Lena Evins1
SKB1
When considering spent nuclear fuel (SNF) for direct disposal, it is necessary to estimate the rate of radionuclide release from this waste form in the repository environment. Since the majority of the radionuclides are contained within the UO2-matrix of the spent nuclear fuel pellets, the matrix dissolution rate strongly impacts the rate of radionuclide release from the SNF and therefore also the safety assessment. Radionuclides are also located in and on the SNF cladding and other metallic parts of the fuel, and in the gap between the SNF pellets and the cladding. These radionuclides are released more rapidly than those found in the SNF matrix. It is important to quantify these fractions, especially the so-called Instant Release Fraction (IRF), to estimate the effect of this on the safety assessment. Therefore, efforts are made to fully understand the matrix dissolution as well as the fast release of radionuclides observed in SNF leaching experiments [1,2,3]. As soon as it was clear that direct disposal of SNF was likely to be the preferred strategy in some countries, experiments were started to gather radionuclide release data [4]. These first experiments were mainly performed in aerated conditions. Later, there was a focus on reducing conditions, as these are more relevant to the expected repository environment. Many studies in published in the last decade have shown that hydrogen in the gas phase can strongly reduce the oxidative dissolution rate [1, 5]. However, some questions still remain regarding the mechanisms and conditions for which the hydrogen is most efficient in this respect. The effects of iron in the system is another important issue [6]. In this contribution, experiments performed under hydrogen and/or corroding iron are reviewed and discussed. Recent advances concerning oxidative and non-oxidative dissolution, gap and grain boundary inventories, as well as fuel microstructure are also presented. As an outlook, some aspects concerning the ongoing and future fuel development, such as fuel with various additives, are also discussed.<br/>References<br/>[1] Evins, L.Z., Bosbach, D., Duro. L., Farnan, I., Metz, V. , Riba, O., 2021. Final Scientific Report. Deliverable D1.26, DisCo project (Grant Agreement 755443), Euratom Research and Training Programme on Nuclear Energy, Horizon 2020 Framework Programme, European Commission.<br/>[2] Ekeroth, E., Granfors, M., Schild, D. and Spahiu, K., 2020. The effect of temperature and fuel surface area on spent nuclear fuel dissolution kinetics under H2 atmosphere. Journal of Nuclear Materials, 531, p.151981.<br/>[3] Lemmens, K., González-Robles, E., Kienzler, B., Curti, E., Serrano-Purroy, D., Sureda, R., Martínez-Torrents, A., Roth, O., Slonszki, E., Mennecart, T. and Günther-Leopold, I., 2017. Instant release of fission products in leaching experiments with high burn-up nuclear fuels in the framework of the Euratom project FIRST-Nuclides. Journal of Nuclear Materials, 484, pp.307-323.<br/>[4] Forsyth R., 1983. The KBS UO2 leaching program Summary Report 1983-02-01. SKBF KBS Technical Report 83-86, Swedish Nuclear Fuel Supply Co/Division KBS.<br/>[5] Puranen, A., Barreiro, A., Evins, L.Z. and Spahiu, K., 2020. Spent fuel corrosion and the impact of iron corrosion–The effects of hydrogen generation and formation of iron corrosion products. Journal of Nuclear Materials, 542, p.152423.<br/>[6] Odorowski, M., Jegou, C., De Windt, L., Broudic, V., Jouan, G., Peuget, S. and Martin, C., 2017. Effect of metallic iron on the oxidative dissolution of UO2 doped with a radioactive alpha emitter in synthetic Callovian-Oxfordian groundwater. Geochimica et Cosmochimica Acta, 219, pp.1-21.