Plutonium metal production

W. C. Hazen, Remote Control Equipment for Plutonium Metal Production, LA-1387, Los Alamos Scientific Laboratory, N.M., 1951. 

M. W. Gibson and D. H. Nyman, Recent Plutonium Metal Production Experience at Hanford, WHC-SA-0616, Westinghouse Hanford Co., Wash., 1989. 

An optimum molten salt extraction process at Rocky Flats would use the minimum amount of salt required to obtain (1) a desired removal of americium, (2) a minimum transfer of plutonium to the salt, and (3) a minimum take-up of magnesium by the plutonium metal product. The product salt must be compatible with subsequent chemical processes for the recovery of americium and plutonium contained in the salt. To minimize the number of glove-box operations, time in the gloves, and operator radiation exposure, the operations must be simple and easy to conduct. By using the minimum amount of salt feed, a minimum amount of waste will be generated that ultimately must be sent to long-term storage. 

Fig. 21.17. Uranium metal reduction process a similar process is used for plutonium metal production. (Courtesy USDOE.)...

The plutonium extracted by the Purex process usually has been in the form of a concentrated nitrate solution or symp, which must be converted to anhydrous PuF [13842-83-6] or PuF, which are charge materials for metal production. The nitrate solution is sufficientiy pure for the processing to be conducted in gloveboxes without P- or y-shielding (130). The Pu is first precipitated as plutonium(IV) peroxide [12412-68-9], plutonium(Ill) oxalate [56609-10-0], plutonium(IV) oxalate [13278-81-4], or plutonium(Ill) fluoride. These precipitates are converted to anhydrous PuF or PuF. The precipitation process used depends on numerous factors, eg, derived purity of product, safety considerations, ease of recovering wastes, and required process equipment. The peroxide precipitation yields the purest product and generally is the preferred route (131). The peroxide precipitate is converted to PuF by HF—O2 gas or to PuF by HF—H2 gas (31,132). 

Preparation of Plutonium Metal from Fluorides. Plutonium fluoride, PuF or PuF, is reduced to the metal with calcium (31). Although the reactions of Ca with both fluorides are exothermic, iodine is added to provide additional heat. The thermodynamics of the process have been described (133). The purity of production-grade Pu metal by this method is ca 99.87 wt % (134). Metal of greater than 99.99 wt % purity can be produced by electrorefining, which is appHcable for Pu alloys as well as to purify Pu metal. The electrorefining has been conducted at 740°C in a NaCl—KCl electrolyte containing PuCl [13569-62-5], PuF, or PuF. Processing was done routinely on a 4-kg Pu batch basis (135). 

The first definite production of plutonium metal was made in November, 1943 by Baumbach and coworkers (1958). Approximately 35 micrograms of PuFi in a small thoria crucible in a high vacuum was reacted with barium metal at 1400 C to yield plutonium metal. The metal was found to have a silvery lustre, a density of about 16 grams j>er cubic centimeter and it rapidly absorbed hydrogen at about 210 C to form a black powder subsequently identified as PUH3 (a proof that metal had been produced). 

Chemical processing activities involve the recovery of plutonium from Rocky Flats Plant scrap, waste materials and residues, and effluent streams. The final product of this recovery and purification effort is high-purity plutonium metal for use in foundry operations. 

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... 

The products of the reaction, once cooled to room temperature, are easily separated. The plutonium metal is sent to the foundry, while the CaF2 salt is stored until it can be dissolved in 9M HNO3 for recovery of residual amounts of plutonium. 

A production process has evolved from this original work, and is presently used for extracting americium from kilogram amounts of plutonium metal. This process is based upon equilibrium partitioning (by oxidation-reduction reactions) of americium and plutonium between the molten chloride salt and the molten plutonium phase. The chemistry of this process is indicated by the following reactions ... 

This PUCI3 also acts as a salt-phase buffer to prevent dissolution of trace impurities in the metal feed by forcing the anode equilibrium to favor production (retention) of trace impurities as metals, instead of permitting oxidation of the impurities to ions. Metallic impurities in the feed fall into two classes, those more electropositive and those less electropositive than plutonium. Since the cell is operated at temperatures above the melting point of all the feed components, and both the liquid anode and salt are well mixed by a mechanical stirrer, chemical equlibrium is established between all impurities and the plutonium in the salt even before current is applied to the cell. Thus, impurities more electropositive than the liquid plutonium anode will be oxidized by Pu+3 and be taken up by the salt phase, while impurities in the electrolyte salt less electropositive than plutonium will be reduced by plutonium metal and be collected in the anode. 

The basic electrorefining process is now being used on a production scale for the purification of non-specification plutonium metal. The technology is sufficiently well developed to permit 24-hour unattended operation of the electrorefining cells, and the quality of the product metal is highly consistent. This technology is rapidly replacing aqueous chemistry for plutonium metal purification. 

The production of plutonium metal by both fluoride and oxide reduction is well established at Los Alamos. The subsequent purification of this metal by electrorefining is now being performed in production on a 6-kg batch scale. The objective is the production of high-purity plutonium metal. 

The Los Alamos National Laboratory has had a very active program for the production of high purity plutonium metal for both Laboratory and national needs for many years using pyrochemical techniques. 

A. Process Schematic. A schematic of the main process sequence for the conversion of plutonia scrap to high-purity metal is shown in Figure 2. Plutonia scrap is fed to both the direct oxide reduction (DOR) process and the plutonium tetrafluoride production/ reduction process. 

The buttons usually have a film of Pu02 as a result of exposure to glove-box air. Upon casting, this Pu02 floats and remains in the skull along with trapped plutonium metal. This portion of the skull is recycled back into the production sequence. 

Molten Salt Extraction Salts Plutonium and Americium Recovery. We have demonstrated the ability to successfully strip the plutonium from the MSE salts. The resulting metal product now contains as much as 10% americium and as a result cannot be fed directly into the metal processing sequence. To use the plutonium we must remove the americium. 

Sohn, C. L. Thorn, C. W. Christensen, D. C. "Enchanced Production of Plutonium Metal Using Pu02 in the PuF4/Bomb Reduction Process," Los Alamos National Laboratory LA-UR-82-1230, May 1982. 

Perhaps the most straightforward of examinations of the behaviour of plutonium with water was that of Lai and Goya 147) who showed that plutonium metal interacts with seawater to form two types of solid reaction products ... 

At Rocky Flats, the metal composition is held essentially constant because the americium content (200 to 2000 ppm) and the amount of magnesium produced by Equations 2 and 3 are small and purified plutonium metal is the metal extraction product. Variables that can be manipulated and that influence the value of the distribution coefficient (Kd) are the salt composition and the temperature. 

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