Understanding the Corrosion Behaviour of Used Mixed Oxide (MOX) Fuels: Insights from Post-leaching Characterisation

The disposal of spent nuclear fuels (SNF) in a deep geological repository (DGR) is regarded as the best practical waste management option in many countries. The long-term safety of a DGR over an assessment time frame of up to one million years necessitates a comprehensive understanding of the corrosion behaviour of SNF once the waste canister is breached and groundwater comes into contact. Although various studies have addressed this topic in the last decades, some of the processes contributing to the (radiolytic) matrix corrosion of SNF in the generally reducing repository environment are not fully understood. Furthermore, only limited efforts were deployed to study the corrosion behaviour of irradiated MOX fuels. To examine the effects of environmental conditions on SNF corrosion, the SF-ALE project (Spent Fuel Autoclave Leaching Experiments) was started as a collaboration between the Belgian Nuclear Research Centre SCK CEN and the Forschungszentrum Jülich GmbH. Within SF-ALE, irradiated MOX fuel rod segments (burn-up between 29&nbsp;GWd/tHM and 52&nbsp;GWd/tHM) were leached in bicarbonate water at neutral pH and in synthetic cementitious water at pH 13.5 under reducing atmosphere (4 vol% H2 in Ar at 40 bars pressure) in order to assess the release of various radionuclides and fission gases over a timeframe of 3.5 years. Following the leaching phase, a post-leaching characterisation of the fuel rod segments was initiated. Scanning electron microscopy analyses revealed that the structure of the MOX fuel matrix was affected differently by the exposure to the varied environmental conditions. Furthermore, secondary phases and alteration products were observed on the fuel surfaces. This contribution introduces initial results of the post-leaching characterisation and their implications to the general understanding of the corrosion behaviour of SNF under repository relevant conditions.</p