Nitric acid neptunium oxidation

Tochiyama, O., Nakamura, Y., Hirota, M., Inoue, Y. 1995. Kinetics of nitrous acid-catalyzed oxidation of neptunium in nitric acid - TBP extraction system. J. Nucl. Sci. Technol. 32 118-124. 

Co-extraction of Np and Pu The waste from the uranium extraction battery is adjusted to 5 M LiNO and then passed through an IRA-400 column at a high flow rate (30 L/h). The americium is not sorbed. Neptunium and plutonium are stripped by a dilute nitric acid solution and precipitated as a hydroxide, and calcined to yield mixture of oxides. 

Dissolution of the calcium fluoride in aluminum nitrate-nitric acid oxidizes the plutonium to the tetravalent hexanitrate complex (3), while the transplutonium nuclides remain in the trivalent state. The only actinides retained by a nitrate-form anion-exchange column are thorium, neptunium, and plutonium. The uranium distribution coeflBcient under these conditions is about ten, but uranium should not be present at this point since hexavalent uranium does not carry on calcium fluoride (4). 

Nitric Acid Media. The photo-redox reactions of neptunium in nitric acid are, as stated earlier, complicated by the NO3 photo-reduction which prevents the complete photo-oxidation of neptunium to the (VI) state. At first this might seem to pose a dilemma which ought to be avoided, but recent results have shown how this apparent complication is essential to a photo-separation process. Two photochemical reactions working together will be shown to be necessary in order to reach what we have accepted as a practical result. 

Neptunium dioxide, Np03, is the most stable of the neptunium oxides. It crystallizes with the fluorite structure of all the actinide dioxides, with a crystalline density of 11.14 g/cm . It can be formed from the thermal decomposition of other neptunium compounds, such as the hydroxide, the nitrate, or the oxalate, in the temperature range of 600 to 1000°C. High-fired NpOj can be dissolved in hot concentrated nitric acid containing small amounts of fluoride. 

Oxidation of pentavalent neptunium by nitric acid. Oxidation of pentavalent neptunium to hexavalent by nitric acid requires catalysis by nitrous acid. The kinetics of this reaction have been studied by Siddall and Dukes [S16], Swanson [S24] and Mouline [M9]. Siddall and Dukes reported that the reaction was first order in neptunium concentration, independent of nitrous acid concentration if greater than 5 X 10" M, and depended on temperature T (K) and nitric acid molarity Xh as can be represented by Eq. (10.33) ... 

These reaction rates are too low to explain the appreciable extraction of neptunium obtained in the short-residence-time HA contactors used at Hanford and elsewhere. Swanson [S24] reported that radiolysis reaction products of TBP and nitric acid present in Purex solutions increased the neptunium oxidation rate and provided a possible explanation. He found that the oxidation rate could be increased several orders of magnitude by adding a synthetic rate-accelerating material (RAM) produced by reacting the aciform of nitropropane, CjHs(CH)(NO)(OH), with nitric and nitrous acids, and recommended addition of such a catalyst to Purex feed if increased neptunium extraction were desired. 

Swanson, J. L Oxidation of Neptunium(V) in Nitric Acid Solution—Laboratory Study of Rate Accelerating Materials (RAM), Report BNWL-1017, Apr. 1969. 

The chemistry of uranium in the Purex process is simpler than that of plutonium or neptunium. In the dissolver solution, U is present exclusively. The lower oxidation states (IV) and (V) are both unstable in nitric acid media, U " disproportionating rapidly and U being formed only in... 

The plutonium fraction is then purified by several cycles comprising oxidation to Pu(IV), extraction, scrub, and back-extraction by reduction to Pu(III). This purification removes neptunium and the last traces of fission products. Finally, Pu(IV) is back-extracted into 0.2 M nitric acid. The plutonium nitrate is then converted to Pu02 or Pu metal. 

The uranium fraction is concentrated by evaporation and nitric acid is added to -S M. Uranium is then extracted into a TBP solution that is scrubbed with a reducing solution to remove traces of plutonium and neptunium. This process is repeated twice. Then uranium is back-extracted into 0.01 M nitric acid and converted to uranium oxide. 

The redox potential of the Np /Np couple has been measured in alkaline media, the neptunium(vi) species in 0.1—1.5M sodium hydroxide being present as the ion Np02(OH)4 oxidation to Np is considered to involve the loss of two hydroxide groups. Neptunium(vi) is also formed in the reaction of Np with nitrate ion in aqueous nitric acid ... 

Pu, Zr(+Nb), and Ce and iTithenlum complexes. Neptunium, plutonium, and cerium are made less extractable by reduction to lower oxidation states. Favorable separation of tiranlum from the other elements may be achieved by control.of the nitric acid and saltlng-out agent concentrations. Free halogens are extracted. These elements may be eliminated from solution prior to uranliim extraction. The halogens also combine chemically with a number of solvents eg,. 

---