Nuclear Technology / Volume 164 / Number 3 / December 2008 / Pages 433-441
Technical Paper / Reprocessing
Previous studies for electrochemical reduction using uranium oxide have shown that reduction was completed within several tens of hours when particles or powders of oxide were used for the cathode material. In the case of mixed oxide (MOX) fuel prepared for fast reactors, there are two significant differences with respect to uranium oxide fuel for light water reactors. The MOX fuel contains ~30% Pu and a small amount of Am. The density of the uranium oxide pellet and MOX pellet is ~98% and ~85% with respect to theoretical values, respectively. These differences decrease the electroconductivity of oxide and the reaction rate. Also, the behavior of transuranic elements has not been certified. In the present study, electrochemical reduction of MOX pellets was performed by setting the pellets directly on the cathode in a molten lithium chloride bath. Reduction was completed after ~15 h, even when using MOX pellets. This value compares closely to the previous values for uranium oxide particles or powders. Current efficiency was varied at ~60%, which is slightly higher than in the previous study. The lower density of MOX allows better diffusion of the molten salt into the pellet and contributes to efficient electrolysis. Concerning actinide behavior during electrolysis, the uranium and plutonium concentrations in the molten salt bath were lower than their detection limits. Although a small amount of americium was dissolved in the molten salt bath and gradually accumulated, the amount was <1% with respect to the initial amount. The oxygen concentration in the molten salt decreased gradually during electrolysis. These variations in the salt hardly affected the current efficiency and the actinide recovery ratio. These observations indicate that the electrochemical reduction of MOX pellets is applicable to industrial processes.