Plutonium Diffusivity in Compacted Bentonite

None

出光 一哉*; 石黒 勝彦; 湯佐 泰久; 佐々木 憲明; 角田 直己

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

本論文は,1988年10月57日にスウェーデンのルンドで開催された「高レベル廃棄物処分場のための粘土系人工バリア」の会議(SKB***主催)で発表したものである。 目的水で飽和した圧縮ベントナイト中のプルトニウムの拡散係数を測定する。 方法直径20mm,高さ20mmの円柱試験中にプルトニウムを約1カ月間拡散させ,その濃度分布から拡散係数を算出した。用いた材料は,山形県月布産,青森県黒石産のベントナイトである。月布産ベントナイトは更に1)無処理,2)精製物(Na-型),3)塩酸処理物(H-型)に分けられた。H-型はNa-型に比べpH・膨潤圧が低く拡散係数が大きくなると予想され,試料中の濃度分布を測定するのに都合が良いので標準試料として用いた。試料のバルク密度は12001800kg/M3である。 結果試料中のプルトニウム濃度分布は一定濃度源からの拡散で説明できた。得られた拡散係数は10-1310-12m2/s(H-型,黒石産),10-14m2/s(Na-型,月布産)であった。H-型ベントナイト中の拡散係数は,バルク密度の上昇と共に減少する傾向があるが,石英砂(50%まで)とヘマタイト(1%まで)の混合による影響は認められなかった。

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.