DETERMINATION OF OXYGEN TO URANIUM RATIO IN IRRADIATED URANIUM-DIOXIDE BY CONTROLLED POTENTIAL COULOMETRY

This work reports the determination of oxygen to uranium (O/U) ratio i n irradiated UO2+x fuel pellet of burnup of ca 34 GWd/t by controlled potential coulometry. The method is based on the dissolution of the nu clear fuel in strong phosphoric acid (SPA) at 180-190 degrees C under an inert atmosphere. After dissolution, 8% sulphuric acid is added in order to obtain a 20% SPA in 8% sulphuric acid. A controlled potential coulometric determination of uranium(VI) is carried out at -0.60 V vs . ferri-ferrocyanide. The uranium(IV) contained in an aliquot of the f uel solution is oxidised to uranium(VI) with cerium(IV) sulphate, and the total uranium content is then determined by coulometry. Optimum ex perimental conditions have been established using simulated irradiated fuel solution containing various fission products which include ceriu m, tellurium, palladium, ruthenium, molybdenum and zirconium. Interfer ence of the fission products and the possible removal of their interfe rences by preelectrolysis at +0.5 V vs. saturated calomel electrode (S CE) have been investigated. The accuracy of the coulometric method is confirmed by polarographic measurement using several unirradiated UO2x fuel of known stoichiometry.