Pyrochemical processes electrorefining

An overview is presented of plutonium process chemistry at Rocky Flats and of research in progress to improve plutonium processing operations or to develop new processes. Both pyrochemical and aqueous methods are used to process plutonium metal scrap, oxide, and other residues. The pyrochemical processes currently in production include electrorefining, fluorination, hydriding, molten salt extraction, calcination, and reduction operations. Aqueous processing and waste treatment methods involve nitric acid dissolution, ion exchange, solvent extraction, and precipitation techniques. 

Figure 1 shows a simplified flow sheet for plutonium-239 recovery operations at Rocky Flats. Impure plutonium metal is sent through a pyrochemical process, called molten salt extraction (MSE), to remove the elemental impurity americium. The product plutonium metal, if it meets plant purity requirements, is sent to the foundry. Metal that does not meet foundry requirements is processed further, either through an aqueous process using ion exchange, or through a pyrochemical electrorefining process. The waste chloride salt from MSE is... 

Anode Residue Recovery. Approximately 10% of the plutonium present in a Pu- lwt% Ga electrorefining feed, ends up in the anode residue. This residue also contains most of the impurities which were present in the metal feed. The following pyrochemical processes have been considered for recovering plutonium from this residue. 

Recent process development efforts have been devoted to more expeditious and less costly pyrochemical reprocessing of residues created by the metal preparation and purification process. We intend to establish an internal recycle which yields either reusable or discardable residues and recovers all plutonium for feed to the electrorefining purification system. This internal recycle is to be performed in a more timely and less costly operation than in the present reprocessing mode. 

Electrorefining (ER) Salt Recovery. Approximately 8% (500 g) of the plutonium in an electrorefining run is lost to the salt phase. Historically, this waste stream was recovered by aqueous processing. The salts are now recovered pyrochemically by salt stripping. This operation consists of contacting the molten salt residue with calcium metal, resulting in the reduction of PuCls to metal and the coalescence of metallic plutonium shot. The process results in the formation of a metallic plutonium button (96% yield), a large white salt phase, and a small black salt phase. 

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