System Engineering Design

Two major pieces of electrometallurgical process equipment are the Electrorefiner and the Cathode Processor. NE personnel have been involved in the conceptual design, final design, procurement, manufacture, installation and testing of these two units which now reside in FCF. These two units required the development of first-of-a-kind large-scale systems and components with complex requirements including corrosive and hazardous materials, high-temperature structural integrity, vacuum and pressure conditions, and remote handling operations.

The Electrorefiner

The electrorefiner is an apparatus used for electrometallurgical treatment of spent nuclear fuel to facilitate storage and ultimate disposal. The input material is introduced at the anode of an electrolytic cell. The desired materials are electrochemically oxidized in a molten lithium and potassium chloride salt and transported to a cathode where they are again reduced to metallic form and collected either as a dendritic deposit on a solid mandrel or as particulate or dissolved material in a crucible of molten cadmium. After removal, these materials are sent on to the cathode processor.

The electrorefining process is conducted in a vessel that is radiatively coupled to a resistance-heated furnace. Handling of the electrodes is performed by dedicated mechanisms mounted on the electrorefiner using general-purpose in-cell remote handling equipment. All electrorefiner functions are computer controlled and the entire system is qualified for full remote operation and maintenance.

Operational Requirements

• Process temperature: 930 deg-F
• Electrode deposition current: 600 A
• Batch size (dendritic deposit): 20 kg
• Process time: ~ 24 hr
• Heater power: 11 kW

The Cathode Processor

The cathode processor is a large-scale high-temperature retort for processing metals under vacuum or pressure conditions. The input charge is the solid product of the electrorefiner process. The furnace region is heated with an internal passively cooled electrical induction heater, to distill the cadmium and salts in a vacuum. The process metal ingots are formed after melting at temperatures up to 2500 deg-F. An internal condenser region was designed to remain cool to collect the distillate. The entire operation is computer controlled and designed for remote operation and maintenance.

Operational Requirements

• Crucible temperature: up to 3000 deg-F
• Vessel pressure: vacuum to 15 psia
• Induction Heater: 30 kW
• Process Batch:
  • Cadmium: up to 33 kg
  • Salt: up to 20 kg
  • Process metal: up to 70 kg

Experiments

The prototype cathode processor has conducted a wide range of experiments, focusing primarily on studies of crucible materials and coatings. The uranium process metal, in conjunction with the electrorefiner salt that is present, poses a difficult challenge for crucible materials at the high process temperatures. Graphite, ceramics, and several novel composite materials are being explored.

In addition, numerous tests have been conducted to learn more about the capabilities and limits of the cathode processor. These have included tests of such things as the effect of different backfill gasses on cooldown rate, distillation rates of salt with different process conditions, and comparison of alumina and graphite as structural components.

The versatility of the prototype cathode processor has allowed it to perform tests that were not originally planned. Numerous tests in support of the metal waste form furnace have been conducted at temperatures as high as 3000 deg-F. This is well beyond the original design temperature of 2500 deg-F. And the retort aspect of the cathode processor has allowed the prototype unit to purify, by distillation, several hundred kilograms of uranium chloride salt produced by the Chemical Technology Division.