Depth Trend of Chemical Characteristics of Bentonite Deposit in Contact with River Water; Sampling and Analytical Results

Saji, Shinichi*; Ito, Masakazu*; Shibata, Masahiro ; Jintoku, Takashi*; Isogai, Takeshi*

Porewater chemistry in Buffer Material is one of the most important information for assessment of corrosion behavior on the overpack, and of solubility and sorption of nuclides in the buffer etc. In the Second Progress Report of HLW in Japan (H12 Report), porewater chemistry is estimated by batch modeling calculation based on the batch experiment. However, it is expected that, in the disposal vault, temporal-spatial evolution of porewater chemistry occurrs. After H12 report, JNC started experimental study and modeling work for further understanding of the chemical evolution. However, modeling for long term evaluation of porewater chemistry is calculated on the basis of the short term laboratory experiment. Thus, the long term reliability of the model needs to be verified by comparing natural analogue.In this study, bentonite deposit under the river bed was sampled as function of depth by boring. At the sampling point, the river improvement was carried out in 1987 and river water contact with bentonite deposit since then. Mineral composition, chemical composition, inter layer cation concentration, CEC and methylene blue adsorption etc. of the sample are measured.As a result, erosion of the smectite, decreasing of pH and leaching out of SO4 are obsereved. However, clear trend of the depth direction is not obtained for the reason of natural inhomogeneous system.