Natural analogue studies of long term alteration of volcanic glass under the high level radioactive waste disposal conditions. Document prepared by other institute, based on the trust contract

The alteration characteristics of volcanic glass bentonite in geological medium are investigated as natural analogues of geological disposal of high-level radioactive wastes. For the study of alteration of volcanic glass, literature survey, petrographical observation by using optical microscope, chemical analysis of pore water and groundwater, and microbiostratigraphical study were carried out. The volcanic ashes were collected from a borehole, which had been drilled in the Kanto region by the National Research Institute for Earth Science and Disaster Prevention. The volcanic glass was wholly altered in thin sections. Chemical analysis of pore water indicate the chloride-rich composition. And chemical analysis of groundwater, which are pumped up near the studied borehole, indicate the mixing product of connate marine water and fresh groundwater. Fossil evidence suggests that the studied volcanic glass is 3.65 ∼ 4.2 Ma. in age, and is correlated with upper Miura Group. This study leads to following conclusion. The volcanic glass, under the temperature condition 70degC, was wholly altered to montomorillonite and clinoptilolite. This case must be occurred in the connate marine water at last 0.5 Ma.. (author)