Understanding the corrosion behaviour of used mixed oxide fuels under repository relevant conditions: Initial results of autoclave leaching experiments
Abstract Disposal in a deep geological repository (DGR) based on a multibarrier concept is considered as the safest and most sustainable option for the management of spent nuclear fuels (SNF) in many countries. Demonstrating the long-term safety of a DGR for SNF over assessment time frames of up to one million years requires a profound understanding of the corrosion behaviour of SNF coming into contact with groundwater, when the waste canister is eventually breached. During the last decades, a number of studies addressed this topic, leading to a good phenomenological understanding of the long-term behaviour of SNF in a DGR. However, various processes contributing to the (radiolytic) matrix corrosion of SNF in the generally reducing repository environment are still not fully understood, and corrosion data on used mixed oxide (MOX) fuels are scarce to date. To investigate the effects of environmental conditions on the corrosion of SNF, the SF-ALE project (Spent Fuel Autoclave Leaching Experiments) was initiated as collaboration between the Belgian Nuclear Research Centre SCK CEN, Forschungszentrum Jülich GmbH, and the Belgian Agency for Radioactive Waste and Fissile Materials (NIRAS/ONDRAF). Within SF-ALE, the release of various radionuclides and fission gases from well-characterised, irradiated MOX fuel segments (burn-up between 29 GWd/tHM and 52 GWd/tHM) exposed to bicarbonate water as reference groundwater at neutral pH and a synthetic cementitious water at pH 13.5 under reducing atmosphere (4 vol% H2 in Ar at 40 bars pressure) is investigated. The first project phase lasted for about two years and was completed in September 2020. The leaching solutions were renewed and a second leaching phase was started on the same MOX fuel segments, aiming for a total duration of about 3.5 years. In this contribution, we report the results obtained during the first phase and describe the transition to phase two of the project.</p
