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

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

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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Category:Chemistry, Inorganic & Nuclear

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