Measurement of Bentonite-water Potential Using a Thermocouple Psychrometer

Bentonite, a kind of swelling clay is favorably used for the buffer of a high-level waste (HLW) repository, because it has the ability to prevent water intrusion into a repository, to retard the release of radionuclides from wastes, and to support containers encapsulating the wastes from external impacts. The bentonite buffer will remain unsaturated for a certain period of time after its emplacement, due to a combinational process of decay heat from the wastes and the inflow of groundwater, and, for that period, the movement of groundwater and radionuclides through it is dominated by bentonite water potential. Therefore, the water potential measurement is inevitably needed to assess the barrier performance of the buffer of a HLW repository. The water potential has been widely studied for plant soils since 1960's, using the direct and indirect techniques of measurement (piezometry, tensiometry, thermocouple psychrometry, and measurements with electrical resistance sensor, heat dissipation sensor, filter paper technique, vapor equilibration etc.). However, not much data has been reported on the water potential of bentonite as a buffer material of a HLW repository. The present study intends to establish an optimal procedure of measurement for the thermocouple psychrometry using a domestic bentonite, and also to investigate the dependency of water potential on water contents which can be expected under disposal environments