Performance of Pyroprocess Equipment of Semi-Industrial Design and Material Balance in Repeated Engineering-Scale Fuel Cycle Tests Using Simulated Oxide/Metal Fuels

Fuel cycle tests using uranium and simulants with process equipment of 1 ton HM/yr throughput were conducted to develop an equipment design for long-term and hot-cell operation with stable performance, and to investigate the influence of impurities on the behavior of sensitive materials such as molten chlorides and active metals on material mass balance during repeated engineering-scale operations. These cycle tests were performed in two phases. The first phase simulated the introduction of spent oxide fuel into the metallic fuel cycle by the sequential operations of the UO₂ electroreduction, electrorefining of the reduction product, salt distillation using the electrorefining product, and injection casting of U-Zr alloy using the recovered uranium metal. The second phase, consisting of electrorefining, salt distillation, and injection casting, simulated the repeated metallic fuel cycle. The major achievements and results in these cycle tests are summarized as follows:

1. Simulated metallic fuel (U-Zr alloy rods) was successfully fabricated using UO₂ as the starting material.

2. The electrorefining, product transfer, salt distillation, and injection casting equipment operated satisfactorily, and their performance was sufficiently high, taking the target processing rate of 5 kg/day into account.

3. Regarding electroreduction, the reduction rate was approximately half the target value, and the cathodic current efficiency was also low. The reasons for the unsatisfactory result are considered to be Li₂O stagnancy at the cathode, the parasitic generation of lithium and the subsequent oxidation out of the cathode, and possibly the reaction between the reduced uranium and the oxygen gas evolved at the anode. Improvement of equipment design should be continued to moderate the influence of these factors on the electroreduction performance.

4. Favorable material mass balance of uranium, zirconium, and ruthenium (simulated fission products) was kept during the cycle tests, including the electrorefining, product transfer, salt distillation, and injection casting steps. No influence of three-time repetition of the fuel cycle tests was found from this viewpoint. The representativity of the anode residue and cathode product samples from the electrorefining step, which strongly influences the material mass balance evaluation, would be improved by performing anode residue treatment including metal waste consolidation and cathode processing for all the cathode products.