Spent fuel matrix oxidation studies under dry storage conditions
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Spent fuel matrix oxidation studies under dry storage conditions Jone M. Elorrieta1, Laura J. Bonales1, Nieves Rodríguez-Villagra1, Valentín G. Baonza2 and Joaquín Cobos1 1
Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT, Avenida Complutense 40, 28040, Madrid, Spain. 2 MALTA-Consolider Team. Departamento de Química Física I, Facultad de Ciencias Químicas, Universidad Complutense, 28040, Madrid, Spain. ABSTRACT A good understanding of the spent fuel matrix (UO2) behavior under predisposal activities conditions is required for the proper performance assessment of a final repository. Hence, the oxidation evolution of UO2 under dry interim storage conditions, as a main predisposal action within the Spanish strategy, needs to be addressed. For this aim, in this work a detailed in situ Raman spectroscopy study of the surface oxidation of a UO2.00 disk heated in the presence of synthetic air at 573 K is presented. The spectra analysis required two previous studies. In the first one, UO2+x powder samples with controlled degree of non-stoichiometry were identified by thermogravimetric analysis and subsequently characterized by Raman spectroscopy. The equations obtained from this study enable estimating the oxidation degree of any UO2+x sample (for x < 0.20) at atmospheric conditions. The second one was performed in order to use these equations for the in situ experiments (at 573 K), since the shift of the bands due to temperature needs to be taken into account. Thus, the behavior of the Raman spectra as a function of temperature was analyzed and a correction term thereafter introduced in the initial equations. INTRODUCTION It is of great significance to understand the oxidation behavior of the spent nuclear fuel matrix (UO2) under dry storage conditions in order to correctly assess the feasibility of interim repositories. If any shielding failure occurs while storing this irradiated fuel, the UO2 matrix may oxidize in the presence of atmospheric oxygen and the high temperatures caused by decay heat [1], what entails an increase in volume of around 36% whether it becomes U3O8 [2]. The swelling produced under these conditions might make fuel pellets lose their integrity, what implies a risk for the future handling of such nuclear waste. UO2 presents a fluorite-type crystal structure (Fm-3m space group, f.c.c.) [3]. The UO2 lattice can easily accommodate additional oxygen ions owing to the presence of numerous empty interstitial sites, thus leading to hyperstoichiometric UO2+x [4]. The cubic phase is then preserved up to around x = 0.25, usually referred to as U4O9 [5]. Further oxidation produces a change from the cubic to the tetragonal structure, corresponding to U3O7, and then to orthorhombic U3O8 [6]. Micro-Raman spectroscopy is one of the main characterization techniques that are currently employed on this matter [7-14], due to the fact that it offers a series of advantages over other conventional techniques [15]; perhaps the most important is the possibility of performing in situ characteri
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