Uranium recovery via electrochemical deposition with a liquid zinc cathode followed by electrochemical oxidation of rare earth metals

Highlights

• Recovery of U/transuranic (TRU) elements from LiCl-KCl salt was attempted.

• U was used as a surrogate for U/transuranic (TRU) elements in this study.

• The method involved two sequential steps; the first was electrochemical deposition.

• This was followed by selective oxidation of rare earth (RE) metals.

• Feasibility of a liquid Zn cathode and behavior of RE dissolution were analyzed.

Abstract

Uranium recovery with electrochemical deposition followed by selective oxidation of rare earth (RE) metals was examined to confirm the feasibility of liquid Zn cathode and figure out the behavior of RE dissolution. U was used as a surrogate for U/transuranic (TRU) elements in this study. About 1.6 wt% UCl₃, 2.9 wt% NdCl₃ and 3.1 wt% CeCl₃ was dissolved into the LiCl-KCl eutectic, and the U, Nd, and Ce was co-deposited into a liquid Zn cathode. About 3.3 g of U/RE alloy was successfully consolidated by subsequent removal of the Zn with the distillation. U was segregated and agglomerated with each other in a matrix alloy consisting of Nd and Ce, confirming the co-deposition of RE along with U. The U/RE alloy was electrochemically oxidized to remove the RE, applying a constant voltage of −1.7 V (vs. Ag/AgCl) at the anode. Approximately 2.2 g of final product in a granular shape was ultimately obtained after the subsequent removal of residual salts. Characterization revealed that the concentration of U was larger than that of the REs, even though a large amount of REs still remained, confirming the selective dissolution of REs. The results suggest a new method, i.e., electrochemical deposition followed by selective oxidation of RE, to recover U/TRU from the LiCl-KCl salt using a liquid Zn cathode.

Introduction

Electrorefining is one of the most important pyroprocessing techniques for the recovery of useful elements such as Uranium and the transuranic elements (TRUs) from spent fuel [1,2]. In this process, spent fuel dissolves in a molten LiCl-KCl eutectic salt via electrolysis, and U is selectively recovered onto the surface of a cathode electrode [[3], [4], [5], [6]]. The U/TRUs remaining in the eutectic salt are electrochemically recovered into a liquid Cd cathode (LCC) [[6], [7], [8]].

Cd is used as a liquid cathode in pyroprocessing because TRUs have low chemical activities and stabilize in the liquid Cd phase [9,10]. Moreover, it has a major role in nuclear proliferation resistance, since the TRUs co-deposited with rear earth elements (RE) in the LCC [11]. Unfortunately, Cd is hazardous to the environment. Bi and Ga have been reported as promising alternative liquid cathode materials, but the high boiling point (1837 K for Bi, 2673 K for Ga) hinders the recovery of U/TRU from the cathode.

We previously examined the feasibility of liquid Zn for U recovery by electrochemical deposition [12] and distillation [13]. It was found that a U-Zn alloy formed during electrochemical deposition into the liquid Zn cathode and U was successfully recovered by distillation at a relatively mild temperature of 1073 K. These previous results suggested that Zn could be promising for U/TRU recovery from the LiCl-KCl eutectic. Therefore, the feasibility of using Zn as a liquid cathode throughout electrolytic deposition to Zn distillation was tested.

One of the merits and/or drawbacks of U/TRU recovery by using a liquid cathode is the large amount of RE co-deposition as mentioned above [8,11]. The co-deposition gives a high advantage on a nonproliferation. However, the REs in U/TRU act as contaminants when recycling for nuclear fuels. Thus, the conditions needed for U/TRU recovery, such as the TRU/RE ratio, is controlled to avoid depositing REs into the liquid cathode, but a large amount of TRUs inevitably remain in the salt and are lost. Another method to deter contamination is through the removal of REs from the products obtained after using a LCC.

In this study, we introduce electrochemical oxidation of REs after U/TRU recovery to remove the contaminants from product. The objective is firstly to confirm the feasibility of Zn as a liquid cathode for U/RE recovery from the LiCl-KCl eutectic, and secondly to figure out the behavior of RE dissolution by electrochemical oxidation. Fig. 1 shows a schematic of the experiment in this study. We used U for the simulated test instead of U/TRU. Nd and Ce were used as representatives for the REs, since the composition of both materials are above 60 wt% of RE metals in the spent fuel [14,15]. U, Nd, and Ce were deposited into liquid Zn, followed by Zn distillation to obtain the U/RE product. The U/RE product was next immersed into the LiCl-KCl-LaCl₃ salt, and electrochemically oxidized by controlling the potential in order to selectively dissolve the RE. The product obtained after oxidation was distilled to remove residual salts, and the product was analyzed. Both the feasibility of Zn as a liquid cathode material and electrochemical oxidation for selective RE removal have been studied in detail.