Diffusion of radioisotopes in Fe-montmorillonite relevant to geological disposal of HLW

Abstract

Bentonite clay is proposed as backfill or buffer materials in the engineered barrier system for high-level nuclear waste (HLW) disposal in a deep geological repository (DGR). The major clay mineral of bentonite, montmorillonite (Mt), is expected to undergo alteration owing to its interaction with corrosion products from overpack/canister over a long period of time. As a typical and simple altered clay mineral, we used Fe(II)−Mt and Fe(III)−Mt for preliminary investigation on the diffusion behavior of various radioisotopes relevant to HLW. Instantaneous planar source method was used to determine the apparent diffusion coefficient (Da) using compacted (ρd = 1.6 g/cm3) and water saturated Fe−Mt. In Fe(III)−Mt, the Da values in m2/s for 137Cs+ = (3.9 ± 0.2) × 10−12, 133Ba2+ = (1.1 ± 0.6) × 10−12, 152+154Eu3+ = (2.2 ± 0.5) × 10−12 and 131I− = (5.5 ± 0.1) × 10−12. In Fe(II)−Mt, the Da values in m2/s for 22Na+ = (2.0 ± 1.2) × 10−11, 137Cs+ = (3.0 ± 0.4) × 10−12, 133Ba2+ = (1.4 ± 0.4) × 10−11, 152+154Eu3+ = (4.0 ± 0.7) × 10−12 and 131I− = (4.8 ± 1.0) × 10−12. In general, similar Da values are obtained for Fe(II)−Mt and Fe(III)−Mt. This is the first extensive study to report the diffusion behavior of radioisotopes in Fe−Mt.