Plutonium Diffusivity in Compacted Bentonite

Idemitsu, Kazuya*; Ishiguro, Katsuhiko; Yusa, Yasuhisa; Sasaki, Noriaki; Tsunoda, Naomi

Measurement on plutonium diffusivity in water-saturated compacted bentonite was carried out. Representative specimens of sodium bentonite were taken from Tsukinuno and Kuroishi mines situated in northeast Japan. Tsukinuno bentonite was divided into three types; raw type, purified Na-type, and H-type which was prepared by treating Na-type bentonite with hydrochloric acid. Kuroishi bentonite contained chlorite as impurity. H-type bentonite was used as reference for the convenience of profile measurement in bentonite, since plutonium diffusivity in H-type bentonite was considered to be larger than that in Na-type bentonite because of low pH and low swelling pressure of H-type bentonite. Sampled bentonite was compacted into pellet of 20 mm in diameter and 20 mm in height. Bulk densities of these specimens were 1200 to 1800 kg/m for purified Na-type and H-type bentonite and 1600 kg/m for raw type bentonite. Plutonium profiles obtained in H-type bentonite can be explained by diffusion equation with constant concentration source. Diffusivity ranges from 10 to 10 m/s for H-type and Kuroishi impure sodium bentonite. Diffusivity in both raw type and purified Tsukinuno bentonite was estimated to be less than 10 m/s. Diffusivity in H-type bentonite showed a tendency to decrease with increasing density. Influence of quartz and hematite contents in bentonite was also studied. Quartz content up to 50% or hematite content up to 1% did not influence diffusivity significantly in H-type bentonite. The chemical species of plutonium in pore water of Na-type and H-type were estimated Pu(OH) and PuO respectively.