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(CO)(OH) and those for Pu in the presence of fulvic acid where the Pu was stable as fulvic complexes. These experiments were performed under Ar ( 10 atm). Effective diffusivity (De) values of (1.810.03)10, (1.80.8)10, (5.10.8)10 and (9.04.1)10 m s were obtained for HTO, Cs, I and Np(CO)(OH), respectively. The ratio of the De to the diffusivity in bulk of the water was around 0.1 for Np(CO)(OH), 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(CO)(OH) 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.