Experimental investigation and mathematical simulation of coupled T-H-M processes of the engineered buffer materials, the TC3 problem

Abstract Evaluation of coupled heat and water transfer and stress changes due to swelling / shrinkage in engineered clay barriers is an important issue in the safety assessment of engineered clay barriers. Comparison of the observed results from experimental investigation using a large scale laboratory test and simulated values of coupled thermo-hydro-mechanical (T-H-M) processes within engineered buffer material using numerical models was performed in Phase III of the DECOVALEX project. This problem has been identified within the project as the Test Case 3 (TC3). The large scale experiment was conducted using the Big-Bentonite facility (BIG-BEN) at Tokai Works, Power Reactor and Nuclear Fuel Development Corporation (PNC). The BIG-BEN is a physical model of a disposal borehole (pit) included in the current design of a repository concept considered by PNC. The experiment consists of an electric heater surrounded by glass beads, a carbon steel overpack, buffer material and man-made rock. The buffer is a mixture of bentonite and sand. The heater was operated at 0.8 kW. Water was injected at the buffer / man-made rock interface at a pressure of 0.05 MPa. The duration of the experiment was five months. Four variables were of interest in this study: the temperature, moisture content, vertical stress and horizontal stress. The temperature changes and swelling pressures were continuously monitored and moisture content measured gravimetrically at the end of experiment (five months). The coupled T-H-M processes were simulated by three Research Teams: Clay Technology (CLAY TECH.)/SKB, Center for Nuclear Waste Regulatory Analysis (CNWRA)/NRC and Kyoto University-PNC-Hazama Corporation (KPH)/PNC using different models. The results of the validation suggested that the changes in temperature and moisture content within the buffer can be simulated well; simulated values by all three teams matched the observed data. Observed values for vertical and horizontal stresses were not available and comparison was made between the values simulated by CLAY TECH. and KPH. The results showed a discrepancy in the simulated stress values. These discrepancies may be explained by the different parameters used in the two models although problems associated with the conceptual models can not be ruled out. It appears that further investigations are needed to improve numerical models to simulate adequately the coupled T-H-M processes in the buffer.