Abstract: To evaluate the retardation capability of buffer materials in an underground repository for high-level radioactive waste, it is necessary to understand the performance of densely compacted bentonite block and joint combination under coupled thermo-hydro- mechanical (THM) condition. For this purpose, a testing apparatus for coupled THM responses of combined bentonite blocks and joints is developed. It consists of five parts, including the model test box, the combined block-joint sample, the temperature control system, the hydraulic control system, and the data measurement and acquisition system. Two compacted blocks composed of the Gaomiaozi (GMZ) bentonite and one joint filled with sand and bentonite powder were used to make the composite sample. To examine the feasibility of the design of this apparatus, the test was carried out under a thermal boundary condition of 90 ℃ and a hydraulic boundary condition of 0.1 MPa for 266 days. Based on test data of temperature, volumetric moisture content and expansive force, the coupled THM responses and their spatiotemporal evolution rules were obtained. A new definition formula of joint-healing- degree was proposed on basis of variation of dry density in test sample. The joint-healing-degree was calculated through the measured change in joint thickness during the test. Regression analysis was conducted according to the relationship between the change in joint thickness and the coupled THM responses including temperature, volume moisture content and expansive force, and the regression formula was accordingly set up. It has shown that this apparatus can be used in the indoor physical simulation test of buffer materials for deep nuclear waste disposal.

Key words: high-level radioactive waste disposal, bentonite block, joint, THM coupling, model test