The principal function of the Mark IV electrorefiner (Mk-IV ER) is electrometallurgical treatment of used sodium-bonded metallic nuclear driver fuel. Examples include alloys of uranium-zirconium (U-Zr) fuel, uranium-fissium fuel, uranium-plutonium-zirconium (U-Pu-Zr) fuel, and related experimental fuels. The electrorefiner vessel contains a lithium chloride/potassium chloride (LiCl-KCl) eutectic molten salt electrolyte with a nominal concentration of uranium trichloride (UCl3) that floats above a pool of molten cadmium. Chopped irradiated fuel segments are loaded in an anode basket and submerged in the molten salt electrolyte. An electrical potential (typically less than 5 VDC) is applied between the anode basket and cathode mandrel by an external power supply. Under these conditions, uranium in the used fuel is oxidized at the anode basket and reduced on the cathode mandrel as eletrorefined uranium. Those metals more noble than uranium (the transition metal fission products) remain in the anode basket and are disposed of in a metal waste ingot. Those metals more reactive than uranium (Group 1, Group 2, and lanthanide fission products along with the transuranics) accumulate in the electrolyte as metal chlorides at the expense of the uranium trichloride concentration. For each unit of electrical charge (called a coulomb) that passes between the anode and cathode, a unit of charge of uranium cations is oxidized at the anode and reduced at the cathode. These separations produce an electrorefined uranium metal that can be used to make new nuclear fuel. Electrical current continues to flow during this process until the anode basket is nearly fully depleted of uranium. The electrorefined uranium is harvested from the cathode mandrels and subsequently treated in the cathode processor (a vacuum retort furnace) to consolidate the uranium into a metal ingot and separate the salt that is returned to the electrorefiner vessel. The uranium trichloride concentration in the salt is maintained by adding cadmium chloride that reacts with uranium metal to form uranium trichloride and cadmium metal. Hence, the pool of cadmium in the bottom of the vessel.
The MK-IV ER is used to electrochemically dissolve the sodium-bonded metallic fuel previously irradiated in Experimental Breeder Reactor-II and separate the uranium metal from the fission products in the used fuel elements. The separated uranium metal is recovered and removed from the vessel while the majority of the fission products and bond sodium form stable chlorides with the electrolyte and remain in the vessel. The separation and resulting stabilization of the bond sodium neutralizes the reactive characteristics of the sodium, facilitating the disposition of the irradiated materials.
Electrolyte temperature: 500°C
Cadmium temperature: 500°C
Vessel wall temperature: 500°C
Power supplies: 300 A dc current, 10 V dc max voltage