The Future of Nuclear Energy: Electrochemical Reprocessing of Fuel Takes Center Stage

Uranium is a key element used for nuclear energy production. Some advanced reactor designs, specifically molten salt reactors, continue to use uranium as the fissile material for energy production. These new technologies require an intimate understanding of uranium chemistry both during and after energy production. This review covers contemporary research on the coordination chemistry and behavior of uranium with the coolant and pyroprocessing salts as proposed for use in future reactor designs. Discussed topics include the nature of U redox reactions involving the reduction of U(III) to U metal and oxidation of U(III) to U(IV). These systems have been interrogated using cyclic voltammetry, chronopotentiometry, and optical and X-ray absorption spectroscopies. Insights obtained into the electrode potentials, the uranium species, and their diffusion coefficients in alkali halide melts from decades of research are summarized selectively. Perspectives are provided on the importance of unifying studies for comparison across multiple institutions. The application of synchrotron radiation research and multimodal approaches involving two (or more) probes, such as the widespread combination of UV-Visible spectroscopy and electroanalysis known as spectroelectrochemistry, can provide new knowledge about the main process of uranium electrorefining—diffusion, as will be demonstrated here through the lack of comparable results.

Section: Core Conceptsmentioning

confidence: 99%

Section: Electrochemistrymentioning

confidence: 99%

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Review—Fundamental Uranium Electrochemistry and Spectroscopy in Molten Salt Systems

Hege¹,

Jackson²,

Shafer³

2023

“…In spent nuclear fuel reprocessing, the electrochemical recovery of actinides and fission products from molten salts takes advantage of the high conductivity, wider working potential ranges, and radiochemical stability of molten salts compared to conventional aqueous solutions. 1 The spent fuel treatment process from EBR-II was the first pilot-scale demonstration of pyroprocessing where a product stream of metallic uranium was successfully recovered, leaving behind the stainless steel cladding, fuel metal wastes and leftover salt wastes from the electrorefiners that were fixed and isolated into glass-bonded sodalite ceramic. [2][3][4] During this electrorefining process, actinides are separated from unwanted fission products to be reprocessed back into nuclear fuel (U, Pu).…”

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confidence: 99%

Electrochemical Deposition with Redox Replacement of Lanthanum with Uranium in Molten LiCl-KCl

Eakin

Molina

Guo

et al. 2023

Electrochemical recovery of dilute concentrations of actinides from spent nuclear fuel would reduce the longevity of storing high-level nuclear waste. Electrochemical deposition with redox replacement (EDRR) is used in a molten salt medium for the selective electrochemical recovery of uranium in the presence of excess concentrations of lanthanum. In each EDRR cycle, after a short electrodeposition pulse, the deposited lanthanum is spontaneously replaced by uranium at open circuit. After repeated cycles, uranium metal was obtained on a tungsten electrode immersed in LiCl-KCl melt that contained 1 wt.% lanthanum chloride – 0.15 wt.% uranium4+ chloride. Scanning electron microscopy and energy dispersive X-ray spectroscopy analysis revealed uranium particles approximately 0.5 - 1 μm with well-defined rectangular shapes; and with 20 – 60 times more uranium recovered on the surface of the electrode than lanthanum.

“…Based on the above considerations, momentum is now building to examine in detail the LiF-CaF 2 eutectic molten salt system for the separation of actinides from lanthanides or rare earth elements from the spent nuclear fuel. 6,7 Consequently, it has now become essential to understand the electrochemistry of Zr 4+ in this molten salt for the reprocessing of U-Zr and U-Pu-Zr metallic spent fuels and because zirconium, which is also produced as a fission product, needs to be extracted from the spent fuel in Gen-IV molten salt reactors. In the longer term, the knowledge will assist in the evaluation of the potential of an MSR as a breeder system based on 232 Th/ 233 U cycle with lower radioactive waste level compared to that in a U based nuclear reactor.…”

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confidence: 99%

Cyclic Voltammetric Experiment-Simulation Comparisons of the Complex Zr⁴⁺ to Zr⁰ Reduction Mechanism at a Molybdenum Electrode in LiF-CaF₂ Eutectic Molten Salt

Fabian

Bond

2022

Nuclear energy represents an important option for generating largely clean CO2-free electricity. Zirconium is a fission product in the nuclear reaction that needs to be extracted from irradiated fuels used in Gen-IV molten salt reactors. The present investigation addresses the electrochemical reduction of solution soluble Zr4+ (soln) to surface confined Zro (s−c) at a molybdenum electrode in a LiF—CaF2 eutectic molten salt at 840 °C using DC cyclic, square-wave and AC voltammetry. Cyclic voltammograms simulated by the reaction scheme: Zr4+ (soln) + 4e− → Zro (soln); Zro (soln) ↔ Zr*(s−c); Zr*(s−c) → Zr4+*(s−c) + 4e−; Zr4+*(s−c) → Zr4+ (soln); Zr*(s−c) + Zr4+ (soln) ↔ 2Zr2+ (soln); Zr2+ (soln) ↔ Zr2+*(s−c) and Zr2+*(s−c) → Zr4+*(s−c) + 2e− provided excellent agreement with experimental data over the scan rate range of 50 to 500 mV s−1. The interpretation of the simulation is that the reduction of Zr4+ (soln) to Zro (metal) takes place via a transiently soluble Zro (soln) in an overall 4-electron essentially reversible diffusion-controlled process having a reversible formal potential (Eo f) of −1.22 V (vs Pt). A minor oxidation process observed at −0.455 V (vs Pt) on the reversing the potential scan direction is simulated via the reaction step Zr(s−c) + Zr4+ (soln) ↔ 2Zr2+ (s−c) followed by Zr2+ (s−c) → Zr4+ (sol) + 2e−. The sharply rising initial component where reduction of Zr4+ (sol) commences, contains evidence of a nucleation-growth mechanism associated with the electrocrystallisation of zirconium metal. This initial rapid growth of current is not fully accommodated in the simulations, but all features found beyond the peak potential are supported by the theory. A comparison with theory based on a direct reduction of Zr4+ (soln) to the metallic state having unit activity is provided. It is proposed that an analogous mechanism applies at a Ni electrode, except that a Ni-Zr alloy formation occurs instead of Zr metal.