URANIUM-DIOXIDE LEACHING MECHANISM IN A SYNTHETIC GRANITIC GROUNDWATER IN OXIC CONDITIONS IN FUNCTION OF S V RATIO AT 96-DEGREES-C/

UO2 dissolution experiments under oxic conditions have been performed for one, two and five months at 96 degrees C in function of surface to volume (S/V) ratio. The leachant is representative of a groundwater i n equilibrium with a granitic medium. Two different results in functio n of time and S/V ratio were found. On the one hand, no sign of precip itation of secondary phases has been observed and uranium concentratio n is around 10(-9) mol . l(-1). On the other hand for the five months lest, yellow perfectly crystallized phases, identified as schoepite, h ave been found on the surface of the UO2 pellets correlated to an uran ium concentration equal to 2.77 . 10(-4) mol . l(-1).