MIGRATORY PROPERTIES OF SOME NUCLEAR WASTE ELEMENTS IN GEOLOGIC MEDIA

Nuclear waste can be disposed of in geologic repositories. To aid in assessing the suitability of geologic disposal, the authors have examined the interactions of trace quantities of cesium, plutonium, neptunium, and americium in aqueous solutions with rocks from formations that may be suitable for waste repositories. The results indicate that many geologic formations are barriers to the movement of these elements in flowing water. However, reactions that retard element migration are varied and do not lend themselves to simplified descriptions. In experiments with plutonium and americium, kinetics of reactions were seen to differ for each trace element and rock studies. In rock infiltration experiments with radioactive cesium, plutonium, neptunium, and americium, often most of the activity moved slowly compared to the water stream, but small quantities of the trace elements moved downstream from the main peaks of activity because of the slow reaction rates seen in static experiments, or possibly because of multiple speciation, colloid formation, movement of particles with adsorbed nuclides, or other causes. These fast-moving components of the trace elements may present a radiological hazard from a breached repository, even though they contain only a small fraction of the activity leaving the repository; therefore, detailed characteristics of nuclide migration need to be considered in the design of a nuclear waste repository.