The isotopes of uranium have the same chemical properties because they all have the same number of protons, 92. However, variation in the number of neutrons gives the isotopes different radiological properties. The radioactivity of isotopes can be compared using specific activity, a measurement of the number of nuclear transformations (disintegrations) per second per unit mass (Box 4.1). The most abundant naturally occurring uranium isotope, ²³⁸U, has the lowest specific activity (0.33 microcuries per gram [μCi/g]) (U.S. AEPI, 1995). The high specific activity of ²³⁴U (6,200 μCi/g) contributes to more than half of the radioactivity of natural uranium even though by weight its percentage is extremely small (Table 4.1). Enriched uranium is quantified by its percentage of ²³⁵U (specific activity 2.2 μCi/g) which can range from 2 percent to more than 90 percent (U.S. AEPI, 1995). Because of the high percentage of ²³⁸U in natural uranium (Table 4.1) and that isotope’s low specific activity, natural uranium is considered a low-level radioactive element.

Uranium is also categorized as a heavy metal (i.e., any metal with a specific gravity of 5.0 or greater). The chemical toxicity of a uranium compound varies depending on the nature of the compound, its solubility, and its route of exposure.

There are a number of radiological protection regulations and guidelines. The U.S. Nuclear Regulatory Commission’s (U.S. NRC’s) regulations for occupational dose to individual adults state an annual limit of the total effective dose equivalent of 5 rem per year (50 millisieverts [mSv] per year) (10 CFR 20). For members of the general public, the U.S. NRC’s regulations require that the total effective dose equivalent to individual members of the general public not exceed 0.1 rem in a year (1 mSv), exclusive of the dose contributions of background radiation (10 CFR 20).

Depleted uranium (DU) is a by-product of the enrichment process used to make reactor-grade uranium. Because of the different percentages of uranium isotopes in depleted uranium (Table 4.1), its specific activity (14.8 mBq/μg) is