Effects of materials and design on the criticality and shielding assessment of canister concepts for the disposal of spent nuclear fuel

• Published in: Applied radiation and isotopes Vol. 139; pp. 201 - 208
• Main Authors: Gutiérrez, Miguel Morales, Caruso, Stefano, Diomidis, Nikitas
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.09.2018
• ISSN: 0969-8043; 1872-9800
• DOI: 10.1016/j.apradiso.2018.05.016

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 system, aiming to provide containment of the SNF for thousands of years. This study evaluates the criticality safety and shielding of candidate disposal canister concepts, focusing on the fulfilment 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 were carried out. 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. Therefore, the application of a burnup credit methodology for the PWR case is required, being currently under development. Relevant is also the influence of canister material and internal geometry on criticality, enabling the identification of safer fuel arrangements. For a final burnup of 55MWd/kgHM and 30y cooling time, the combined photon-neutron surface dose rate is well below the threshold of 1 Gy/h defined to limit radiation-induced corrosion of the canister in all cases. •Evaluation of a multi-barrier system for spent nuclear fuel geological disposal.•Development and analysis of advanced disposal canisters concepts.•Performed criticality safety assessment for proposed canister configurations.•Shielding assessment to prevent radiolysis processes.•Results very promising for long term safety assessment.