CRITICALITY AND SHIELDING ASSESSMENT OF CANISTER CONCEPTS FOR THE DISPOSAL OF SPENT NUCLEAR FUEL

摘要

According to the Swiss disposal concept, the safety of a deep geological repository for spent nuclear fuel (SNF) is based on a multi-barrier system. The disposal canister is an important component of the engineered barrier concept, providing containment of the SNF for at least 10,000 years. This study evaluates the criticality safety and shielding of candidate disposal canister concepts, focusing on the fulfillment of the sub-criticality criterion and on limiting radiolysis processes at the outer surface of the canister which can enhance corrosion mechanisms. The effective neutron multiplication factor (k-eff) and the surface dose rates are calculated for three different canister designs and material combinations for boiling water reactor (BWR) canisters, containing 12 spent fuel assemblies (SFA), and pressurized water reactor (PWR) canisters, with 4 SFAs. For each configuration, individual criticality and shielding calculations are carried out using the SCALE6.2 package (KENO-VI for criticality; MA VRIC for shielding). The results show that k-eff falls below the defined upper safety limit (USL) of 0.95 for all BWR configurations, while staying above USL for the PWR ones. For a final burnup of 55MWd/kgHM and 30y cooling time, the combined photon-neutron surface dose rate is well below the defined limit of IGy/h in all cases.