Effects of matrix composition and sample preparation on instant release fractions from high burnup nuclear fuel
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Effects of matrix composition and sample preparation on instant release fractions from high burnup nuclear fuel O. Roth1, J. Low1, K. Spahiu2 1 Studsvik Nuclear AB, Hot Cell Laboratory, SE-611 82 Nyköping, Sweden 2 SKB, Box 250, SE-101 24, Stockholm, Sweden. ABSTRACT The rapid release of fission products segregated either to the gap between the fuel and the cladding or to the UO2 grain boundaries from spent nuclear fuel in contact with water (often referred to as the instant release fraction - IRF) is of interest for the safety assessment of geological repositories for spent fuel due to the potential dose contribution. In September 2012 a study was initiated with the aim of comparing the instant release behavior of fuels with and without additives/dopants. Preliminary results from this (ongoing) study indicate that the release of uranium during the first contact periods was higher than during the tests with fuel segments, even though the fuel was cut open recently [1]. This could be due to the sample preparation method which included axial cutting of the cladding in order to remove the fuel fragments used in the study. In the present work, leaching data from both studies are presented and the releases are discussed comparing the two sample preparation methods and considering the effect of matrix composition. The leaching studies have been performed in air using 10 mM NaCl + 2 mM NaHCO3 as leaching solution. INTRODUCTION The performance assessment of geological repositories for direct disposal of spent fuel requires understanding of the processes involved in the release of radionuclides from the spent fuel upon contact with ground water. The majority of the radionuclides are distributed within the UO2 matrix of the fuel and their release will be governed by the matrix dissolution. However, a fraction of certain volatile and segregated fission products will be leached from the gap between the fuel and the cladding, from the UO2 grain boundaries or from separate phases within the fuel. The leaching rate varies depending on the origin of the nuclide (matrix/gap/grain boundary etc.) and the leaching behavior of a specific fission product will be strongly dependent on its chemical state. Elements leached from the gap and grain boundaries (i.e. I and Cs) are generally released rapidly upon contact with water and therefore referred to as the “Instant Release Fraction” (IRF). Previous studies have shown that the IRF is often comparable to the released fission gases (FGR) as measured in gas release testing of fuel rods [2-5]. This relationship can be explained by the fact that under reactor operation temperatures volatile and segregated fission products are usually in gaseous state which facilitates their migration to the grain boundaries and the fuel/cladding gap. The new fuel types used today (i.e. additive fuels) generally have lower FGR than traditional standard fuels when operated under the same conditions and it is reasonable to believe that also the IRF should differ between the two fuel types. In order to investigate
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