Tomoo Fujita

Experiment and validation of numerical simulation of coupled thermal, hydraulic and mechanical behaviour in the engineered buffer materials

Evaluation of the coupled heat transfer, water flow and stress changes in the engineered clay barrier is an important issue in the performance assessment of the high-level radioactive waste disposal. To demonstrate the function of the engineered barrier system, the large-scale experiment is conducted, which is called Big Bentonite facility (BIG-BEN). The facility consists of an electric heater surrounded by glass beads, carbon steel overpack, buffer material and man-made rock. The buffer is a mixture of bentonite and sand. The heater is operated at 0.8 kW. Water is injected from the interface between the buffer and the man-made rocks at the pressure of 0.05 MPa. The duration of the experiment is 20 months. The change in temperature and swelling pressure are continuously monitored and gravimetric water content is measured by sampling. The coupled thermal, hydraulic and mechanical processes are simulated with a finite element code THAMES, which can simulate the fully coupled phenomena in the saturated and unsaturated clay under anisothermal condition. To examine the validity of the code, all the parameters used in the model are evaluated from the other laboratory tests. The simulated results are compared with the measured ones without calibration of the parameter values using the results from the BIG-BEN experiment. It can be concluded that the changes in temperature and gravimetric water content within the buffer can be simulated reasonably well and that the mechanical effect such as swelling pressure is difficult to realize.

Characterization of Hydraulic Properties of Fractured Rock Mass with Numerical Model

The flow aspect in the fractured rocks is examined by using the general theory by Barker. It is found from the examination that the correlation between the flow dimension and hydraulic conductivity is positive. This observed correlation is related to the geometry of the flow network and hydraulic properties of flow paths. To examine the characteristics of hydraulic properties of fractures, the discontinuous fracture network model is used for the parametric study of the statistical properties. The variety in the relation between flow dimension and hydraulic conductivity is examined. The observed relation of flow dimension to hydraulic conductivity cannot be realized when the fracture length is independent of the fracture aperture. It is expected from the examination that the fracture aperture increases exponentially with the fracture length.