Americium purity

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... 

At Los Alamos National Laboratory in New Mexico the Analytical Chemistry Group (C-AAC) supports the Pu-238 Heat Source Project that fabricates heat sources for use in the space industry. These heat sources have been used on NASA s deep-space probes and on instruments exploring the surface of Mars. The chemical and isotopic purity of the heat sources are critically controlled to ensure dependable service. The Radiochemistry Task Area performs analyses of the heat source material for four radioisotopes americium-241, plutonium-238, neptunium-237, and uranium-235. 

A nearly white precipitate of Am and Ca oxalates was obtained while most of the metallic contaminants (e.g., Fe, Cr, Al) remained in solution as stable oxalato complexes. The precipitate was filtered off, dissoved in boiling concentrated nitric acid to destroy the oxalate, neutralized with ammonia to pH = 2.5 to 3, and the Am was extracted from the strongly salted aqueous ammonium nitrate solution by 0.5 M TCMAN/Solvesso. The loaded organic solvent was scrubbed with concentrated ammonium nitrate solution, and the americium was back-extracted with dilute nitric acid, precipitated as the oxalate, and converted into An by calcination at 800°C. Multi-gram amounts of 241 have been prepared with this procedure, with Am purities > 99%. 

At present, americium is separated and purified in Kg/yr quantities at the new plutonium facility at Los Alamos (Figure 1). The feed for the americium production comes from a line which produces high purity ceramic grade Pu02 for the Fast Flux Test Facility (FFTF) at Richland, Washington. The feed for this FFTF PuO is aged plutonium metal which contains sizable amounts of 24 Am... 

The plutonium metal feed stock contained about 5 ppm natural lead which was not removed by the process. Recovery of americium in the finishing process (oxalate precipitation and calcination) averaged 98.5%. Most of the residual chromium contaminant was removed from the oxalate in decanted supernate and washes. The finished oxide product purity exceeded specifications i.e., >95% Am02 By selective blending, impurities in the shipped product, predominantly lead and nickel, were kept below 2%. 

Precipitations were made by adding sufficient 0.9M oxalic acid to bring the final oxalate concentration to 0.3M. After a digestion period and decanting the filtrate, the oxalate precipitate was washed four times with 0.2M H2C2O4-O.7M HNO3, and once with 0.2M (NH4>20204. On the initial runs, the washed oxalate precipitate was calcined to americium carbonate to allow easier acid dissolution if impurity analysis indicated recycle was required. As all product batches exceeded the purity guidelines, the low-temperature calcination step was eliminated and all products were calcined at 700°C. 

The analysis of the composite sample from the same series of curium-americium oxide production runs is presented in Table II. The content of carbon, the major impurity, is inferred rather than directly analyzed. The analyses of curium-americium oxide products generally reflect the purity of the feed, except for carbon and sulfur from the resin and a few potential corrosion products. 

By thermal dissociation of intermetallic compounds with noble metals (Pt, Ir), the volatile metals americium, curium and californium have been obtained in high purity (4,5) ... 

Chemicals of normal reagent-grade purity suffice for the separation of analytes that do not occur in nature there should be no important impact on the analyses of samples of neptunium, plutonium, americium, or curium, provided reagent volumes are kept small so that mass is not introduced into the final samples. For all other analytes, reagents that... 

Americium metal has been prepared by the following methods (1) reduction of AmF3 with barium (or lithium) metal (2) reduction of Am02 with lanthanum metal (3) bomb reduction of AmF4 with calcium metal (4) thermal decomposition of Pts Am. Lanthanum reduction of Am02 in tantalum equipment and subsequent distillation of the americium metal from the reaction mixture yields americium of very high (>99.9%) purity. There is about 10 -fold difference in americium-lanthanum volatility. Extensive application of this technique by the Euratom group has led to important new measurements of the physical properties and thermodynamic properties of americium metal [81,342], Rocky Flats workers have reported similar success with vacuum distillation [333]. 

Preparation of americium metal by thermal decomposition of the intermetallic compound PtsAm is a recent development. Muller, Reul, and Spirlet [343] produced high-purity americium metal by thermal decomposition of 4 g of PtsAm at 1550 C and 10 torr, followed by further distillation. 

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