keff behaviour of a potential PWR assembly loaded disposal canister in simplified corrosion scenarios

The Swiss National Cooperative for the Disposal of Radioactive Waste (Nagra) and the Center for Nuclear Engineering and Sciences at the Paul Scherrer Institute (PSI) are investigating how potential long-term changes to the geometry and material composition of the current final disposal canister (BE-ELB) design may affect the neutron multiplication factor (k_eff) of a canister loaded with PWR spent nuclear fuel assemblies (FAs). Several conservative corrosion scenarios were formulated, modeled, simulated and analyzed using the Monte-Carlo codes MCNP6.2® and Serpent2.2. The corrosion-induced effects, such as the replacement of moderator by magnetite, differ significantly from typical fuel lattice configurations used in reactor or storage pool safety analyses. This paper therefore focuses on underlying physical phenomena causing the observed k_eff changes, including analyses of neutron currents and spectra, the ‘6-factor formula’, and the sensitivity of k_eff to specific regions, materials, and nuclides. These analyses showed the canister wall and the corrosion product magnetite to act as a heavy reflector, enhancing the backscattering of neutrons in the epithermal and fast energy ranges. Furthermore, the critical role of the water distribution in all the potential scenarios was revealed. Water inside the FAs clearly increases reactivity by moderation, water outside the remainders of the steel basket, however, has an inhibiting effect on neutron multiplication. All simulation results heavily depend on the specific preliminary ELB design. To that end, the results of this study can help to optimize the ELB design and the Swiss concept for the final disposal of high-level radioactive waste.