Powder Leaching Study for Grain Boundary Inventory of Two High Burnup Fuels
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2018.683
Powder Leaching Study for Grain Boundary Inventory of Two High Burnup Fuels Alexandre Barreiro Fidalgo1, Olivia Roth1, Anders Puranen1, Lena Z. Evins2, Kastriot Spahiu2, Charlotta Askeljung1 1
Studsvik Nuclear AB, Nyköping, Sweden
2
The Swedish Nuclear Fuel Waste Management Company (SKB), Stockholm, Sweden
ABSTRACT
In the context of safety assessment, the fraction of inventory that is expected to rapidly dissolve when water contacts the spent fuel is called the Instant Release Fraction (IRF). Conceptually, this fraction consists of radionuclides outside of the uranium dioxide matrix and therefore the fraction can be further divided into the radionuclides in the fuel/cladding gap and radionuclides in the grain boundaries. The relative importance of these two fractions is investigated here for two Swedish high burnup fuels through simultaneous grinding and leaching fuel fragments in simplified groundwater for a short period of time. The hypothesis is that this will expose grain boundaries to leaching solution and provide an estimate of the release of the grain boundary inventory upon contact with water. The studied fragments were used in previous leaching experiments and thus pre-washed to remove any pre-oxidized phases. The results showed a significant release of iodine, cesium and rubidium and to a lower extent molybdenum and technetium. The fraction of inventory in the aqueous phase of actinides and lanthanides was 1-2 orders of magnitude lower than for the elements associated to the IRF. Both fuels displayed a very similar behavior and no correlation as a function of burnup or fission gas release was found.
INTRODUCTION: Understanding the mechanisms and release rates of radionuclides is critical for the safety assessment of geological repositories of spent nuclear fuel. In an event of canister failure and water intrusion, a fraction of the inventory, consisting of volatile elements that were mobile during reactor operation, will be rapidly released. This rapidly
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leached fraction of the radionuclide inventory is usually referred as the Instant Release Fraction (IRF). These radionuclides are expected to have migrated out of the UO2 matrix into the fuel/cladding gap and, to some extent, grain boundaries (GB). These two contributions are however difficult to differentiate in leaching experiments. After this initial release, the corrosion and dissolution of the fuel matrix is expected to govern the release of most radionuclides present in the fuel. Correlations have been used to predict the IRF from the Fission Gas Release (FGR), i.e. the fraction of fission gas that has migrated to the fuel/clad gap and can be measured during Post-Irradiation Experiments via puncturing of the cladding. Previous
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