Surface Site Densities of Uranium Oxides: UO 2 , U 3 O 8
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6XUIDFH6LWH'HQVLWLHVRI8UDQLXP2[LGHV 8282 F. Clarens, J. de Pablo, I. Casas, J. Giménez and M. Rovira. Chemical Engineering Department. Universitat Politècnica de Catalunya. 08028 Barcelona. Spain $%675$&7 The estimation of the surface site density for UO2 (main component of the SF matrix) is an important aspect to take into account in the development of radiolytical models for the dissolution of the spent nuclear fuel (SF). Also, other oxides can be formed on the SF surface due to the effect of radiolytically-formed oxidizing species. Due to the lack or reliable data in literature we have studied the surface site densities of two uranium oxides: UO2 and U3O8. For this determination, the knowledge of both the reactive surface area and the surface charge of the solid are necessary. In this work the reactive surface area of the two solids was determined by the BET method while the surface charge was determined by potentiometric acidbase titrations of suspensions of the solids at different ionic strengths (0.001, 0.01, and 0.1 mol·dm-3) under N2 atmosphere. The amount of adsorbed protons was calculated by subtracting blank titration from the solid suspension titration. The single-site nonelectrostatic model (NEM) was used to describe titration data. Uranium speciation in solution was included in the model as well. The surface area values obtained were 0.15 ± 0.01 m2· g-1 for UO2 and 0.77 ± 0.02 m2· g-1 for U3O8, while the surface acidic site densities were determined to be 165 ± 10 sites·nm-2 and 48 ± 3 sites·nm-2 for UO2 and U3O8, respectively. ,1752'8&7,21 The dissolution rate of spent nuclear fuel (SF) is a key process in the Performance Assessment (PA) of the High Level Nuclear Waste (HLNW) final disposal. A key parameter that controls both dissolution/precipitation and sorption processes in the near-field when the groundwater interacts with the SF is the surface sites density of the solid. For the determination of this parameter it is necessary to know both the surface area and the surface charge of the solid. The SF is mainly composed of UO2, however, other oxides with a higher uranium oxidation state can be formed when the groundwater is in contact with the SF due to the formation of oxidizing species by the radiolysis products of water. For this reason, in this work we have studied both the surface area and the surface charge of two different uranium oxides, UO2 and U3O8, in order to determine the value of their surface sites density. The surface area has been determined by the N2-BET method [1], while the surface charge has been obtained by potentiometric titrations of suspensions of the solids, as described by other authors for different solid phases [2-9]. However, when studying the uranium oxides, an additional problem is found due to the low surface area of these oxides. As it can be seen in Table 1, the surface area of UO2 or U3O8 are between one and two orders of magnitude lower than the determined for the other solids. This low surface areas can lead to problems of nonconvergence of the pro
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