Assessing the role of spent fuel surfaces during leaching in presence of hydrogen by using Cr(VI) as a redox marker
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Assessing the role of spent fuel surfaces during leaching in presence of hydrogen by using Cr(VI) as a redox marker A. Puranen1, E. Ekeroth1, M. Granfors1, J. Low1 and K. Spahiu2 1 Hot Cell Laboratory, Studsvik Nuclear AB, SE-611 82 Nyköping, Sweden 2 Swedish Nuclear Fuel and Waste Management Co., 10124 Stockholm, Sweden ABSTRACT In many deep repository concepts spent nuclear fuel (SNF) will be disposed in canisters containing large amounts of iron. Intrusion of groundwater in a failed canister may occur under the presence of hydrogen, expected to be produced by the anoxic corrosion of iron. Compelling evidence now exists that hydrogen inhibits oxidative dissolution of SNF, the mechanism is however not fully understood. Hydrogen generally requires a catalyst in order to operate as a reductant. The metallic inclusions (ε-particles) present in SNF are a likely catalyst for this process due to their noble metal content. There is also evidence that the SNF UO2 matrix or doping of the UO2 with fission products can activate hydrogen. In most spent fuel experiments carried out under hydrogen, a decrease in concentration of all redox sensitive nuclides originating from a pre-oxidized layer is observed. Given their low concentrations and abundance in the fuel, it has however been difficult to detect any reductive precipitation on the fuel surfaces. In this study, Cr(VI) oxyanions were employed as a redox sensitive marker, as Cr(VI) is expected to precipitate as Cr(III) oxide on the catalyst that activates hydrogen. In the experiments PWR spent fuel (43 MWd/kgU) was leached in simulated groundwater (10 mM NaCl, 2 mM NaHCO3) at 25 and 70 ◦C under 5 MPa of hydrogen and dissolved Cr(VI). Dark green, Cr(III)-oxide was found to precipitate; mapping by electron microscopy (SEMWDS) evidenced a Cr rich layer covering the fuel, suggesting that the whole fuel surface is catalyzing the reduction of chromium. INTRODUCTION In a deep repository, such as the proposed Swedish KBS-3 concept, spent nuclear fuel (SNF) dissolution following the intrusion of groundwater in a failed canister is likely to occur under the presence of large concentrations of dissolved hydrogen, produced mainly through the anoxic corrosion of iron [1, 2]. The experimental data on spent fuel and alpha doped UO2 leaching under hydrogen show that their radioactive surfaces play an important role [3]. The decrease of the concentration of redox sensitive nuclides, observed in practically all tests under hydrogen, seems to be due to their reductive precipitation on the fuel surface itself. However, a direct proof for this reductive precipitation is missing. Indirect confirmation for the precipitation on the fuel surface was reported e.g. in [4] finding less than 1% of the precipitated uranium in the vessel strip, while in [5, 6] almost no U was found in the metallic iron powder nearby the fuel pellet. The noble metal inclusions present in SNF are likely catalysts for this process [7-10]. There is also evidence that the SNF UO2 matrix itself or doping of the UO2 matrix with
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