The effect of NaNO3 on diffusion of CO32-, Cl- and I- ions in tuff

The migration data of 14C, 36Cl and 129I nuclides are required for safety assessment of TRU waste disposal. The specific phenomena for TRU waste disposal such as the influence of NaNO3 concentration and the hyper alkaline groundwater due to degradation of cementitious materials should be considered for reliable safety assessment. In this study, the effective diffusion coefficient (De) for CO32-, Cl- and I- ions in tuff were determined with various NaNO3 concentrations under hyper alkaline condition. The obtained De values for CO32-, Cl- and I- ions were on the order of 10-12 - 10-11 m2/s and decreased with increasing NaNO3 concentration. The De values for HDO and HTO were also decreased with increasing NaNO3 concentration. Because the degradation of tuff samples was not observed by X-Ray diffractometry, SEM and EPMA, the decreases in the De values were considered to be due to the decreases in the diffusivities in free water. The apparent diffusion coefficient (Da) values for Cl- ion with 5 mol/dm3 NaNO3 concentration and for CO32- ion were lower than those of the other conditions. These low Da values suggested retardation in tuff samples. (author)