Laboratory-scale work has been conducted to investigate the limits for reduction of PuO2, elucidate the kinetics of UO2 reduction, develop an electrowinning cathode for handling metallic lithium, and evaluate possible anode materials. Available thermodynamic information on both Pu2O3 and PuCl3 introduces uncertainties about the extent of reduction of Pu2O3 and the behavior of the resulting PuCl3 in the electrorefiner.
Several anode materials have been tested. Platinum performs well and has low overpotential but is expensive and reacts with both metallic lithium and gaseous chlorine. Iron oxide (Fe3O4) is cheap and leads to an already existing corrosion product but presents thermal shock and fabrication difficulties. Doped tin oxide (SnO2) is cheap and is commercially available, but results in a high cell overpotential. The desired anode reaction is production of gaseous oxygen from Li2O. Use of a high cell overpotential can result in production of gaseous chlorine, which would react with the anode material. Although several anode materials have been tested, including platinum, iron oxide, and doped tin oxide, the committee believes that further progress will be enhanced through analysis of published material in this area.
Work is in progress to study oxide fuel reduction kinetics, optimum fuel basket design for oxide fuel reduction and electrorefining, and development of methods for handling metallic lithium.
Based on the six engineering-scale tests conducted to date, the committee agrees that lithium electrowinning has been successfully demonstrated using a cathode consisting of stainless steel screen wrapped on a stainless steel rod, that progress has been made on understanding factors important in oxide fuel reduction kinetics, and that the reduction step can be interfaced successfully with the electrorefining step without carryover of metallic lithium or Li2O.
The feasibility of electrometallurgical treatment of aluminum alloy spent fuels has been demonstrated in laboratory-scale experiments. 18 The key step is electrorefining of the aluminum, which represents about 90% of the spent fuel volume and which can potentially be discarded as low-level waste.
ANL has developed a flow sheet in which initial separation of the aluminum as a metal waste is followed by separation of metallic uranium from fission products and transuranic elements.
An engineering-scale aluminum electrorefiner has been installed for further testing. Although the laboratory-scale work is promising, significant development problems remain to be resolved before the process can be adapted to engineering practice. Many of the difficulties being encountered with performance of the anode-cathode module are apt to be encountered with aluminum alloy fuels.
C. C. McPheeters, ANL-W, presentation to the committee, October 26, 1998, Chicago, IL.