Review—Metallic Lithium and the Reduction of Actinide Oxides

Abstract

Extensive research and process development has been conducted on the electrolytic reduction of actinide oxides in molten LiCl-Li 2 O. It is now accepted that the reduction of these metal oxides occurs via two separate reduction mechanisms: direct electro-chemical reduction and mediated chemical reduction by metallic lithium. The deposition of metallic lithium at the cathode (mediated chemical reduction mechanism) during the process is known to be essential in order to achieve high process throughputs and reduction yields, and yet a knowledge gap exists regarding the nature of metallic lithium in this system. This review summarizes the formation of lithium during the process and its dispersion into the molten salt electrolyte. Previously reported aspects of the physical chemistry of the LiCl-Li 2 O-Li system are presented with a specific focus on the dispersion of Li in the solution. Finally, issues regarding the effect of the presence of lithium on the electrolytic reduction process are discussed. Evidence shows that electrochemically generated metallic lithium is likely a significant source of experimental uncertainty, low current efficiency and Li 2 O consumption in the oxide reduction process. The reduction of uranium, plutonium and minor actinide oxides to a metallic form is an important nuclear fuel cycle process.2-9 The ability of metallic lithium to reduce various uranium oxides, importantly UO 2 and U 3 O 8 , has been known for many years.10 A molten salt metalothermic reduction process employing metallic lithium (Li) as a reductant dissolved in molten LiCl was first developed by Argonne National Laboratory (ANL) to consolidate a variety of forms of actinide oxides for integration into a single electrometallurgical reprocessing system. 11-13 An electrolytic process, often referred to as electro-deoxidation or simply oxide reduction, was subsequently developed as a more controllable method of reducing metal oxides. A feasibility study conducted by Poa et al. demonstrated that oxides of uranium and plutonium could be reduced electrochemically in molten salts as long as the reduction potential of the metal oxide in question was more noble than the cation of the electrolyte.14,15 This approach was further developed by Gourishankar et al. to better understand processes chemistry and develop the electrolytic cell technology. 13,[16][17][18][19][20] Independent research conducted by Fray, Farthing, and Chen (FFC) applied the principles of electrochemical reduction in molten salt electrolytes to a variety of metal oxide reduction processes. 21,22 The FFC process has been the subject of several literature reviews.23-25 While these review articles address the processes associated with the reduction of nuclear fuel in LiCl, they focus on the electro-deoxidation phenomena and the CaCl 2 -CaO system. Additionally, the process engineering of the electrolytic reduction of nuclear fuel, and experience in incorporating oxide fuel into pyroprocessing, has been the focus of a recent review article. 26 The current review wi...

Identification of a chemical fingerprint linking the undeclared 2017 release of 106 Ru to advanced nuclear fuel reprocessing

“…That a small proportion of the 106 Ru contaminant is composed of polychlorinated 106 Ru(III) species is direct evidence that fuel reprocessing was the origin of the 2017 environmental release. Plausible reprocessing activities that could lead to such compounds are limited to either the reductive trapping of oxidatively generated RuO 4 in hydrochloric acid ( 29 – 31 ) or the electrochemical reduction and metallization of uranium in spent fuel from molten alkali chloride mixtures ( 44 – 46 ). Fortunately, the compositions formed from both approaches differ substantially.…”

“…In nitric acid solution, what is denoted commercially as Ru(NO)(NO 3 ) 3 is actually a mixture of ruthenium (III/IV) complexes varying in proportion of coordinated and exchangeable nitrate, nitrite, and water molecules, depending on solution conditions (16,43); moreover, it is directly representative of the nitric acid-based oxidative mixtures used to dissolve spent fuel early in the PUREX process (16,17). Subjecting these model compounds to the reaction conditions used to synthesize ttpyRuCl 3 (Materials and Methods), with the notable exception of using saturated ethanolic and aqueous ethanolic solutions of potassium chloride in the case of Ru(NO)(NO 3 ) 3 , produced no discernable trace of ttpyRuCl 3 ( (29)(30)(31) or the electrochemical reduction and metallization of uranium in spent fuel from molten alkali chloride mixtures (44)(45)(46). Fortunately, the compositions formed from both approaches differ substantially.…”

Nuclear Fuels and Reprocessing Technologies: A U.S. Perspective

“…The process is based on the greater stability of lithium oxide compared to that of uranium and plutonium oxides. [623][624][625][626][627] This lithium-based chemical oxide reduction process is similar in principle to lithium-based electrochemical oxide reduction process cited earlier. In the former process, lithium metal is directly introduced as the reducing agent.…”

Automated Development of Molten Salt Machine Learning Potentials: Application to LiCl

“…55 Next, we use the improved computational efficiency of GAP MD relative to AIMD to examine the dynamical properties of molten LiCl. First we examine the diffusion constants of Li + and Cl − , which are important in nuclear fuel reprocessing, 59 in which Li metal chemically reduces UO 2 to its metal form via the electrochemical reduction of Li + , the reaction of which is usually diffusion-limited. The self-diffusion coefficient of Li + and Cl − are computed by fitting the slope of the mean squared displacement vs. time and using the Einstein relation (SI Equation 5).…”