Method for measuring the saturated permeability coefficient of compacted bentonite at temperatures exceeding 100 °C

Abstract

In deep geological repositories for high-level radioactive waste, the bentonite buffer layer is considered an important artificially engineered barrier between nuclear waste canisters and the host rock. Low permeability requirements of the bentonite must be met to prevent groundwater infiltration and radionuclide leakage. During the repository operation, gases will inevitably be generated through various physicochemical processes around the canister, and the continuous accumulation of these gases will result in a high-pressure state in the repository. In addition, the bentonite buffer layer may experence temperatures exceeding 100 °C because of the continuous release of decay heat from the nuclear waste in the canister, which can also affect the permeability of the bentonite buffer layer. Hence, it is necessary to obtain the saturated permeability coefficient of compacted bentonite at temperatures exceeding 100 °C. However, the conventional methods fail to consider the pressure effect, because when the temperature exceeds 100 °C under standard atmospheric pressure, free water will boil violently to form steam, which hinders the measurement of the saturated permeability coefficient of bentonite. Therefore, an innovative measurement method for the saturated permeability coefficient of compacted bentonite at temperatures exceeding 100 °C was proposed in this study, based on the relationship between the boiling point of water and the pressure. As the boiling point was increased by raising the pressure, the saturated permeability coefficients at 25 °C and 105 °C were sequentially measured for the same compacted bentonite specimen with the dry density of 1.65 g/cm3 to verify the feasibility of the proposed method. The results at 25 °C showed that the difference between the permeability coefficients measured by the proposed method and the conventional method was no more than 1 × 10−13 m/s. Thus, the proposed method can be applied to measure saturated permeability coefficients at temperatures exceeding 100 °C. Furthermore, the results at 105 °C indicated that it was more reasonable to calculate the permeability coefficients according to the amount of water seepage at the outlet. The proposed method can provide technical support for future studies on the effect of temperatures exceeding 100 °C on permeability and allow evaluation of the long-term performance of bentonite buffer layers in deep geological repositories.