Masataka Sawada

New Rapid Evaluation for Long-Term Behavior in Deep Geological Repository by Geotechnical Centrifuge. Part 1: Test of Physical Modeling in Near Field Under Isotropic Stress-Constraint Conditions

The objective of this study is to evaluate the long-term geomechanical behavior of a geological repository for high-level radioactive waste disposal, using the centrifugal near-field model test. The model consisted of a sedimentary rock mass, bentonite buffer, and model overpack, and was enclosed within a pressure vessel. Tests were conducted with a centrifugal force field of 30 G under isotropic stress-constraint conditions with confining pressures of 5–10 MPa and injection of pore water up through a time period equivalent to about 165 years in the field. Our results showed that the measured values and the temporal changes in the displacement of the overpack, the soil pressure of the bentonite, and the strain of the rock mass were clearly dependent on the confining pressure. These data were not convergent during the test. Our data experimentally revealed that long-term behavior in the near field was changed by the geomechanical interaction between the deformation stress of the bedrock/disposal hole and the swelling behavior of the bentonite buffer.

New Rapid Evaluation for Long-Term Behavior in Deep Geological Repository by Geotechnical Centrifuge—Part 2: Numerical Simulation of Model Tests in Isothermal Condition

Abstract In high-level radioactive waste disposal repositories, there are long-term complex thermal, hydraulic, and mechanical (T–H–M) phenomena that involve the generation of heat from the waste, the infiltration of ground water, and swelling of the bentonite buffer. The ability to model such coupled phenomena is of particular importance to the repository design and assessments of its safety. We have developed a T–H–M-coupled analysis program that evaluates the long-term behavior around the repository (called “near-field”). We have also conducted centrifugal model tests that model the long-term T–H–M-coupled behavior in the near-field. In this study, we conduct H–M-coupled numerical simulations of the centrifugal near-field model tests. We compare numerical results with each other and with results obtained from the centrifugal model tests. From the comparison, we deduce that: (1) in the numerical simulation, water infiltration in the rock mass was in agreement with the experimental observation. (2) The constant-stress boundary condition in the centrifugal model tests may cause a larger expansion of the rock mass than in the in situ condition, but the mechanical boundary condition did not affect the buffer behavior in the deposition hole. (3) The numerical simulation broadly reproduced the measured bentonite pressure and the overpack displacement, but did not reproduce the decreasing trend of the bentonite pressure after 100 equivalent years. This indicates the effect of the time-dependent characteristics of the surrounding rock mass. Further investigations are needed to determine the effect of initial heterogeneity in the deposition hole and the time-dependent behavior of the surrounding rock mass.