Corrosion Behavior of High Burnup Spent Fuel in Highly Alkaline Solutions
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Corrosion Behavior of High Burnup Spent Fuel in Highly Alkaline Solutions A. Loida 1, R. Gens 2, V. Metz 1, K. Lemmens 3, C. Cachoir 3, T. Mennecart 3 and B. Kienzler 1 1 Forschungszentrum Karlsruhe, Institut fur Nukleare Entsorgung (FZK/INE), P.O. Box 3640, D-76021 Karlsruhe, Germany 2 ONDRAF/NIRAS, Avenue des Arts 14, B-1210 Brussels, Belgium 3 SCK•CEN, Boeretang 200, B-2400 Mol, Belgium ABSTRACT This study is focused on the alteration behavior of spent nuclear fuel when exposed to highly alkaline groundwater. Contact of highly alkaline solution with the waste product is considered in the Belgian concept for disposal in the Boom Clay formation. According to the “supercontainer design” the fuel will be encapsulated in carbon steel canisters, surrounded by a concrete overpack. After saturation of the engineered barriers by porewater, interactions with the concrete will result in solutions rich in NaOH, KOH and Ca(OH)2. Using this type of solution at pH 12.5, spent nuclear fuel corrosion experiments were conducted over 378 days. Under anoxic conditions, parallel experiments were performed (a) in the absence of Fe phases and (b) in the presence of solid Fe phases representing container (corrosion) products. Both types of experiments resulted in relatively low matrix dissolution rates, around 10-7 per day, according to the fractional release of Sr. Solution concentrations of actinides are close to or below the detection limit, indicating an effective retention of these radioelements in the system. The observed precipitation of a Ca rich phase onto the surfaces of the corroded fuel samples may be related to the inhibited release of actinides, Sr and other matrix bound radioelements. INTRODUCTION In Belgium, the so-called “supercontainer design” is envisioned for the final disposal of spent nuclear fuel (SNF) in a clay formation [1]. It comprises the encapsulation of the waste products in carbon steel overpacks surrounded by huge amounts of concrete. In the case of porewater intrusion into the repository and after re-saturation of the engineered barriers, the solution composition will be dominated by interactions with concrete. Main components of the altered solution will be NaOH, KOH and Ca(OH)2, while the pH will be shifted to the highly alkaline region (pH > 12). Corrosion of the Fe-based overpacks and fuel containers is significantly reduced under very alkaline conditions. Potentially, solution may penetrate through containers and react with the SNF rods. In a previous study, the corrosion behavior of depleted UO2 and UO2 doped with α-emitters 233 ( U and 238Pu) was studied at 11.7 pH 13.5. In most of these experiments, an aqueous concentration of uranium around 10-8 mol•L-1 was observed. Based on the observed U release, a UO2 dissolution rate ranging from 37 to > 1900 µg m-2•d-1 was derived [2,3]. Different to corrosion of (α-doped) UO2, the corrosion behavior of spent nuclear fuel is controlled by a mixed α-, β- and γ-radiation field producing considerable amounts of oxidizing and reducing radiolysis products i
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