The electroreduction of oxide fuel is carried out by removing oxygen from the oxide fuel at the cathode and generating oxygen gas at the anode. Most studies are mainly focused on obtaining design parameters for the anode and cathode, to achieve high throughput and high reduction yield. In the case of a cathode to charge spent oxide fuel; however, the basket has con­tradictory design requirements. According to visual examination of incom­plete reduction products, the reaction proceeds from the outside to the inside of the oxide particle. This suggests that diffusion in the grain bound­ary or in cracks in the particle is the rate-determining step. To complete the reaction earlier, a cathode basket with smaller holes is better for charging smaller oxide particles. On the other hand, a cathode basket with larger holes is required to accelerate the circulation of molten salt to diffuse oxygen ions from the inside to the outside of the basket. With regard to the importance of particle size, KAERI has developed an engineering-scale electroreduction cell using a porous MgO crucible (18 cm ID x 42 cm H) as the cathode basket, surrounded by six platinum anode rods (3 cm OD x 30 cm H). Figure 10.25 shows an electroreduction cell of 50 cm ID, which has been used only for on-irradiated materials. When 10 kg of U3O8 powder (10-30 ^m) was charged in this electroreduction cell, more than 99% of the reduction reaction was completed in about 100 h (Jeong, 2008). As the concentration of Li2O in the molten salt decreased with time, the permeabil-

ity of oxygen ions through the MgO crucible was found to be very slow, suggesting that the holes in the MgO crucible were too small. However, INL employed a percolated stainless-steel basket to charge crushed spent oxide fuel particles with 0.45-2.8 mm diameter (Herrmann, 2005) but no decrease in Li2O concentration was observed, and it took 36 h to complete the reduction reaction of 41 to 50 g of spent fuel. INL continued reduction experiments using voloxidized spent oxide fuel powder of less than 45 ^m diameter in small baskets made of stainless-steel mesh, with sintered stain­less-steel frit (Herrmann, 2007). With either mesh or sintered frit stainless — steel baskets, a higher current can be passed for the powder form than for crushed particles. As an alternative, CRIEPI has proposed processing porous chunks from the voloxidized fuel powder for charging in a stainless — steel basket with a large opening. According to experiments, using roughly sintered UO2 pellets with a porosity of approximately 30%, the time to complete the reduction reaction was less than 10 h and was unaffected by an increase in the amount of fuel pellets from 10 g to 100 g (Sakamura, 2008). Although the procedure is more complex, it is one possible method for realizing practical throughput.

With regard to anode design for generating oxygen gas, developments have been mainly focussed on the durable anode material, which will be described in the next section. An effective evacuation of the oxygen gas from the vicinity of anode is another key issue for anode design.