Modelling preferential paths of gas migration application to the gas migration test

The gas migration test simulates an underground storage concept for medium or low level radioactive waste. A concrete silo was isolated from a granitic host rock by means of a 80/20 compacted sand bentonite mixture. The test was emplaced in a cavern excavated in the Grimsel Rock Mechanics Laboratory in Switzerland. The comprehensive instrumentation installed (piezometers, TDR probes, total pressure cells) has provided information on the barrier performance during the successive stages of the test: emplacement, natural and forced saturation, water injection test and, finally, gas injection tests. Due to the high number of interfaces (buffer-concrete, buffer-rock, surfaces of compacted layers), natural preferential paths are available for gas migration during the final stage of the test. The paper presents a computer simulation of all the test stages and compares actual field measurements with calculated values. Special attention is given to the simulation of interfaces. This is achieved by means of special continuum elements which simulate gas flow through planar openings. The sand/bentonite mixture was described by means of an elasto-plastic model (BBM). Model parameters were found by means of a back analysis of suction controlled odometer experiments on the sand/bentonite compacted samples. Hydraulic laboratory tests (water retention, permeability) were also performed in order to determine the appropriate material parameters. (authors)