Inverse Modeling of Gas, Water, and Heat Flow in Bentonite/crushed Rock Backfill

Swelling clays play a major role in current concepts for the underground disposal of high-level nuclear waste in deep geological formations. In one of the multi-barrier concepts for preventing the escape of radioactive substances from a high-level nuclear waste repository, the barrier consists of a copper container, compacted bentonite as buffer and backfill (the engineered barrier), and the repository host rock. Corrosion of the copper canister and radiolysis of water both produce hydrogen. When the buffer and backfill are saturated with water and the permeability of the bentonite is reduced by swelling, any hydrogen that is produced can accumulate in the space between the container and the engineered barrier. This will result in pressures exceeding the entry pressure of the bentonite and passage of gas through the engineered barrier. An experimental program was developed to investigate the thermal and hydraulic properties of the buffer and backfill under conditions expected to exist in a permanent repository for radioactive waste. Water retention curves were measured from low to high saturation using a pressure cell and a thermohygrometer. Non-isothermal drainage experiments were also designed. The measured water outflow, pressure and temperature values were used for inverse modeling using iTOUGH2. The capillary pressure curves obtained with iTOUGH2 were consistent with those obtained with the pressure cell and thermohygrometer. The influence of salinity on the capillary pressure curves was investigated.