Reexamining the Dissolution of Spent Fuel: A Comparison of Different Methods for Calculating Rates
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Reexamining the Dissolution of Spent Fuel: A Comparison of Different Methods for Calculating Rates Brady D. Hanson and Ray B. Stout1 Radiochemical Science & Engineering Group, Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A. [email protected] 1 Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. ABSTRACT Dissolution rates for spent fuel have typically been reported in terms of a rate normalized to the surface area of the specimen. Recent evidence has shown that neither the geometric surface area nor that measured with BET accurately predicts the effective surface area of spent fuel. Dissolution rates calculated from results obtained by flowthrough tests were reexamined comparing the cumulative releases and surface area normalized rates. While initial surface area is important for comparison of different rates, it appears that normalizing to the surface area introduces unnecessary uncertainty compared to using cumulative or fractional release rates. Discrepancies in past data analyses are mitigated using this alternative method. INTRODUCTION Leaching experiments on UO2 and spent nuclear fuels have been performed to determine the rate of radionuclide release from fuel exposed to water in proposed geologic repositories. Since only the atoms at the surface of the specimen are actually subjected to contact with water, dissolution rates have typically been reported as normalized to the exposed surface area (e.g., mg m-2 d-1). The exposed surface area is usually determined as the geometric surface area, typically calculated from particle-size distribution measurements and often multiplied by a surface roughness factor [1], or measured using the Brunauer, Emmett, and Teller (BET) [2] method. Virtually all dissolution tests on UO2 and spent fuel exhibit decreasing rates as time and the extent of reaction increase [3-8]. For static or low-flow tests, such behavior can be explained by the formation of alteration products on the surface of the specimen that limit water contact with the matrix, decrease the sites for O2 reduction [9], or incorporate radionuclides and delay or prevent their release. However, for flowthrough tests where no back reactions, at least for uranium, occur, similar decreases have been observed [1, 6-7]. It is often assumed that the early initial releases are due to an oxidized layer of the fuel. Serrano et al. [8] have shown that this effect appears to be relatively transient and short-lived and does not explain the continuing decrease in reaction rate over time. Röllin et al. [5] reported that the decreases were a result of corrosion of the stainless steel frits used to hold the fuel in the column, leading to reduction of radionuclides by iron. While the reductive capacity of iron may be an issue, it does not explain the lack of reproducibility in tests, especially since corrosion of stainless steel under these conditions should be minimal. For example, Oversby [10] has stated that there is a difference in dissolution rates for SIMFUEL of up to a factor of 10
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