(Invited) Consumable Fe Anode for Use in Direct Electrolytic Reduction of Spent Oxide Fuel
Authors: Gonzalez Mario Alberto , Simpson Michael 
Published: September 2023

The direct electrolytic reduction (DER) process can be used to reduce UO2 to U metal in a molten LiCl-Li2O electrolyte. In the baseline process, UO2 is loaded in a steel basket that serves as the cathode, while O2 gas is generated at a Pt anode. The use of Pt for anodes is non-optimal because of its cost and tendency to corrode via lithium platinate formation during the DER process. Other inert metals and cermets for DER anodes have been extensively researched to reduce the cost. Alternatively, reactive anodes may be considered that form disposable metal oxides rather than O2 gas. Thus, we investigated the behavior of pure iron rods as anodes for this process. Experiments were performed using a LiCl-2wt%Li2O electrolyte at 650oC in an Ar atmosphere glove box with <10 ppm O2 and H2O. UO2 powder was loaded into a stainless-steel cathode basket. A 7.56-mm diameter iron rod was used as the anode. Anode and cathode potentials were measured relative to a Ni/NiO reference electrode encased in a MgO tube with a porous MgO plug. The cell was run via controlled anode potential with increments from 0.2–1.0 V versus Ni/NiO. The headspace gas was sampled during experiments and analyzed for O2. Throughout the experiment, the cathode potential was consistent with either direct reduction of UO2 or reduction of Li2O, which forms Li that will also reduce UO2 to U metal. LECO oxygen analysis indicated 32% reduction of UO2 with a cell efficiency of 37%. Li2O concentration remained constant in the salt; thus, net oxidation must have occurred at the anode. The Fe anode was pulled out of the salt and photographed after stages of holding the potential at 0.2–1.0 V versus Ni/NiO in 0.1 V increments for about 11-12 min at each stage. Yet unidentified metal oxides were observed on the anode surface after holding at potentials of 0.4 V and 0.6–1.0 V. O2 was detected in the headspace gas when the anode potential was increased to 1.0 V. Thus, iron oxides form at lower potentials, while O2 gas formation occurs at a threshold potential. O2 formation may be enabled by the oxide layer passivating the iron against further oxidation. Results of ICP-MS of salt samples taken throughout the experiment showed no increase in Fe concentration in the salt. A metallic deposit formed on the outside of the cathode basket, implying some electrotransport occurred from the anode to the cathode. The results of this study show that cheap, disposable iron anodes are promising for the DER process. After an initial stage of formation of an iron oxide phase on the surface of the anode, O2 gas forms at a sufficiently high potential (1.0 V versus Ni/NiO). Further work is needed to achieve a more detailed mechanistic understanding of the effect of anode potential on specific reactions at the anode. Optimization of the anode potential may serve to promote O2 formation and minimize the rate of consumption of iron anodes.
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