TAB C Properties and Characteristics of DU
Natural uranium (extracted from uranium ore) is processed to form enriched uranium for nuclear power or nuclear weapons. 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) in the following proportions: 99.28 percent 238U, 0.71 percent 235U, and 0.0058 percent 234U by mass (See Figure C-1). Isotopes of an element have essentially the same chemical and physical properties because they have the same number of protons (92 for uranium) 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. Uranium isotopes differ in the types of radiation emitted during the nuclear decay process, in their decay rate, in their interactions with nuclear particles, and in their ability to undergo nuclear fission.
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 discrete particles, whereas gamma radiation consists of a photon of energy similar to a x-ray, but emitted 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 an external hazard. Alpha particles, however, can be a health hazard if alpha-emitting radionuclides are 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 a few millimeters of skin 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 very penetrating, and can be both an internal and external health hazard.
238U -- which by weight makes up almost 99.8 percent of DU -- is an alpha emitter. The radiological half-life of 238U or the time required for 238U to lose 50 percent of its activity by decay is 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 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 thorium-231 (231Th, half-life 25.5 hours), which decays into protactinium-231 (231Pa, half-life of 3.25 x 104 years) and then to actinium with a half-life of 21.8 years, resulting in alpha and beta particles and gamma rays. The 238U and 235U chains continue through a series of isotopes before terminating in stable, non-radioactive lead isotopes 206Pb and 207-Pb, respectively.
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 for specific activity is microcuries per gram, with a microcurie equal to the mass of material in which 3.7 x 104 atoms disintegrate per second. The isotopes 234U, 235U and 238U are all alpha particle emitting radionuclides. Although 234U is only 0.0058 percent by weight of the natural uranium, it accounts for 48.9 percent of the radioactivity of uranium because of its relatively high specific activity (6200 �Ci/g). 235U and 238U account for the remaining 2.3 percent and 48.8 percent of the radioactivity of uranium, respectively. The specific activity of natural uranium is about 0.67 �Ci/g.
To be used as nuclear fuel or weapons grade uranium, natural uranium is enriched through the gaseous diffusion process to increase the 235U content to 3 to 90 percent; reducing the 238U content to 10 to 97 percent. "Depleted uranium," the byproduct of the enrichment process, has about 0.002 percent 234U, 0.2 percent 235U and 99.8 percent 238U, and about 60 percent of natural uranium's radioactivity. In the gaseous diffusion process, uranium hexafluoride (UF6), a gaseous compound of uranium and fluorine, 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 stock. This is the first stage at which the depleted material is in the state necessary for further processing by ammunition manufacturers.
Figure C-1. Isotopic weight percentages in uranium forms
The Nuclear Regulatory Commission (NRC) defines "depleted uranium" as uranium in which the percentage of the 235U isotope's weight is less than 0.711 percent. Military specifications mandated by the Department of Defense (DoD) require that the percentage of 235U be less than 0.3 percent, but in actual practice, DoD uses DU with a 235U content of approximately 0.2 percent. DU is 40 percent less radioactive than the uranium in the raw uranium-bearing ores found in nature; but its material content is still uranium. The specific activity of DU is, therefore, about 0.4 �Ci/g. 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.
The Department of Energy has recently reported that the DU used by DOD in its armor plates (found only on "Heavy Armor" Abrams tank models) may contain trace levels of transuranics (neptunium, plutonium, and americium) and fission products (technetium-99). The DU used in munitions may also contain these materials. The military services are testing the stocks of DU munitions and parts. The levels of transuranics and fission products found during testing of the material used for producing armor packages are in minute quantities (the picocurie/gram range) and result in less than a one percent increase in the internal radiation dose. These evaluations indicate that measures designed to protect personnel from the DU itself are adequate to protect them from the traces of transuranics and fission products as well.
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