Speaker
Description
In several countries, direct disposal of spent nuclear fuel (SNF) in a deep geological repository (DGR) is the preferred waste management option. Understanding SNF corrosion if the waste canister is breached and the fuel comes into contact with groundwater is required to evaluate the long-term safety of a DGR over an assessment timeframe of up to one million years. While various studies have addressed this issue in the past, some open questions remain with respect to the processes contributing to the radiolytic corrosion of SNF in a reducing repository environment. Furthermore, little research has been conducted on the corrosion behaviour of irradiated mixed oxide (MOX) fuels, and only limited knowledge exists on the impact of corrosion on the microstructure of SNFs.
In order to investigate the impact of environmental conditions on SNF corrosion, three irradiated Zircaloy-4 clad MOX fuel rod segments with burn-ups ranging from 29 GWd/tHM to 52 GWd/tHM were leached in bicarbonate water at circumneutral pH and in synthetic cementitious water at a pH of 13.7(2), under a reducing atmosphere (4 vol% H₂ in Ar at 40 bar pressure) for approximately 3.5 years. Following the leaching phase, the fuel rod segments were examined by scanning electron microscopy (SEM) and fragments of these leached segments were analysed by energy-dispersive X-ray spectroscopy (EDS). The post-leaching analyses revealed that the exposure to the diverse environmental conditions affected the SNF microstructure differently. A deposit containing several leachate constituents (e.g., potassium, sodium, and calcium), as well as zirconium and titanium, was identified on surfaces exposed to the synthetic cementitious water [1]. The presence of titanium in the surface deposits indicates that the titanium-coated stainless steel autoclave liners corroded during the leaching experiment. In contrast, the surface of fuel segments exposed to bicarbonate water showed significantly less amounts of surface deposits; in these deposits, no titanium could be detected by EDS.
This contribution presents initial findings of the post-leaching characterisation of the fuel segments and discusses their implications for the corrosion behaviour of SNF under repository-relevant conditions.
- C. Schreinemachers, G. Leinders, J. Van Eyken et al. "Characterisation of high burnup spent mixed oxide fuel after leaching at hyperalkaline pH in a reducing environment". MRS Advances 9, 351-356 (2024). https://doi.org/10.1557/s43580-024-00791-7