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

Abstract

Diffusive transport of neptunium, plutonium, tritiated water (HTO), 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 is stable as NpIV(CO3)2(OH)22-and those for Pu in the presence of fulvic acid where the Pu is stable as fulvic complexes. These experiments were performed under Ar (pO2< 10-6atm). Effective diffusivity (De) values of (1.81 ± 0.03) × 10-10, (1.8 ± 0.8) × 10-10, (5.1 ± 0.8) × 10-11and (9.0 ± 4.1) × 10-11m2s-1were obtained for HTO, Cs+, I-and Np(CO3)2(OH)22-, respectively. The ratio of theDeto the diffusivity in bulk of the water was around 0.1 for Np(CO3)2(OH)22-, 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(CO3)2(OH)22-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.