Modelling a novel technique to remove excess decay heat from a geological disposal facility using a closed-loop geothermal system

Decay heat released from radioactive waste residing in a future geological disposal facility (GDF) poses many challenges, including long-term geological integrity and high temperature gradients in the rock. This paper presents a novel technique to remove decay heat from a GDF, by utilising a 1 km U-loop closed-loop geothermal system. A lower-strength sedimentary rock (LSSR) environment was adopted using the T2Well-ECO2N/TOUGH2 software suite to represent a notional UK high-heat producing waste (HHPW) disposal programme. Within the LSSR environment, three source terms were assessed representative of: pressurized water reactor (PWR), advanced gas-cooled reactor (AGR), and European pressurized reactor (EPR) types. These were assessed for static (no wellbore flow) and dynamic (wellbore activated) scenarios. Sensitivity analyses for the latter involved varying fixed mass flow rate and source term distance from the wellbore lateral to predict excess decay heat removal and end lateral temperatures. An initial software comparison was made against code developed in MATLAB and the OpenGeoSys (OGS) software, to assess a dynamic 10-year setup for 30 fuel canisters. The comparison revealed relative errors of 7.8 % (MATLAB) and 2.3 % (OGS) for end lateral temperatures after 10 years against T2Well. The sensitivity analyses revealed that at 1.2 m below the wellbore, inclusion of the 1 km single lateral closed-loop geothermal system (CLGS) saw a temperature reduction of 26.6 ° C (PWR), 22.8 ° C (AGR) and 32.6 ° C (EPR) from the rock after 11 years. Reducing peak temperatures of the source term could improve overall safety of the GDF by mitigating unwanted processes. • U-loop CLGS within GDF environment for excess heat removal from SNF. • Hypothetical UK GDF case study in LSSR rock for three types of SNF (PWR, AGR, EPR). • Static/dynamic and sensitivity analyses (mass flow rate and source term distance). • Discussion on end lateral temperatures and energy flow rates from CLGS design. • Discussion on heat removal to reduce peak temperatures to improve GDF safety.