Electrochemical Reduction of UO₂ to U in LiCl-KCl Molten Salt Eutectic Using the Fluidized Cathode Process

Abstract

The electrochemical reduction of UO 2 to U metal has been investigated in both Fluidized Cathode (FC) and Metallic Cavity Electrode (MCE) cell arrangements. Differences in the local concentration of O 2− where the reduction takes place influnces the reduction potential. The fleeting contact of UO 2 particle contact with the current collector in case of the FC results in much less O 2− buildup compared to MCE. Consequently, UO 2 reduction occurs over a range of potentials in the FC and may involve separate two 2-electron steps compared to one apparent 4-electron step in the MCE. It is proposed that there are three discrete periods during the FC reduction process. The first is an induction period during which reduced uranium particles gradually adhere to the tungsten current collector. The second is reduction associated with a rapid growth in electrode area and consequent increase in current. The third is a slower reduction of the remaining oxide in the melt. Complete reduction of metallic U is achieved at −2.2 V (vs. Ag/Ag + ) with an estimated faradaic current efficiency of >92%. There are significant drivers for pursuing the next generation of nuclear reactors. Generation IV reactors offer both advancements in reactor design and also feature fully integrated fuel reprocessing capabilities. There are six types of Generation IV nuclear reactors and power plants under development. These are: the very-high-temperature reactor (VHTR), the sodium-cooled fast reactor (SFR), the supercriticalwater-cooled reactor (SCWR), the gas-cooled fast reactor (GFR), the lead-cooled fast reactor (LFR), and the molten salt reactor (MSR). The key features of each are summarized in Table I. The fuel type used in the majority of these reactors is metal. Given that most current reactors employ metal oxide (MOX or UOX) fuel and the majority of legacy waste is also metal oxide, the conversion of metal oxides to metals is an important component in a future nuclear power flow sheet.High temperature molten salt reprocessing technology (pyroprocessing) 2 for spent nuclear fuel offers a range of advantages when compared to aqueous reprocessing techniques. This is due to: an improved proliferation resistance (local processing reduces the likelihood of material diversion, no separation of plutonium); using facilities with a smaller footprint; a shorter cooling period for irradiated fuel, and also improved criticality safety margins. Different process systems and salts have been studied in pyroprocessing, most of which are summarized by the Nuclear Energy Agency (NEA). Conceptual flow sheets for the pyrochemical treatment of used nuclear fuel are described by Argonne National Lab (ANL). 4 The spent nuclear fuel, in the form of oxide pellets, is decladded and chopped, then passed on for an electrolytic reduction step. The uranium is in the form of UO 2 ; hence, the reduction of UO 2 to U metal is being investigated here. The reduced species then undergo electrorefining, [5][6][7][8][9] where U product and other transuranic species are separated an...

Recoveryof actinidesand fission products from spent nuclear fuel via electrolytic reduction: Thematic overview

“…This electrochemical operation is carried out using a working electrode and a reference electrode (Figure 2). This reference electrode (Ag/Ag + ) is often used in laboratory experiments to recover UO 2 18 . A more stable reference potential can be expected when using a Ni/NiO reference electrode for electrochemical studies in the LiCl‐Li 2 O system.…”

“…This reference electrode (Ag/Ag + ) is often used in laboratory experiments to recover UO 2 . 18 A more stable reference potential can be expected when using a Ni/NiO reference electrode for electrochemical studies in the LiCl-Li 2 O system.…”