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Diffusive transport of neptunium and plutonium through compacted sand-bentonite mixtures under anaerobic conditions

低酸素濃度条件下におけるネプツニウム及びプルトニウムの砂混合ベントナイト圧縮体透過拡散

山口 徹治; 中山 真一; 長尾 誠也*; 木崎 實

Yamaguchi, Tetsuji; Nakayama, Shinichi; Nagao, Seiya*; Kizaki, Minoru

砂混合ベントナイト圧縮体中におけるネプツニウム,プルトニウム,トリチウム水,Cs$$^{+}$$,I$$^{-}$$の拡散を透過法で調べた。Npの実験はNpがNp$$^{IV}$$(CO$$_{3}$$)$$_{2}$$(OH)$$_{2}$$$$^{2-}$$として安定な炭酸イオン共存系で行った。Puの実験はPuがフルボ酸錯体として安定なフルボ酸共存系で実施した。これらの実験はアルゴン雰囲気(${it p}$ $$_{O2}$$ $$<$$ 10$$^{-6}$$ atm)において行った。HTO, Cs$$^{+}$$, I$$^{-}$$, Np$$^{IV}$$(CO$$_{3}$$)$$_{2}$$(OH)$$_{2}$$$$^{2-}$$の有効拡散係数はそれぞれ、(1.81$$pm$$0.03)$$times$$10$$^{-10}$$, (1.8$$pm$$0.8)$$times$$10$$^{-10}$$, (5.1$$pm$$0.8)$$times$$10$$^{-11}$$及び(9.0$$pm$$4.1)$$times$$10$$^{-11}$$ m$$^{2}$$ s$$^{-1}$$であった。Deとバルク水中における拡散係数との比はNp$$^{IV}$$(CO$$_{3}$$)$$_{2}$$(OH)$$_{2}$$$$^{2-}$$, HTO及びCs$$^{+}$$については約0.1であり、これは細孔拡散モデルと整合している。拡散によるPuの移行は、HTO, Cs$$^{+}$$, I$$^{-}$$及びNp$$^{IV}$$(CO$$_{3}$$)$$_{2}$$(OH)$$_{2}$$$$^{2-}$$に比べてはるかに小さく、これはPuがコロイドの形態で存在し、砂混合ベントナイト圧縮体内の狭隘な間隙内ではコロイド形態のPuは移動できないためと考えられる。

Diffusive transport of neptunium, plutonium, tritiated water, Cs$$^{+}$$ and I$$^{-}$$ in compacted sand-bentonite mixtures was studied by a through diffusion method. Experiments for Np were performed in the presence of carbonate where Np was stable as Np$$^{IV}$$(CO$$_{3}$$)$$_{2}$$(OH)$$_{2}$$$$^{2-}$$ and those for Pu in the presence of fulvic acid where the Pu was stable as fulvic complexes. These experiments were performed under Ar (${it p}$ $$_{O2}$$ $$<$$ 10$$^{-6}$$ atm). Effective diffusivity (De) values of (1.81$$pm$$0.03)$$times$$10$$^{-10}$$, (1.8$$pm$$0.8)$$times$$10$$^{-10}$$, (5.1$$pm$$0.8)$$times$$10$$^{-11}$$ and (9.0$$pm$$4.1)$$times$$10$$^{-11}$$ m$$^{2}$$ s$$^{-1}$$ were obtained for HTO, Cs$$^{+}$$, I$$^{-}$$ and Np$$^{IV}$$(CO$$_{3}$$)$$_{2}$$(OH)$$_{2}$$$$^{2-}$$, respectively. The ratio of the De to the diffusivity in bulk of the water was around 0.1 for Np$$^{IV}$$(CO$$_{3}$$)$$_{2}$$(OH)$$_{2}$$$$^{2-}$$, HTO and Cs$$^{+}$$, which is consistent with the pore diffusion model. Observed diffusive transport of Pu was much smaller than those of HTO, Cs$$^{+}$$, I$$^{-}$$ and Np$$^{IV}$$(CO$$_{3}$$)$$_{2}$$(OH)$$_{2}$$$$^{2-}$$ probably because Pu was present as colloidal forms and that confined pore space in the compacted sand-bentonite mixtures does not allow diffusive transport of colloidal plutonium.

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パーセンタイル:53.53

分野:Chemistry, Inorganic & Nuclear

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