Thermal analysis of reference repository for RBMK-1500 spent nuclear fuel in crystalline rocks

Total amount of RBMK-1500 type spent nuclear fuel (SNF) in Lithuania is approximately 22 thousands of fuel assemblies. All these assemblies should be stored for 50 years and then disposed of. International consensus prevails that SNF and long-lived high-level radioactive wastes are best disposed of in geological repositories using a system of engineered and natural barriers. Disposed nuclear waste induces a number of coupled thermo-hydro-mechanical processes around the repository. Thermal analysis of a deep geological repository could provide temperature distribution which is required for the repository’s design and for evaluation of thermal integrity of the engineered barriers. One of the most critical parameters for the repository is peak temperature at the outer surface of the canister. This temperature cannot exceed 100 °C; otherwise, unfavourable groundwater chemistry can adversely affect chemical stability of the engineered barriers. Thermal behaviour of the conceptual Lithuanian repository for RBMK-1500 type SNF in crystalline rocks was modelled using numerical codes ANSYS FLUENT and COMPASS. Very similar temperature distributions around the disposal canister were determined using both modelling tools. The modelling results revealed the importance of coupled heat and hydrodynamical processes for peak temperature in the engineered barriers, whereas the impact of mechanical processes evaluation was insignificant. It was also determined that peak temperature at the outer surface of the disposal canister does not exceed the permitted 100 °C.