Plutonium diffusivity in compacted bentonite

Measurements on plutonium diffusivity in water-saturated compacted bentonite were carried out. Representative specimens of sodium bentonite were taken from the 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 pellets of 20 mm in diameter and 20 mm in height. Bulk densities of these specimens were 1200-1800 kg/m³ for purified Na-type and H-type bentonite and 1600 kg/m³ 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^-13 to 10 ¹² m²/s for H-type and Kuroishi impure sodium bentonite. Diffusivity in both raw type and purified Tsukinuno bentonite was was estimated to less than 10^-14 M²/s. Diffusivity in H-type bentonite showed a tendency to decrease with increasing density. Influence of 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)₃^-; and PuO₂^-, respectively.