Criticality assessments for long-term post-closure canister corrosion scenarios in a deep geological repository

Abstract

Evaluating the criticality safety of a final disposal concept for high-level waste (HLW) in the repository post-closure phase is a regulatory requirement for many national radioactive waste management organisations. The Swiss National Cooperative for the Disposal of Radioactive Waste, Nagra, is presently elaborating the technical foundation for a criticality safety assessment for the Swiss HLW deep geological repository concept. The very long timescale (one million years in Switzerland) that must be considered in the assessment represents one of the main challenges. In this context, the evolution with time of the system defined by the final disposal canister (ELB) filled with spent nuclear fuel (SNF) and its potential impact on the system’s neutron multiplication factor must be considered and evaluated. One of Nagra’s key safety requirements for the ELB design is a canister lifetime of 10,000 years. Various interactions between the natural and engineered barriers can occur over the long timeframe, leading to alterations of the initial geometry and material composition of the canister and the surrounding backfill material. To study their potential effect on the system criticality change in terms of Δkeff (referred to in the following also as “reactivity change”), several conservative simplified ELB corrosion scenarios have been formulated by Nagra and investigated in collaboration with the Paul Scherrer Institute (PSI). The reactivity effects of the postulated ELB corrosion scenarios were studied at PSI by means of detailed criticality simulations carried out with the MCNP® code. This paper presents the conservative ELB corrosion scenarios considered in the analysis and their implementation. The observed relative impact on the system reactivity is then discussed. Additional preliminary considerations concerning the expected variability in porewater composition and canister manufacturing tolerances are also addressed.