Leaching behaviour of medium and high burn-up spent UOX and (U, Pu)OX fuels under anoxic and reducing conditions – Release of fission gases and their impact on the instant release fraction and matrix dissolution

Several countries worldwide foresee the direct disposal of spent nuclear fuel (SNF) in deep geological formations combined with a resilient multi-barrier concept, in order to safely isolate and confine the highly radioactive waste from the biosphere. Regarding the long-term safety of a deep geological repository, the intrusion of water into the underground facility and the subsequent contact of solution with the SNF, after failure of the canisters and loss of cladding integrity, has to be considered. Upon contact with intruding solution, a fast release of radionuclides from accessible locations within the fuel, i.e., the fuel gap and fractures, can be observed. This initial release of radionuclides, denoted as instant release fraction (IRF), is of significant importance in safety assessments as the involved radionuclides are long-lived and geochemically mobile thus contributing or even dominating the calculated dose exposure. Moreover, in Germany, the IRF plays a decisive role for the assessment of the safe containment of radioactive waste in the repository near field according to the German safety requirements ordinance (EndlSiAnfV, 2020). Subsequent to the IRF, a slower release from the dissolution of the UO_X matrix itself is observed (Ewing, 2015). Intertwined with the release of aqueous radionuclide species, fission gases are released from the breached fuel rod throughout the dissolution process.

In the present study, we show results obtained from (partly ongoing) leaching experiments with medium and high burn-up UO_X as well as mixed-oxide (U, Pu)O_X SNF. All tests were performed in either highly alkaline cement water solutions (pH 13.7) or bicarbonate water (pH 7.9) under anoxic or reducing conditions induced by dissolved hydrogen. The SNF samples used in this study were irradiated in commercial nuclear power reactors in Germany and Switzerland during the 1970s and 1980s. Each experiment was periodically sampled and solution as well as gas aliquots were analysed.

The results of our contribution point to the interrelation between the fission gas release throughout the leaching experiments and the release behaviour of IRF radionuclides (e.g., ¹²⁹I, ¹³⁷Cs) into solution. For the fission gases, iodine and caesium, a fast initial release is observed, which reaches its maximum at around 200 days of leaching and slows down throughout the experimental duration, although a continuous release is ongoing. Furthermore, the aqueous concentrations of actinides approach the respective solubility limits of the respective An(IV) (U, Np, Pu) and An(III) (Am, Cm) solid phases in the long-term under reducing conditions. The inhibition of the radiolysis driven oxidative matrix dissolution is attributed to the presence of hydrogen. In addition, an attempt is made to correlate the long-term release behaviour of fission gases with the dissolution of the SNF matrix.

Acknowledgements

The authors thank all (former) colleagues from KIT-INE for their scientific and technical contributions. The research leading to these results has received financial support by ONDRAF/NIRAS as well as funding from the European Commission Horizon 2020 Research and Training Programme under grant agreement n° 755443.

References

EndlSiAnfV, Endlagersicherheitsanforderungsverordnung, 2020.

Ewing, R. C.: Long-term storage of spent nuclear fuel, Nature Materials, 14, 252–257, doi:10.1038/nmat4226, 2015.