Diffusion and sorption of Cs in the mudstone matrix at the Horonobe URL; Comparative experimental and modeling analysis between in situ and laboratory conditions

武田 匡樹 ; 大野 宏和 ; 伊藤 剛志*; 四辻 健治*; 深津 勇太  ; 佐藤 智文*; 舘 幸男  

Takeda, Masaki; Ono, Hirokazu; Ito, Tsuyoshi*; Yotsuji, Kenji*; Fukatsu, Yuta; Sato, Tomofumi*; Tachi, Yukio

Diffusion and sorption of radionuclides in potential host rocks are key processes controlling the safe geological disposal of radioactive waste. To set reliable parameters of diffusion and sorption relevant to safety assessment, it is necessary to understand these processes under laboratory and in situ conditions, and to confirm the consistency or the relationship of these parameters (e and d) between both conditions. This paper focused on comparative experiments and modeling of diffusion and sorption of Cs for both in situ and laboratory conditions in siliceous mudstone in 350 m Gallery at the Horonobe Underground Research Laboratory (URL) in northern Hokkaido, Japan. The e values for Cs were consistent between in situ and laboratory results, indicating typical cation excess and porewater salinity effects. The effect of the diffusion direction, parallel or perpendicular to the bedding plane was not significant. The d values for Cs were also consistent between in situ and laboratory results, indicating the porewater salinity effect. The clay-based modeling approach, which considers the major contributions of clay minerals through cation excess diffusion by electrostatic interaction at the clay surface, and the component additive cation exchange reactions, could interpret the diffusion and sorption behavior of Cs under different salinity conditions and for both in situ and laboratory conditions.