SORPTION BEHAVIOR OF URANIUM ONTO YUCCA MOUNTAIN ALLUVIUM

Yucca Mountain, Nevada is currently being studied as the proposed geological repository for disposal of high-level nuclear wastes in the United States. In the event of a failure of engineered barriers in the repository, the saturated alluvium down gradient of Yucca Mountain is expected to serve as a natural barrier to the migration of radionuclides to the accessible environment. Therefore, it is necessary to study radionuclide sorption characteristics of the saturated alluvium. In this study, batch sorption experiments were carried out to determine the apparent distribution coefficients, K_d (ml/g) of ²³³U(VI) (half-life=1.592*10⁵ years) onto alluvium materials from three different wells located south of Yucca Mountain at varying depth intervals. We used three different particle-size fractions of the alluvium (500-2000, 75-500 and <75 micrometers) and tracer solutions that were prepared by dilution of ²³³U(VI) hydrochloric acid stock solution with the corresponding well groundwater. The resulting solutions were analyzed for ²³³U(VI) before and after contact with the alluvium by Liquid Scintillation Counting. Our results showed that ²³³U(VI) K_d values varied from 2 to 23 ml/g, with a tendency toward smaller K_d values at greater depths below land surface and larger K_d values for smaller alluvium particle size fractions (<75 micrometers). The latter result is not surprising given that the smallest particles have the largest surface area as well as the largest weight percentage of clays and zeolites, with their characteristically high adsorption capacities. The desorption behavior of ²³³U(VI) from alluvium and the dependence of both ²³³U(VI) sorption and desorption on alluvium mineralogy are currently being investigated.