Coupled thermal-hydro analysis of unsaturated buffer and backfill in a high-level waste repository

• The sensitivity analysis was carried out for the thermal-hydro model parameters.
• The temperature distribution was sensitive to a change in the degree of saturation.
• The degree of saturation distribution was sensitive to a variation in temperature.
• A coupled TH behavior in the buffer and backfill of KRS repository was analyzed.

The buffer and backfill are the major components of an engineered barrier system (EBS) for a high level waste (HLW) repository. Reliable EBS performance assessment requires the delicate numerical modeling of the buffer and backfill. This study carried out the sensitivity analysis of thermal-hydro (TH) model parameters, and based on its results, the coupled TH analysis of an unsaturated buffer and backfill in an HLW repository. The temperature distribution was sensitive to a change in the degree of saturation and thus the thermal conductivity, and the degree of saturation distribution was sensitive to a variation in temperature and thus water viscosity. The decay heat of HLW from a canister dissipated out through the buffer and backfill into the surrounding rock. The temperature was higher closer to the canister and was lower farther from the canister. The temperature in the backfill was overall lower than that in the buffer, but both temperatures were approached slowly over a long time. The peak temperature was located at the center of the interface line between the canister and buffer, and was 107 °C at the elapsed time of 0.47 year. Re-saturation occurred in the order of the backfill and then a buffer as groundwater was intruded from the surrounding rock. The wetting of the backfill was initiated from the wall of the tunnel and the upper wall of deposition hole, and it then proceeded toward the inside. The buffer was wetted from the wall of the deposition toward the canister. The latest re-saturation location was a bit above from the center axis of the canister, at which the re-saturation period of time was estimated to be about 10 years.