An impact statement of secondary minerals assemblage and minerals reaction rate for the long-term alteration of the buffer material

Sasaki, Ryoichi; Oda, Chie  ; Honda, Akira

Radioactive waste which includes TRU nuclide (TRU waste) possesses wide radionuclide concentration distribution. Some kinds of TRU wastes (hulls-endpieces, asphalt solidification package, spent ioine adsorber etc.) should be disposed in the geological repository because of relatively high radioactivity. Most waste package will be filled with cementitious material. It is considered that waste packages will be isolated to the concrete pit constructed in a drift and filled space with cementitious material filler. I-129 and C-14 cannot be expected solubility limits and storong adsorption to the cementitious material, therefore, the bentonite-sand mixture which possesses very low permeability will be employed as a component of multi-barrier system which installs. The function of engineered barrier system(EBS) which consists of waste package, cementitious filler and buffer material are required to be maintained for a long term. Hence, it is important to analyze the EBS alteration based on available knowledge coming from both experiments and natural analogue. EBS alteration analyses were inplemented with the simplified diffusion transport system which consisted of cement region and bentonite buffer using PHREEQC code (the ver.2.10). In the analysis, variation of secondary mineral assemblage is considered. It is known that the dissolution rate of montmorillonite decreaes as its saturation index increase, so instantaneous dissolution equilibrium and the empirical equation for the dependency of dissolution kinetics on S.I. which was proposed by Cama et al. (2000) were assurned. In most cases, calculation results showed that the funcion of EBS will be maintained over ten southand years. In some cases (instantaneous equilibrium case, modified logK case, SRHP case), significant montmorillonite dissolution was shown. But, such results are inconsistent with the natural analogue knowledge which shows natural bentonite exists for 2 million years under seawater similar ...