Low Current Efficiency in MOX Deposition Tests

A series of MOX deposition tests has been performed since 2001 at RIAR to clarify its complex phenomena and to improve its poor current efficiency. In the 2001 tests, the cathode current efficiency was between 60 and 100% but the Pu fraction in the MOX was between 5 and 20%. In 2002 tests, the fraction was raised to more than 30% by modifying the test conditions but the current efficiency fell to between 20 and 60%. A new method was proposed to simulate the parasitic current due to the electrode reactions of UO22+/UO2+, Pu4+/Pu3+ and Fe3+/Fe2+ at the cathode. It was found that the parasitic current due to the UO22+/UO2+ reaction significantly lowers the current efficiency especially when the cathode potential is kept near the equilibrium value during the electrolysis to increase the Pu fraction in the MOX deposit.