Neptunium purity

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. 

The actual situation with regard to the purity of most of the actinide metals is far from ideal. Only thorixun (99), uranium 11,17), neptunium 20), and plutonium 60) have been produced at a purity > 99.9 at %. Due to the many grams required for preparation and for accurate analysis, it is probable that these abundant and relatively inexpensive elements (Table I) are the only ones whose metals can be prepared and refined to give such high purities, and whose purity can be verified by accurate analysis. The purity levels achieved for some of the actinide metals are listed in Table II. For actinium (Ac), berkelium (Bk), californium (Cf),... 

Dissolution of a Primary Metal. This method is not likely to be used by many radioanalytical laboratories because of special facility requirements. Dedicated hoods and glove box facilities are used to handle the usual types and quantities of special nuclear materials. Samples of metallic plutonium, uranium, or neptunium can be purchased with certification of both purity and mass for use of the metal as the primary standard material. The metal is dissolved in acid and then diluted to prepare the solution of desired concentration. 

Reduction and oxidation (redox) steps are major process steps in the Purex process. Use is made of redox reactions to alter the valency of plutonium, uranium or neptunium with the object of producing these metals with a high degree of purity. 

The aim of the present improvement work on the PUREX process is to make the separations more selective and to create effluent streams of high purity. Thus, modifications are performed to make neptunium end up in a fraction for later transmutation in a reactor or accelerator-driven system. This can be achieved by a better control of redox conditions in the process. Today neptunium is partially co-exlracted with plutonium and uranium. There are also suggestions to withdraw product streams with Tc and respectively, i.e., long-lived nuclides that might be of interest for transmutation. 

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... 

A neptunium sample in the alpha-phase did not show a maximum in the resistivity and although its purity was probably more than ten times worse than that of the plutonium specimens, the residual resistance was much smaller (2.7 ). 

For experimentalists, measurements of the transport properties of metals are also of great practical interest, since they are very sensitive to the purity of the studied materials. It is known that the residual resistivity ratio p(300 K)/p(4.2 K) is commonly used as a global purity test. Values as high as 1000 have been obtained for ultra-pure thorium refined by electro-transport (Peterson et al. 1967). Even in the purest lanthanides this ratio is below 300. A value of 50 has been reached for electro-refined plutonium (Arko et al. 1972) and one of 65 for electro-refined neptunium (Fournier and Amanowicz 1992). 

Neptunium. Np is in a class with Pa no efforts have been made to use it as a fuel solute, but consideration has been given to its formation in and removal from blanket solutions of [30a]. The chemistry of neptunium has been reviewed by Hindman et al. [30b], and the hydrolytic behavior has been reviewed by Kraus [30c]. Continuous separation of Np239 would provide a Pu product of high purity by radioactive decay, whereas plutonium recovered from long-term irradiation of usually contains appreciable amounts of Pu °. Spectrophotometric cells for use at elevated temperatures and pre.ssures in the study of the chemistry of neptunium (and other materials) have recently been developed by Wag-gener [30d] and have been used to measure the absorption spectra of dilute neptunium perchlorate in its six-, five-, four-, and three-valence states, using heavy w ater as the solvent. Dilute solutions of neptunyl nitrate in nitric acid have been so studied at temperatures up to 250°C the pentavalent state was found to be stable under the test conditions [30e]. 

---