Surface site densities of uranium oxides:: UO2, U3O8

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 acid-base titrations of suspensions of the solids at different ionic strengths (0.001, 0.01, and 0.1 mol(.)dm(-3)) under N-2 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 m(2.)g(-1) for UO2 and 0.77 +/- 0.02 m(2.)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.