Effect of pH on Plutonium Migration Behavior in Compacted Bentonite

Abstract

In disposing of high-level radioactive waste, the drop in pH in the repository as the iron overpack corrodes must be considered. Plutonium migration behavior may be affected by the pH of pore water in compacted bentonite barriers in high-level waste repositories. To examine the effect of pH on migration behavior, H-bentonite was prepared by treating Japanese Na-bentonite, Kunipia-F, with hydrochloric acid. Diffusion experiments were performed with mixtures of Na- and H-bentonites. The pH value in the pore water of the water-saturated bentonite mixtures decreased from 8 to 3 as the mixing ratio of H-bentonite increased. Diffusion experiments were carried out by using ²³⁸Pu then apparent diffusion coefficients were determined from the plutonium distribution in the specimens. The apparent diffusion coefficients were on the order of 10^-13 to 10^-12 m²/s at pH values lower than 4, whereas they were less than 10^-14 m²/s at pH values higher than 6.5. These results indicate that plutonium diffused faster as Pu³⁺ or PuO₂²⁺ due to disproportionation at lower pH while plutonium could be retarded as Pu(OH)₄⁰ by sorption on bentonite at higher pH.