The multibarrier system for deep geologic disposal of high-level, long-lived and heat generating radioactive waste has been devised in order to keep them away from humanbeing. This system consists of engineering barrier and geological natural barrier. After burying the waste, an “artificial geothermal system” may be constructed because of heat from radioactive waste, and the artificial geothermal system would make some hydrothermal minerals and alteration zone, in host rocks.
Hosokura ore deposits were made by an “old geothermal system”, and have impermeable zones developed along some ore veins. In some active geothermal area in Japan, geothermal alteration zones seem to play as caprocks. That is, a part of geothermal alteration zones sometimes work as hydraulic barrier (alteration barrier), which is very important for hydrology of that area. In this paper, the possibility about formation of the “artificial alteration zone” at geologic disposal is considered.
Smectite, α-cristobalite, clinoptilolite, mordenite and stilbite are possibly formed by the interaction between neutral hot water and volcanic glass at temperatures below 100°C in an artificial geothermal system. These minerals have high adsorption capacity of radionuclides and low permeability. Because of these properties, the alterative minerals may form an “alteration barrier” which supports the multibarrier system.
Nowadays, a large amount of bentonite is necessary for buffer and backfill material. Mixing volcanic glasses with bentonite may reduce the amount of bentonite, because volcanic glasses may change to smectite and form a part of alteration barrier in a long time.