoD) require that the percentage of 235U be less than 0.3%. In actuality, DoD uses DU with a 235U content of approximately 0.2%. DU is 40% less radioactive than the raw uranium-bearing ores found in nature; but its material content is still uranium. All isotopes of uranium are essentially identical chemically and, since depleted and natural uranium are just different mixtures of the same three isotopes, they have the same chemical properties.
All isotopes of uranium are radioactive. Each has its own unique decay process emitting some form of ionizing radiation: alpha, beta or gamma radiation (or a combination). Alpha and beta radiations are actually discrete particles, whereas gamma radiation is essentially a photon of energy similar to an x-ray but from the nucleus. An alpha particle consists of two protons and two neutrons and is positively charged (+2). Most alpha particles are not energetic enough to penetrate skin and are not considered to be an external hazard. Alpha particles, however, can be a health hazard if inhaled or ingested in sufficient quantities. A beta particle is an electron (charge -1) emitted during the radioactive decay of an atom and is more penetrating than an alpha particle. Beta particles are able to penetrate skin a few millimeters and can pose both an internal and external health risk. Since a gamma ray is a photon of energy with no mass and no charge, it is extremely penetrating, and can be both an internal and external health hazard.
238Uwhich by weight makes up almost 99.8% of DUis an alpha emitter. 238U has a half-life of 4.5 x 109 years. 238U decays into two short-lived "daughters:" thorium-234 (234Th, half-life of 24.1 days) and protactinium-234m (234mPa, half-life of 1.17 minutes)which are beta and weak gamma emitters. Because of this constant nuclear decay process, very small amounts of these "daughters" are always present in DU. 235U (half-life of 7.0 x 108 years) decays into protactinium-231 (231Pa, half-life of 3.25 x 104 years), which is an alpha, beta, and gamma ray emitter. The 238U and 235U chains continue through a series of long-lived isotopes before terminating in stable, non-radioactive lead isotopes 206Pb and 207 Pb, respectively.
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