Neptunium dissolution solution

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. 

Other examples of redox-sensitive elements include heavy elements such as uranium, plutonium, and neptunium, all of which can exist in multiple oxidation states in natural waters. Redox conditions in natural waters are also indirectly important for solute species associated with redox-sensitive elements. For example, dissolution of iron (hydr)oxides under reducing conditions may lead to the solubilization and hence mobilization of associated solid phase species, e.g. arsenate, phosphate (see Sections 3.3.2.1, 3.3.3.2, and 3.3.4.1). 

Dissolution, described in Sec. 4.4, produces an aqueous solution of uranyl nitrate, plutonium(IV) nitrate, nitric acid, small concentrations of neptunium, americium, and curium nitrates, and almost all of the nonvolatile fission products in the fuel. With fuel cooled 150 days after bumup of 33,000 MWd/MT, the fission-product concentration is around 1700 Ci/liter. The fint step in the solvent extraction portion of the Purex process is primary decontamination, in which from 99 to 99.9 percent of these fission products are separated from the uranium and plutonium. Early removal of the fission products reduces the amount of required shielding, simplifies maintenance, and facilitates later process operations by reducing solvent degradation from radiolysis. 

In a modern PUREX plant, the fuel pins are first cut into pieces that are 3-5 cm long. The fuel is then dissolved in 6-11 M nitric acid, while the cladding hulls do not dissolve. Sometimes <0.05 M AIF3 is added to the nitric acid to improve dissolution of, e.g., zirconium by complex formation. The solution is then diluted to 3-4 M and nitrite is added to assure that plutonium is present as Pu(IV) and uranium as U(VI). Plutonium and uranium are then selectively extracted into TBP in aliphatic kerosene. Fission products and trivalent actinides remain in the aqueous phase. The extract is scrubbed with nitric acid to remove all contaminants except traces of ruthenium, neptunium, and zirconium. 

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