Uranium Isotopes

Natural uranium consists of three isotopes: U-238, U-235 and U-234, with abundancies of approximately 99.275, 0.72 and 0.054 percent respectively. In fact, however, there is a range of natural variations for each isotope, U234 varies from 0.0059 - 0.0050, U235 from 0.7202 - 0.7198, and U238 99.2752 - 99.2739. Uranium occurs as a significant constituent in more than 150 different minerals and as a minor component of another 50 minerals. Enriched uranium, as used as a fuel in nuclear reactors, has more than 2 percent of U-235 and a higher than the natural content of U-234. Depleted uranium has less than the natural contents of U-235 and U-234. In the Gulf War depleted uranium with 0.2 percent of U-235 and 0.0011 percent of U-234 was used.

All three isotopes are alpha radioactive, as follows.

• U-238 decays by alpha emission (half-life 4.47E9 years, energy 4.196 MeV) into thorium Th-234. Th-234 decays by beta emission (half-life 24.1 days, energy 0.198 MeV) into protactinium Pa-234. Pa-234 decays by beta emission (half-life 1.75 min, energy 2.229 MeV) into U-234 -- the U234 isotope is a disintegration product of the parent U238.
• U-234 decays by alpha emission (half-life 2.446E5 years, energy 4.777 MeV) into thorium Th-230. The decay half time of U-234 is much less than that of U-238, giving an activity 18,275 times that of U-238.
• U-235 decays by alpha emission (half-life 7.038E8 years, energy 4.598 MeV) into thorium Th-231. Th-231 decays by beta emission (half-life 25.52 hours) into protactinium Pa-231.

The uranium decay-series contains several radioactive isotopes. The varied geochemical properties of these isotoples cause nuclides within the chain to be fractionated in different geological environments. There are three naturally occurring radioactive decay chains; each starts with an actinide nuclide (U238, U235 and Th232) having a long half-life (t1/2 > 0.7 Gyr) and ends with a stable isotope of lead. In between is a series of nuclides with half-lives ranging from microseconds to hundreds of thousands of years.

U-series disequilibrium refers to any fractionation between different members within a decay chain resulting in a non-steady state condition (steady state is known as secular equilibrium). Fractionation of the nuclides away from secular equilibrium is generally due to differences in the solubility of the various nuclides.

Each element has distinct chemical properties and thus, the U-series nuclides can become fractionated during processes that discriminate chemical behavior: phase change, partial melting, crystallization, partitioning, dissolution, adsorption, degassing, oxidation/reduction, complexation. This process is generally called fractionation but for the U-series nuclides, the result is transitory disequilibrium. Fractionation can also take place as a result of radioactive decay, especially in the low-temperature environment, and these effects are generally described as recoil effects.

The energy involved in the Alpha-decay of some nuclides within the decay chain leads to the largely physical fractionation process of Alpha-recoil. As the a particle is ejected, the daughter nuclide recoils in the opposite direction and moves a distance of »550 angstroms in a typical mineral. This recoil causes a fraction of the daughter nuclide produced during a decay to be ejected from the host mineral into the surrounding medium. An additional fraction of the daughter is left residing in damaged crystallographic sites within the mineral, from where it can be more readily mobilized. a-recoil therefore gives daughter nuclides of a-decay a tendency to leave their host mineral by a process which is independent of their chemistry. Alpha-recoil is most important in preferentially releasing U234 from minerals over U238, but also plays a role in mobilizing other nuclides.

The natural uranium comprises the isotopes U238, U234 and U235, that have the same geochemical behavior and whole relative proportions, under radioactive equilibrium conditions. U234 is preferentially mobilized in relation to U238 in the course of rock weathering. The preferential release of U234 to the liquid phase is sustained during water-rock/soil interactions. To explain the selective U-234 loss it is postulated that bulk U is in reduced +4 form and a considerable part of the U-234 isotope in easily leachable oxidised +6 form. U-234 in particulates or sediments may be leached out preferentially over U-238 due to recoil, causing an excess in U-234 over U-238 activity.