oxide fuels would have required a separate front end step to convert the oxides into metal for use in the electrorefiner.6 This step consisted of reduction of the metal oxides into metal by Li, and electrochemical regeneration of metallic Li from Li2O in molten LiCl. The process has also been considered as a possible technology for disposing of excess Pu from the U.S. stockpile.7

Pyroprocessing, or molten salt electrochemical processing, has been in general use for many years for purification of materials, including plutonium.8 It involves anodization (oxidation) of a metal into a molten salt electrolyte and then reduction at a cathode to yield a more (highly) purified form. The overall process technology is diagrammed in Figure 2.1. Differences between the generic process and the pyroprocess as carried out by ANL are described below. Subsequent chapters provide additional detail.

ELECTROMETALLURGICAL TREATMENT

In the EBR-II demonstration project for the treatment of EBR-II driver and blanket assemblies, 100 driver assemblies, consisting of 410 kg of 60 to 75% highly enriched 235U, and up to 25 blanket assemblies, consisting of 1,200 kg of depleted uranium, were to be treated. (As originally proposed, the EBR-II demonstration project was to have treated 1 metric ton of driver fuel and 16 metric tons of blanket fuel, the latter in a high throughput electrorefiner.9,10 The demonstration project was limited to the size noted as the result of a revised EA.11)

As indicated in Figure 2.1, chopped driver (or blanket) fuel rod elements are placed in a steel anode basket in an electrorefiner that contains a KCl-LiCl molten salt eutectic system at upwards of 500 °C. The driver fuel is highly enriched uranium alloyed with ~10 wt % Zr; the cladding is stainless steel. The blanket fuel is depleted uranium, with stainless steel cladding. Both the driver and the blanket elements are sodium-bonded to the stainless steel. Two electrorefiners, the Mark-IV and Mark-V, were developed by ANL for use in the EBR-II demonstration project. The Mark-IV, used to electrorefine the driver elements from the EBR-II, contains a molten Cd pool—a holdover from the ALMR/IFR development—while the Mark-V, used for the blanket elements, is Cd free. The Cd pool provides a corrosion-resistant barrier to the mild steel vessel and also acts as a neutron absorber to prevent criticality problems that might result from the highly enriched uranium in the driver elements falling to the bottom of the vessel. However, the Cd pool in the Mark-IV electrorefiner is not used as a cathode, and thus no Pu separation is performed. The Mark-V differs markedly from the Mark-IV, in that it is designed to process much larger batches of material, as needed to treat the blanket elements; it is also a high-throughput electrorefiner (HTER). Nevertheless, the fundamentals of the two electrorefiners are very much the same.

An oxidant, either CdCl2, in the case of the Mark-IV, or UCl3, in the case of the Mark-V, is added to the salt prior to initiation of electrolysis. The CdCl2 oxidizes some of the U (and other active metals) from the anode baskets. Upon passage of a constant electrolysis current between the anode baskets and the steel cathode, U, Pu, transuranic elements (TRU), the alkalis and alkaline earth metals, and rare earths are oxidized into the molten salt as U3+, Pu3+, TRU, alkali and alkaline earth and rare-earth cations (Table 2.1). The stainless steel from the cladding, most of the Zr, and the noble metals remain in the anode baskets. The U3+ is reduced to the metal and

6  

National Research Council, Electrometallurgical Techniques for DOE Spent Fuel Treatment: A Preliminary Assessment of the Promise of Continued R&D into an Electrometallurgical Approach for Treating DOE Spent Fuel, National Academy Press, Washington, D.C., 1995, p. 7.

7  

National Research Council, An Evaluation of the Electrometallurgical Approach for Treatment of Excess Weapons Plutonium, National Academy Press, Washington, D.C., 1996, p. 1.

8  

M.R. Coops, J.B. Knighton, and L.J. Mullins, Plutonium Chemistry, W.T. Carnall and G.R. Choppin, eds., ACS Symposium Series 216, American Chemical Society, Washington, D.C., 1983, pp. 381-400.

9  

Argonne National Laboratory, Proposal for Development of Electrometallurgical Treatment for DOE Spent Fuel, Argonne National Laboratory, Argonne, IL, 1995.

10  

National Research Council, An Assessment of Continued R&D into an Electrometallurgical Approach for Treating DOE Spent Nuclear Fuel, National Academy Press, Washington, D.C., 1995, p. 12.

11  

National Research Council, Electrometallurgical Techniques for DOE Spent Fuel Treatment: A Status Report on Argonne National Laboratory R&D Activity, National Academy Press, Washington, D.C., 1996, p. 5.



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