Uranium

The three natural radioactive decay chains beginning with ²³⁸U, ²³⁵U, and ²³²Th each have comparable half-lives that are much longer than the radioactive isotopes that follow until the production of stable isotopes of ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb, respectively. When undisturbed, the activities of daughter isotopes in each decay chain are equal to their parents and one can measure the accumulation of the stable isotopes of lead to date the time that has elapsed since a mineral became a closed system (a system that does not exchange matter with its surroundings). Rocks formed hundreds of millions to billions of years ago can be dated using this technique [591]. If a mineral is disturbed at some point during the decay and isotopes in the decay chain are preferentially removed from the system, the equilibria in a decay sequence will be disturbed. For example, one can measure the excess of ²³⁰Th (with a half-life of 7.56×10⁴ years) relative to the ²³⁴U parent radionuclide to date carbonates (speleothems or corals) that are less than 5×10⁵ years old [591].

Nuclei of ²³⁵U are split when bombarded by thermal neutrons. The process is known as nuclear fission and can release tremendous amounts of energy per uranium nucleus. The nucleus that splits will release additional neutrons that, if slowed down sufficiently, can cause subsequent fission events. When properly controlled, ²³⁵U fission can be used to generate heat to drive steam turbines, which in turn produces electricity (Fig. IUPAC.92.1). If the fission process is not controlled, then a rapid and explosive release of energy will occur, similar to that of nuclear weapons [599]. Uranium depleted in ²³⁵U by fission in nuclear reactors (and hence greatly enriched in ²³⁸U compared to “natural” uranium) is used in the manufacture of DUCRETE concrete (Fig. IUPAC.92.2). The incorporation of the large ²³⁸U nuclei makes this material an effective absorber of neutrons and gamma rays, and DUCRETE concrete is used to reduce fluxes of neutrons and high-energy photons. The alpha particles produced by the decay of ²³⁸U are effectively absorbed by the concrete and do not pose a health risk. DUCRETE is being proposed as a suitable material for the storage of radioactive waste [600], [601].