TABLE 1-1 Natural Uranium Concentrations


Uranium Concentration



3.2 ppb

Faure and Mensing 2005

Earth’s crust

2.8 ppm

Eisenbud and Gesell 1997

Bulk earth

0.01 ppm

Faure and Mensing 2005

Human blood

0.074-0.94 ppm

Hamilton 1970

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


Uranium fission produces two atoms with unequal atomic mass known as fission products. Most fission products have relatively short half-lives, but a few have half-lives long enough for them to be present in measurable quantities for many years after the initial reaction.


Uranium fission produces neutrons that can be absorbed by 238U or other decay products; this absorption leads to the production of elements that have atomic numbers greater than that of uranium. Those elements are known as transuranic elements and share some chemical similarity to uranium.

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