Demonstration of the Separation of Actinides from Lanthanides By Electrolysis with in-Situ Voltammetry

Abstract

Electrolysis can be used to recover actinides and lanthanides from electrorefiner salt to facilitate fission product removal and salt recycle in pyrochemical flowsheets. High separability between actinides and lanthanides is a key goal in meeting pyrochemical flowsheet requirements such as maximizing actinide recovery and minimizing salt discharge to waste. The core separation unit in a pyrochemical system is the electrorefiner that separates uranium and transuranics (TRU) from a metallic fuel or reduced oxide fuel through oxidation of the metals at an anode and electrodeposition of the metal at a cathode. After this separation, in which uranium and U+TRU are recovered by cathodic deposition, the electrorefiner salt consists of a mixture of dissolved actinide, and lanthanide chlorides and Group 1 and 2 fission product chlorides in eutectic LiCl/KCl. The ratio of actinides and lanthanides are of critical importance to the purity of the actinide product recovered and must be carefully controlled to minimize lanthanide content in the recovered U+TRU product. Therefore periodic drawdown of the accumulated lanthanide chlorides is required. Reduction of the lanthanide chlorides to metals offers the greatest degree of actinide/lanthanide separation. Because the lanthanide chlorides are more stable than actinide chlorides, it is necessary to first drawdown all of the actinide species. Once the actinides are removed, a similar drawdown operating at more reducing conditions can be used to remove lanthanides from the molten salt. Although the actinide free salt could be disposed of in a suitable wasteform, a lanthanide drawdown followed by concentration of Group 1 and 2 fission products allows for recycle of cleaned electrorefiner salt, thus minimizing the volume of high-level waste generated by the overall process. Drawdown of actinides and lanthanides by electrolysis has two advantages over the addition of a chemical reductant. First, addition of a chemical reductant adds to the waste volume. Secondly, electrolysis simplifies recovery of the actinides and lanthanides because reduction of the metals occurs in the localized area of the cathode surface. Thus the goal of this work is to examine the degree of group separability that can be achieved between actinides and lanthanides in an electrolysis drawdown operation. In a set of electrolysis feasibility experiments, deposition of metals occurs at a central graphite cathode and chlorine gas evolves at three graphite anodes placed equidistant from the cathode. The desired depositing metals are selected by careful control of electrode surface area and potential versus a Ag/AgCl reference electrode. An electrolysis cell of novel design was paired with a chlorine gas scrubbing system to perform the experiments. A set of tungsten working and counter electrodes were used to conduct in-situ voltammetry of the bulk. Voltammetry was used to monitor the concentration of actinides and lanthanides present in the bulk salt during electrolysis. The feasibility demonstration was conducted in three phases, first the reduction of uranium from eutectic LiCl/KCl containing UCl3, second the reduction of uranium from the same salt with the addition of GdCl3, and finally the separation of a group of actinide chlorides from a group of lanthanide chlorides in the same salt. The uranium drawdown experiments successfully removed uranium from the salt and generated chlorine gas. Analytical results from filtered salt samples verified a reduction in UCl3 concentration corroborating the change observed by in-situ voltammetry. Comparison with known starting concentrations allows for the calculation of actinide concentrations from voltammetry data. Products from drawdown experiments had nodule like morphology and heavy salt entrainment, which complicated product recovery. Although the initial drawdown experiments achieved consistently low yields with current efficiencies estimated at below 20%, sequential drawdown of the actinides, an actinide – lanthanide mixture and finally lanthanides was achieved. The lower current efficiency may have been caused by design limitations in the test system, back reactions with higher valence uranium ions in the salt, and/or loss of product not firmly adhered to the cathode. Re-chlorination by dissolved chlorine was eliminated as a possible cause due to low solubility of chlorine in the salt. Future testing of electrolysis for actinide - lanthanide separations will focus on improving product recovery, process yield and efficiency. Government License Notice The submitted manuscript was created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). 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