Effect of dry density on 125I diffusion in GMZ bentonite

Abstract

Gaomiaozi (GMZ) bentonite is regarded as the favorable candidate backfilling material for a potential repository in China. It is important to understand the diffusion behavior of 125I in GMZ bentonite and compare the diffusion behavior in GMZ and other types of bentonite like MX-80, Avonlea, etc. Therefore, through- and out-diffusion experiments were conducted to obtain the effective diffusion coefficient (De) and distribution coefficient (Kd). A computer code named Fitting for diffusion coefficient (FDP) was used for the experimental data processing and theoretical modeling. At the dry density of GMZ bentonite from 1600–2000 kg/m3, the De values of 125I were (2.4–20.4) ¢ 10−12 m2/s and Kd values were constants. At dry density above 1800 kg/m3, the diffusion behaviors were almost the same, indicating that the anion exclusion was ineffective. Out-diffusion results showed that the species of 125I may be changed during the diffusion processing. It was probably caused by some organic matters or reducing substances in GMZ bentonite. Since the main composition of bentonite is montmorillonite, similar diffusion parameters were obtained in GMZ and other types of bentonite. The relationship of De and accessible porosity (ɛacc) could be described by Archie’s law with exponent n = 1.2–2.8 for 125I diffusion in bentonite, whereas n = 2.0 in GMZ bentonite. Furthermore, bentonite with the dry density of 1800 kg/m3 was proposed as the backfilling materials used in the construction of high level radioactivity waste repository.