Novel Approach to Extracting Transuranic Elements in Molten Salt Electrorefining

A novel approach to extracting transuranic elements (TRUs) from molten salt into liquid Cd using U metal as a reductant was investigated for the molten salt electrorefining process. We considered two methods of adding U metal: direct extraction (DE) and electrochemical extraction (EE). In the DE method, U metal added to Cd is dissolved and exchanged for TRU ions in the salt. The EE method is based on the principle of a concentration cell. When U metal and Cd separately placed in the salt are electrically connected, the U metal is anodically dissolved in the salt, and U and TRU ions are reduced at the Cd. The advantages of these methods over the conventional electrolytic method are as follows: The container for Cd can be made of steel, dendritic U metal does not form on the surface of the Cd or the crucible, and the operation is simple and stable. It was experimentally demonstrated that Pu and Am could be extracted from LiCl-KCl melt into liquid Cd by both the DE and EE methods when U metal collected at the solid cathode was used as a reductant. Crucibles made of steel could be used as containers for Cd, and a total of ∼3 wt% of U, Pu, and Am in the Cd was collected in 10 h. In the EE tests, the separation factors among U, Pu, and Am were always equal to the values at equilibrium. The rate-determining step for the extraction was not the mass transfer in the Cd or salt phase but the electron transfer at the Cd-salt interface. Then, a concept high-performance electrorefiner equipped with two anode–solid cathode modules and an EE or DE module was preliminarily designed.