Plutonium product stability

Investigations of the solid-state chemistry of the americium oxides have shown that americium has properties typical of the preceding elements uranium, neptunium, and plutonium as well as properties to be expected of a typical actinide element (preferred stability of the valence state 3-j-). As the production of ternary oxides of trivalent plutonium entails considerable difficulties, it may be justified to speak of a discontinuity in the solid-state chemical behavior in the transition from plutonium to americium. A similar discontinuous change in the solid-state chemical behavior is certainly expected in the transition Am Cm. Americium must be attributed an intermediate position among the neighboring elements which is much more pronounced in the reactions of the oxides than in those of the halides or the behavior in aqueous solution. 

Uranous nitrate [U(N03)4] solution is used for the quantitative reduction of plutonium from loaded tributyl phosphate (TBP) phase [8]. Membrane cell technology was investigated for the production of 100% uranous nitrate solution [9], which is to be used in the partition cycle of the PUREX process in the fuel reprocessing plant. The membranes used hitherto have suffered from mechanical instability. A study was carried out at the BARC to obtain 100% uranous nitrate solution using a membrane-based electrolytic cell. The membrane used in this study was a thin polymer film reinforced with a Teflon fabric. The film was used as a separator between the anolyte and catholyte chambers, which are made of perfluorinated polymers, thus offering high thermal and chemical stability. 

Additional information exists concerning the ease of Pu(fV) to Pu(V) oxidation in neutral and subacid solutions [38-42]. Under these conditions, increasing the alkaline concentration results in the strong stabilization of Pu(V). It was found that, on the surface of the Pu(IV) hydroxides, plutonium is oxidized to Pu(V) by atmospheric oxygen and O and OH radicals - products of water decomposition, and possible nitrate- and nitrite-ions. The yield of back reduction of Pu(V) has been calculated as following [43] ... 

Although the redox potentials for aqueous solution indicate that uranium(IV) should reduce plutonium(IV), anions and other complexing agents can change the potentials sufficiently that uranium(IV) and plutonium(IV) can coexist in solution (25). Since one of the products of photochemical reduction of uranyl by TBP is dibutyl phosphate (DBP), which complexes plutonium(IV) strongly, experiments were done to test the photochemically produced urani-um(IV) solutions as plutonium(IV) reductants (26). Bench-scale stationary tests showed these solutions to be equivalent to hydroxylamine nitrate solutions stabilized with hydrazine (27). 

Anion exchange was investigated for application as a small-scale, primary extraction process to recover plutonium directly from dissolved irradiated uranium in 8N HNO3. A 50-kg U/day pilot plant was operated (76). Even with two cycles of ion exchange the fission product activity with the plutonium was undesirably high. Resin stability is another potential problem which was not resolved fully. 

With the Purex process used in the treatment of the irradiated fuels, the final product will be a concentrated solution of plutonium nitrate (100 to 250 g Pu/liter). However, plutonium oxide (PuOa) has been adopted as the material for storage with a view to safety standards because of its characteristics (chemical stability, physical state, etc.) from the viewpoints of preparation, shipping, and storage. ... 

In the PUREX process, the oxidizing property of nitric acid and the formation of nitrous acid are not favorable to maintain plutonium as a trivalent species. The sufficient amount of hydrazine is added to the system to ensure the stability of Pu(III) with the destruction of nitrous acid according to Equation 14.7. Despite the fact that hydrazine is particularly advantageous because the reaction products, N2/ N2O, and H2O, do not contribute to the volume of stored wastes (Schlea et al., 1963), the interaction of hydrazine and nitrous acid can initiate, in a TBP-nitric acid system, and under specific operating conditions, the formation of hydrazoic acid (HN3) which is a hazardous and potentially explosive compoimd (Equation 14.7) (Dukes and Wallace, 1962). Further oxidation leads to the formation of nitrous oxide and nitrogen gases as depicted in Equation 14.8... 

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