Surface Electrochemistry of Uranium Dioxide in Acidic Hydrogen Peroxide Solutions
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Surface Electrochemistry of Uranium Dioxide in Acidic Hydrogen Peroxide Solutions Mayuri Razdan1, David Hall2 and David W. Shoesmith1 1 2
Department of Chemistry, University of Western Ontario, London, ON N6A 5B7, Canada Department of Chemistry, University of Ottawa, Ottawa, ON K1N 6N5, Canada
ABSTRACT The electrochemical reduction of H2O2 on SIMFUEL was investigated over the pH range 1 to 4. The mechanism at pH 4 is known to occur on UV species incorporated into a surface layer of UIV1-2xUV2xO2+x. However, below pH 3, reduction occurs on an adsorbed UVO2(OH) state which is unstable and oxidizes to insulating UVI before dissolving as UO22+. Both schemes are observed at intermediate pH’s. The presence of both low and high acidic regions at the electrode surface is determined by the combination of peroxide concentration, bulk pH and the surface diffusion conditions. INTRODUCTION Long term management of spent nuclear fuel requires the study of its corrosion and dissolution under permanent waste disposal conditions [1], when the spent fuel would be enclosed in corrosion resistant copper containers and buried in the granitic rock of the Canadian Shield surrounded by compacted bentonite clay [2, 3]. These metallic containers are designed to survive until radiation fields become insignificant. However, if failure occurs, water radiolysis will produce hydrogen peroxide, which will drive fuel corrosion and a mixed potential model has been developed to predict the corrosion behaviour [4]. The production of H2O2 will complicate surface redox conditions since it can also decompose at the fuel surface to produce H2O and O2 [5-8]. Electrochemical studies have shown that peroxide reduction occurs in two steps: the creation of U(V) by chemically oxidizing the UO2 surface (1) followed by the electrochemical regeneration of U(IV) (2) 2UIV + H2O2 ĺ 2UV + 2OHí
(1)
2UV + 2eí ĺ 2UIV
(2)
Under natural corrosion conditions dissolution occurs as UO22+, followed by deposition of UO3.yH2O, which suppresses further fuel oxidation by blocking surface sites. In neutral to slightly alkaline ground water the overall corrosion process becomes limited by the slow release rate of UVI into solution [9]. However, the presence of pores or inhomogeneities (surface flaws, sintering porosity) in the corrosion product and the fuel could lead to local acidification at these sites, introducing a pH gradient and a chemical driving force to retain porosity and sustain film growth [9-11]. In acidic solutions (pH < 6) corrosion products would become unstable and redissolve, leading to increased dissolution rates [12]. Thus, though unlikely, corrosion of fuel in acidified peroxide conditions could constitute a rapid release pathway for radionuclides. In this paper, we report a study of the influence of acidic conditions on H2O2 reduction on UO2, in particular the effects of peroxide concentration and surface diffusion conditions.
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EXPERIMENTAL Experiments were performed on 1.5 at% SIMFUEL, an unirradiated analogue of spent nuclear fuel fabricated b
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