similarly in the body and the environment, and decay to stable lead isotopes through a series of radioactive decays. Table 1-2 lists the progeny, decay modes, half-lives, and other relevant characteristics of the two main isotopes, ²³⁵U and ²³⁸U. The half-life of ²³⁸U is much longer than that of ²³⁴Th, and relatively few atoms of ²³⁸U will decay on a human timescale, so ²³⁸U acts effectively as an inexhaustible source of ²³⁴Th. Eventually, an equilibrium at which the rate of production of ²³⁴Th will equal its decay rate will be reached (that is, secular equilibrium will be reached). That situation can be generalized to the entire decay chain, but reaching such an equilibrium takes a few million years.

Of the two main uranium isotopes, only the less abundant ²³⁵U is fissile, that is, capable of supporting a self-sustained chain reaction under particular circumstances. Natural uranium consists largely of ²³⁸U, and it must be processed to increase the percentage of ²³⁵U. The process is called uranium enrichment, and it has two end products: enriched uranium, in which the ²³⁵U concentration has been increased; and depleted uranium, in which the ²³⁵U concentration is lower than 0.72%.

Spent reactor fuel can be reprocessed and reintroduced into the uranium-enrichment process. Contaminants that remain in the uranium after chemical processing—such as fission products,¹ transuranic elements,² and other trace contaminants—can therefore enter the enrichment process. Minor amounts of fission products and transuranic elements were introduced into the uranium-enrichment system in the 1960s and 1970s when the United States reprocessed