Study of the leaching behaviour of sintered UO 2 in groundwater using nuclear microprobe techniques
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Study of the leaching behaviour of sintered U02 in groundwater using nuclear microprobe techniques Patrick TROCELLIER, Jean Paul GALLIEN and Christelle CACHOIR CEA-CNRS, LaboratoirePierreSte, Centre d'Etudes de Saclay, 91191 Gif sur Yvette (France). Pierre TOULHOAT CEA, Section de G~ochimie (DCC/DESD/SESD), Centre d'Etudes de Fontenayaux Roses, 92265 Fontenay aux Roses (France). Abstract The leaching behaviour of sintered U0 2 pellets has been studied in synthetic groundwater at 96 *C under strictly controlled conditions (CO 2 partial pressure, 02 partial pressure, pH and redox potential). Results obtained in oxidizing conditions (02 content varying from 5 to 25 ppm) show a strong initial uranium release in solution during the first five days and a solubility control essentially driven by the growth of U(VI) species such as oxides, hydrates and carbonates. Results obtained in reducing conditions (use of a mineralogical buffer of Ni(0)/Ni(ll) with E0 = -0.257 Volts) show a very weak uranium release in solution close to 10-11 mole/I and a solubility control essentially driven by the formation of U(IV) compounds such as coffinite or uraninite. 1. Introduction The concept of direct disposal of used nuclear fuel in a geological vault is being investigated for a long time in several countries [1, 2, 3]. It is thus of primary importance to well understand the corrosion mechanisms of UO2 to be able to perform long term predictions concerning the behaviour of nuclear fuel. In the work reported here, we studied the corrosion of unirradiated sintered UO2 in groundwater under oxidizing and/or reducing conditions as it was described by Gallien [4]. Composition of both leachates and surfaces have been investigated and analytical results were discussed and compared with theoretical calculations. 2. Experimental 2.1 Samples Uranium dioxide was supplied by the CEA Nuclear Research Centre in Grenoble. It corresponds to isotopically depleted nuclear fuel (0.3 % 235U) with a density of about 10.42 and an average initial stoichiometry around 2.1 [4]. Sampling consisted in cutting 3 mm thick circular pellets (8 mm of diameter) with a diamond saw under flowing water. Polishing using decreasing grain size of alumina (35, 26, 9, 5, 3 and 0.3 pm respectively) is then performed [5]. Surface roughness and U/O ratio are estimated using scanning electron microscopy and X-ray microanalysis. 2.2 Leaching conditions A groundwater solution in equilibrium with its mineral constituents has been synthesized and used as leachant. It corresponds to a granitic groundwater from Pyr6ndes near the France/Spain border named "Exalada" ; its typical composition is indicated in table 1. A dedicated device has been specially built and tested to perform leaching tests in fully controlled conditions. This apparatus described in details elsewhere [61 is shown figure 1. A variable partial pressure of CO 2 has been imposed to the leachant to control its carbonate content, 400 ppm, 9000 ppm, 9 % and 18 % respectively. The temperature has been fixed at 96 °C and the le
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