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

Electro-reduction of solid UO 2 to U has been studied with molten CaCl 2 or LiCl as the electrolyte medium. Electro-reduction of thick (>3 mm), powder compacted and sintered pellets of UO 2 showed incomplete reduction resulting in a mixture of uranium metal and UO 2 . The extent of reduction of UO 2 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 UO 2 pellets in both CaCl 2 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 CaCl 2 (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 UO 2 pellet in CaCl 2 melt resulted in products with lower extent of reduction. Based on the measurements, a probable mechanism on the propagation of reduction through the solid UO 2 matrix during the electrochemical reduction process has been proposed.