Aqueous leaching of high burnup UO 2 fuel under hydrogen conditions
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.273
Aqueous leaching of high burnup UO2 fuel under hydrogen conditions Anders Puranen1, Olivia Roth1, Lena Z. Evins2, Kastriot Spahiu2 1
Studsvik Nuclear, Nyköping, Sweden
2
The Swedish Nuclear Fuel Waste Management Company, Stockholm, Sweden
ABSTRACT
Leaching results on fragments of high burnup (65 MWd/kgU) UO 2 fuel from a commercial pressurized water reactor are presented. The experiment was performed in simplified granitic groundwater under a hydrogen pressure of up to 5 MPa, representing conditions in a water intrusion scenario for a Swedish KBS-3 design spent nuclear fuel repository. The freshly crushed fragments were pre-washed for 6 days to remove preoxidized matrix and part of the instant release fraction of the radionuclide inventory, and then transferred to an autoclave for leaching under hydrogen conditions. Following an initial release of U attributed to dissolution of a pre-oxidized fuel layer caused by the aerated handling mainly during the transfer from pre-washing to autoclave, the U concentration decreased with time to levels of 2-5x10-9 M, which corresponds, approximately, to the solubility of amorphous UO2. The release of radionuclides such as Cs and Sr gradually declined indicating a transition to inhibition of the fuel matrix dissolution. Introduction In this work we continue the study of aqueous corrosion and dissolution of high burnup UO2 under hydrogen conditions in simplified groundwater [1]. Initial results (up to 70 days) from this autoclave experiment were previously presented along with another series of autoclave studies in [1]. High hydrogen pressures can be expected in a water intrusion scenario of a breached spent fuel canister in a deep geological repository, mainly due to formation of hydrogen from anoxic corrosion of iron which constitutes a major part of most deep geological disposal canisters, as well as from radiolysis of water. The reducing conditions imposed by hydrogen decrease the corrosion rates of spent UO2 fuel by preventing the oxidative dissolution of the UO2 matrix [2], which limits the matrix dissolution to its equilibrium with dissolved U(IV). Once the solubility of UO 2 (am) is reached at about 3x10-9 M [3] under conditions of stagnant granitic groundwater (pH ~8) the oxidative matrix dissolution should thus be inhibited. Several studies have
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been performed on this topic [4, 5, 6]; however, the number of studies on irradiated fuel with high burnup are limited, motivating further studies. Experiment A ca. 1 cm long section of a PWR UO2 fuel rod with a local pellet burnup of ~65 MWd/kgU was cut at an axial elevation of ~1 m from the bottom end plug using a pipe cutter. The location is just below the samples used in previous isotopic analysis an
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