Leaching of spent nuclear fuels in aerated conditions: Influences of sample preparation on radionuclide release patterns

The release of radionuclides from spent nuclear fuel upon contact with water is a central issue for the assessment of the safety of geological disposal concepts. Several studies have been conducted aiming at understanding matrix dissolution as well as the rapid/instant release of radionuclides separated from the UO 2 matrix; there are however questions remaining regarding how higher burn-up affects the amount of fission products segregated to the fuel-cladding gap and grain boundaries. In this study we have performed aerated fuel corrosion and leaching experiments using spent nuclear fuels with a range of burnups, from ∼52 to ∼70 MWd/KgU. The samples have been prepared in different manners in order to investigate the release of the segregated fission products from the gap and the grain boundaries. Thus, the fuel samples are prepared either as segments with the cladding attached, as fragments without cladding, or as milled powder. The fuel samples are leached in aqueous solution in aerated conditions and the radionuclide release is monitored during several contact periods for up to five years total leaching time. The results from the leaching of fuel segments reveal congruent release of e.g. europium and neodymium whereas the release of those elements was lower than the U-238 release for the fragment samples. Potential explanations for this are discussed in the paper. The results show that the release rates of elements segregated from the fuel matrix were in general found to be lower from segments samples as compared to fragment samples, which can be attributed to the closed fuel-cladding gap inhibiting the exposure of the gap inventory to water for the high burnup fuels used in this study. Leaching of fuel powder (78 MWd/kgU local BU) by simultaneous grinding and leaching showed a fractional release of Cs-137 and I-129 of 1.5% and 1.8% respectively.