Dissolution of UO 2 by One- and Two-Electron Oxidants
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'LVVROXWLRQRI82E\2QHDQG7ZR(OHFWURQ2[LGDQWV Mats Jonsson, Ella Ekeroth and Olivia Roth Department of Chemistry, Nuclear Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden $%675$&7 In this work, the efficiency of one- and two-electron oxidants in oxidative dissolution of UO2 has been investigated. This was accomplished by measuring the U(VI)-concentration in solution after exposing UO2-powder to controlled amounts of oxidants in aqueous solution. The oxidants used in this study are H2O2, IrCl62-, CO3•- and OH•. H2O2 acts as a two-electron oxidant while the remaining three oxidants are one-electron oxidants. CO3•- and OH• were generated using γradiolysis. The results clearly show that the dissolution yields for one-electron oxidants (per electron pair) are lower than the yields for two-electron oxidants. Furthermore, the yields for oneelectron oxidants increase with increasing amount of oxidant (especially at low amounts of oxidant). The rationale for this is that U(VI) is the main soluble species which is formed directly upon two-electron oxidation. For one-electron oxidants the primary oxidation product is U(V) which can form U(VI) upon reaction with a second oxidant. The probability for a second oxidation is however low at low concentrations of oxidant. ,1752'8&7,21 Spent nuclear fuel will be stored in a deep repository according to the Swedish model, KBS3, with four barriers to prevent radionuclide release to the environment. The innermost barrier is the UO2-matrix of the fuel itself. UO2 has low solubility in reducing ground water [1]. However, radiolysis of ground water will produce oxidants (and reductants) that alter the otherwise reducing conditions. Radiolysis of water will produce both radicals and molecular products e.g. OH•, HO2•, H2O2, O2 (oxidants) and H•, eaq- and H2 (reductants) [2]. With carbonate present in the ground water, also CO3•- will be formed. A key question that has been discussed for several years is if and how the radiolysis products will increase the rate of dissolution of spent fuel [3]. In numerous studies, spent fuel and radionuclide doped UO2 have been used to address this issue [47]. The main problem with these studies is that these systems are far to complex to draw conclusions concerning the reactivity of individual products. This is a prerequisite for reliable theoretical modeling and long-term predictions of spent fuel dissolution. To circumvent this problem, a number of studies on the reactivity of individual radiolysis products towards UO2 have been conducted. Most of these studies have been focused on the primary radiolysis product H2O2 and on O2 [3]. Based on the results of these studies, theories on the mechanism of oxidative UO2 dissolution have been put forward. Recently, we studied the kinetics of UO2 oxidation using a number of one- and two-electron oxidants [8]. Interestingly, the logarithm of the rate constant for consumption of the oxidant was found to depend linearly on the one-electron reduction potential of the oxidant, also f
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