Monitoring of the extent of high temperature and unsaturated zone by resistivity tomography during in-situ heating test in the sedimentary soft rock

Abstract

One of the major issues in disposal of nuclear waste is that the long term behaviors of sedimentary soft rock can be affected by various environmental factors. The effects of heat generated by nuclear waste must be taken into consideration on the occasion of facilities design. The generation of unsaturated zone is also important for design because of a corrosion of overpack or influence on the recovery reduction state from oxidation state. We have conducted in-situ heating test to evaluate the influence of high temperature and unsaturated zone to the surrounding rock mass at a depth of 50 m. During the heating test, we have conducted repeated measurements of resistivity tomography to map the extent of the high temperature and unsaturated zone. The results demonstrated that the resistivity of the rock mass around a heating well decreased and this affected zone was found to gradually expand from the heated zone during the heating. Generally, resistivity of rock mass decreases when temperature increases. Measured resistivity change agreed well with that calculated by the equation using resistivity of the rock sample collected from the test site with temperature. This suggests that high temperature zone was detected by resistivity tomography. On the other hand, in the vicinity of the heating well, the measured resistivity change was smaller than that calculated. This suggests that resistivity was increased by unsaturation of rock mass in addition to the resistivity decrease by high temperature.