Determination of the extent of reduction of dense UO2 cathodes from direct electrochemical reduction studies in molten chloride medium

Electro-reduction of solid UO2 to U has been studied with molten CaCl2 or LiCl as the electrolyte medium. Electro-reduction of thick (>3 mm), powder compacted and sintered pellets of UO2 showed incomplete reduction resulting in a mixture of uranium metal and UO2. The extent of reduction of UO2 to U was determined by employing a novel method called ‘metal estimation by hydrogen sensor (MEHS)’, in which the hydrogen evolved during the reaction of U metal in the reduced product with con. HBr was measured using an in-house developed polymer electrolyte based amperometric hydrogen sensor. The results of our investigations on incompletely reduced UO2 pellets in both CaCl2 and LiCl melts showed that the extent of reduction of different regions of the oxide pellet was different. It varied from 88.3% on the surface of the pellet as against 3.7% towards the centre bulk during electro-reduction in CaCl2 (at 1173 K). The metallisation was found restricted to the surface of the pellets reduced in LiCl melt (at 923 K). Electro-reduction of small chunks of UO2 pellet in CaCl2 melt resulted in products with lower extent of reduction. Based on the measurements, a probable mechanism on the propagation of reduction through the solid UO2 matrix during the electrochemical reduction process has been proposed.