Salt has long been considered as a potential host rock for the geological disposal of radioactive waste because of its favorable properties, including self-sealing, low permeability and high thermal conductivity. The feasibility of nuclear waste disposal within salt formations has been investigated mainly for small-sized canisters, widely considered in many national nuclear waste disposal programs. Direct disposal of larger-sized canisters originally designed for spent nuclear fuel storage and transportation has lately been examined as a promising and cost-effective alternative. However, the amount of decay heat released by large canisters may cause high temperature in the backfill surrounding the canisters and in the host rock. This results in pressure and stress changes that may affect the long-term repository performance. To analyze it, the TOUGH-FLAC simulator is used to conduct fully coupled Thermal-Hydro-Mechanical (THM) simulations of a generic salt repository for large-sized canisters. In this code, large deformation has been considered allowing to accommodate viscous compaction of the crushed salt backfill as well as the long-term stress changes in salt host rock. The simulations show that the peak temperature in the backfill and in the host rock can be reduced by adjusting the distance between the canisters along emplacement tunnels or the spacing between the tunnels themselves. Nevertheless, the thermal pressurization in the salt host rock and in the crushed salt backfill, following the reconsolidation of the backfill, seems to be less sensitive to these spacing adjustments. Consequently, the pore-pressure could potentially exceed the lithostatic stress causing an increase of permeability and a fluid infiltration into the rock mass that may last more than 1000 years after the waste canister emplacement. Thus, it is critically important to analyze and manage these coupled THM processes for a safe and effective disposal of nuclear waste in salt formations.
Geologic disposal of large-sized canisters, originally designed for spent nuclear fuel storage and transportation, has lately been examined as a promising and cost-effective alternative to conventional smaller-scale canisters commonly considered in many national nuclear waste disposal programs (Hardin et al., 2015).
