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Effect of sodium nitrate on the diffusion of Cl$$^{-}$$ and I$$^{-}$$ in compacted bentonite

圧縮ベントナイト中のCl$$^{-}$$, I$$^{-}$$の拡散挙動に及ぼすNaNO$$_{3}$$の影響

石寺 孝充; 宮本 真哉*; 佐藤 治夫

Ishidera, Takamitsu; Miyamoto, Shinya*; Sato, Haruo

TRU廃棄物処分の安全評価においては、$$^{14}$$C, $$^{36}$$Cl及び$$^{129}$$Iといった核種のデータ取得が必要であるとともに、硝酸塩の溶出やセメント材料の溶解による地下水の高pH化の影響を評価する必要がある。よって、ケイ砂混合率及びNaNO$$_{3}$$濃度をパラメータとして、高pH条件でCO$$_{3}$$$$^{2-}$$, Cl$$^{-}$$及びI$$^{-}$$のベントナイト中の透過拡散試験を実施し、実効拡散係数(以下、$$D$$$$_{e}$$の取得を行った。その結果、CO$$_{3}$$$$^{2-}$$, Cl$$^{-}$$、及びI$$^{-}$$$$D$$$$_{e}$$は10$$^{-12}$$$$sim$$10$$^{-11}$$m$$^{2}$$/s程度の値が得られた。これらのイオンの$$D$$$$_{e}$$は、NaNO$$_{3}$$濃度の増加に伴ってそれぞれ増大する傾向が見られ、これは、NaNO$$_{3}$$濃度の増加に伴う陰イオン排除効果の低下によるものであると考えられた。また、CO$$_{3}$$$$^{2-}$$はクニゲルV1中に含まれる方解石との化学平衡により、クニゲルV1中に保持されると考えられた。

For safety assessment of TRU waste disposal, the effective diffusion coefficients ($$D$$$$_{e}$$) for CO$$_{3}$$$$^{2-}$$, Cl$$^{-}$$ and I$$^{-}$$ ions in compacted bentonite (Kunigel V1) were determined as a function of NaNO$$_{3}$$ concentrations from 0.01 to 5 mol/dm$$^{3}$$. The $$D$$$$_{e}$$ values increased from 10$$^{-12}$$ to 10$$^{-11}$$ m$$^{2}$$/s with increasing NaNO$$_{3}$$ concentration. The rock capacity factor $$alpha$$, indicative of the effective porosity, were also increased with increasing NaNO$$_{3}$$ concentration. The maximum $$alpha$$ values of 0.21 for Cl$$^{-}$$ ion and of 0.25 for I$$^{-}$$ ion at 5 mol/dm$$^{3}$$ NaNO$$_{3}$$ concentration were lower than the net porosity of compacted bentonite (0.40). Therefore, the increases in the $$D$$$$_{e}$$ values were interpreted by the decrease of anion exclusion effect. The $$D$$$$_{e}$$ values for Cl$$^{-}$$ and I$$^{-}$$ ions normalized by the diffusivities for diffusants in the bulk water were found to increase in proportion to the $$alpha$$ values on the log-log diagram. The maximum $$alpha$$ value for CO$$_{3}$$$$^{2-}$$ ion was higher than the net porosity. The high concentration of CO$$_{3}$$$$^{2-}$$ ion was also found in the concentration profile in compacted bentonite. This retention of CO$$_{3}$$$$^{2-}$$ ion was possibly accounted for by isotopic exchange of $$^{14}$$C with inactive carbon of calcite in Kunigel V1.

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パーセンタイル:42.18

分野:Nuclear Science & Technology

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