Instant Release Fractions from Corrosion Studies with High Burnup LWR Fuel.
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Instant Release Fractions from Corrosion Studies with High Burnup LWR Fuel. Ella Ekeroth1, Daqing Cui1, Jeanett Low1, Michael Granfors1, Hans-Urs Zwicky2, Kastriot Spahiu3, Lena Zetterström Evins3 1 Studsvik Nuclear AB, Hot Cell Laboratory, SE-611 82 Nyköping, Sweden 2 Zwicky Consulting GmbH, Chilacherstr. 17, CH-5236 Remigen, Switzerland 3 SKB, Box 250, SE-101 24, Stockholm, Sweden. ABSTRACT During irradiation in the reactor, a fraction of the fission product inventory will have segregated either to the gap between the fuel and the cladding or to the grain boundaries in the fuel. Of these nuclides, the behavior of the fission gases is best known. The part of the inventory that is rapidly released upon contact with water is designated the instant release fraction (IRF). Previous studies have shown that IRF and fission gas release (FGR) seem to be correlated. Studies of the instant release fraction from high burnup fuel is of interest for the assessment of the safety of a geological repository. The instant release fractions of 129I and 137Cs from five different light water reactor (LWR) fuel rods with a burnup range of 43 to 75 MWd/kgU and a fission gas release range from 0.9 to 5.0 % were studied. Four types of fuel samples (pellet, fragment, powder and fuel rodlet) have been used in the experiments. The results show that the fuel sample preparation method has a significant impact on the release from high burnup fuel samples over the time period covered by this study. Leaching of high burnup fuel samples with fuel detached from the cladding shows the highest release. The fractional 129I release from such fragment samples is similar to the FGR in the corresponding rod. On the other hand, corresponding fractional release of 137Cs is lower. INTRODUCTION The release of toxic and radioactive species from spent fuel in contact with water is expected to depend mainly on the dissolution rate of the UO2 matrix, since the majority of the inventory is uniformly distributed. Matrix dissolution is considered to be a slow process. However, a small percentage of certain fission products (e.g. I and Cs) migrate to grain boundaries and radially from the hot center to cooler regions close to the pellet periphery and the gap between the fuel and the cladding. In contact with water, these fission products are leached rapidly. This rapidly released part of the inventory is designated as Instant Release Fraction (IRF). Only a few studies with LWR fuel on instant release have been performed [1, 2, 3, 4], whereas the slow matrix dissolution term has been thoroughly studied. For the performance assessment of direct disposal of spent fuel in a geological repository, both matrix dissolution and instant release fractions need to be quantified and the chemical reactions understood. Fission Gas Release (FGR) is dependent on burnup, on local power and in particular on power transients where the thermal shock produces cracks and opens isolated sites. There seems to be a correlation between FGR and IRF. Iodine exhibits a diffusional release behavio
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