Dissolution of spent nuclear fuel fragments at high alkaline conditions under H 2 overpressure
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Dissolution of spent nuclear fuel fragments at high alkaline conditions under H 2 overpressure E. González-Robles1, M. Herm1, V. Montoya1, N. Müller1, B. Kienzler1, R. Gens2, V. Metz1 1
Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal (KIT-INE), P.O. Box 3640, 76021 Karlsruhe, Germany 2 Organisme National des Déchets Radioactifs et des Matières Fissiles Enrichies / Nationale Instelling voor Radioactief Afval en Verrijkte Splijtstoffen (ONDRAF/NIRAS), Avenue des Arts 14, 1210 Brussels, Belgium ABSTRACT The long-term behavior of the UO2 fuel matrix under conditions of the Belgian “Supercontainer design” was investigated by dissolution tests of high burn-up spent nuclear fuel (SNF) in high alkaline solution under 40 bar of (Ar + 8%H2) atmosphere. Four fragments of SNF, obtained from a pellet previously leached during two years, were exposed to young cement water with Ca (YCWCa) under 3.2 bar H2 partial pressure in four single/independent autoclave experiments for a period of 59, 182, 252 and 341 days, respectively. After a decrease of the concentration of dissolved 238U, which is associated with a reduction of U(VI) to U(IV), the concentration of 238U in solution is constant in the experiments running for 252 and 341 days. These observations indicate an inhibition of the matrix dissolution due to the presence of H2. A slight increase in the concentration of 90Sr and 137Cs in the aqueous solution indicates that there is still dissolution of the grain boundaries. These findings are similar to those reported for spent nuclear fuel corrosion in synthetic near neutral pH solutions. INTRODUCTION In Belgium, for spent nuclear fuel (SNF) and vitrified high active waste disposal, a specific engineered barrier system, the so-called “Supercontainer” concept, has been developed. With respect to SNF disposal, the “Supercontainer” comprises fuel assembly canisters in carbon steel overpacks, which are surrounded by an overpack of a Portland cement concrete buffer and an outer stainless steel envelope [1-3].After re-saturation of the engineered barriers, the pore water composition will be altered by interactions with concrete. The altered pore water can be simulated by a "young cement water with calcium (YCWCa)" solution (pH of 13.5) that represents the first degree of concrete alteration. Due to the slow corrosion of the steel canister under highly alkaline conditions [4], the SNF could come in contact with the evolved highly alkaline solution. In parallel, in these systems a certain H2 partial pressure is built-up due to steel corrosion processes. Therefore, it is of interest to study the corrosion behavior of the SNF UO2 matrix in YCWCa in presence of externally applied H2 overpressure. MATERIAL AND METHODS Spent nuclear fuel
4163 Downloaded from https:/www.cambridge.org/core. University of Arizona, on 04 May 2017 at 09:57:36, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1557/adv.2017.204
The SNF material was irradiated during four cycles
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