Dissolution of UO 2 (S) in MgCl 2 -Brines Under Different Redox Conditions.
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DISSOLUTION OF UO 2 (S) IN MgCl -BRINES UNDER DIFFERENT REDOX CONDITIONS.
IGNASI CASAS, J.GIMENEZ, J.DE PABLO AND M.E.TORRERO. Department of Chemical Engineering, Universitat Polit~cnica Catalunya E-08028 Barcelona, Spain.
ABSTRACT The dissolution of unirradiated UO 2(s), with a particle size of 1 mm, has been studied in MgCl brines at 298 K under both reducing and oxidizing conditions. Results obtained under reducing conditions (H atmosphere in the presence of a palladium catalyst) show an initial increase of the total uranium concentration in solution and a subsequent decrease until equilibrium (or steady state) values are reached. Results obtained under oxidizing conditions (nominal oxygen partial pressures of 0.05, 0.21 and 1 atm) show two different trends. A relatively fast initial dissolution rate and, after approximately two or three weeks, a slower dissolution rate. X-Ray Photoelectron Spectroscopy (XPS) has shown that the UO 2 surface composition changes during the experiment. INTRODUCTION Disposal of spent nuclear fuel in salt formations is being evaluated in several countries like Germany and Spain. The knowledge of the UO matrix dissolution in salt brines is very important in order to assess the saf~ety of this kind of repository. Previous results have shown some discrepancies concerning whether the brine media are or not very aggresive for the UO 2 and the spent fuel dissolution. Wang and Katayama [I ] reported the formation of a thin layer on the unirradiated uranium dioxide surface in NaCl-brines which passivates the dissolution in some cases. They obtained a dissolution rate of 1.5 10-7 mol h-' m- 2 which was much lower than the one obtained in hydrogen carbonate solutions. On the other hand, Brodda and Merz [2] studied the dissolution of spent HTR fuel in Q-brine. They indicated that this medium is very aggresive, giving a uranium dissolution rate of 1.3 10-5 mol h1 M-2 in Q-brine equilibrated with air. The comparison of dissolution rates reported by different authors who used different materials is sometimes difficult for several reasons. One of them is that the surface area used in the calculations can be estimated from several methods: geometric area, BET measurement etc. The differences are sometimes of orders of magnitude. In contrast, irradiation of the material seems to be less important as shown by Gray [3]. Another important and controversial parameter is the redox potential. However, the influence of redox conditions and the effect of the different oxygen partial pressures have not been widely studied in highly saline environments. The objective of this work is to ascertain the effect of both reducing and oxidizing conditions on the dissolution of unirradiated uranium dioxide in MgCl 2 brines as well as to determine the influence of the oxygen partial pressure on the dissolution rates.
Mat. Res. Soc. Symp. Proc. Vol. 294. '1993 Materials Research Society
68
EXPERIMENTAL The solid phase used in this work is an unirradiated crystalline UO2 with 1 mm particle size. This particle size w
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