Neptunium, with its rich redox chemistry, has a special position in the chemistry of actinides. With a decades-long history of development of aqueous separation methods for used nuclear fuel (UNF), management of neptunium remains an unresolved issue because of its not clearly defined redox speciation. Neptunium is present in two, pentavalent (V) and hexavalent (VI) oxidation states, both in their dioxocation O=Np=O neptunyl form, which differ greatly in their solvent extraction behavior. While the neptunium(VI) dioxocation is being very well extracted, the dioxocation of pentavalent neptunium is practically non-extractable by an organic solvent. As a result, neptunium is not well separated and remains distributed in both organic and aqueous extraction phases. The aim of this study was to develop or enhance the understanding of several key topics governing the redox behavior of neptunium in nitric acid medium, which are of vital importance for the engineering design of industrial-scale liquid-liquid separation systems.
In this work, reactions of neptunium(V) and (VI) with vanadium(V) and acetohydroxamic acid - two redox agents envisioned for adjusting the neptunium oxidation state in aqueous separations – were studied in order to determine their kinetic characteristics, rate laws and rate constants, as a function of temperature and nitric acid concentration. Further were analyzed the interactions of neptunium(V) and (VI) with nitrous acid, which is formed as a product of radiolytic degradation of nitric acid caused by high levels of radioactivity present in such systems. Once HNO₃ is distributed between both the aqueous solutions and organic solvent, nitrous acid is also formed in both phases and has a key influence on redox speciation of neptunium; therefore, the effects of gamma-radiation on the redox speciation of neptunium were investigated. The work also includes the results of examination of scavenging of nitrous acid by hydrogen peroxide, which is generated along with nitrous acid during radiolysis of aqueous solutions of nitric acid, and also by chemical reactions with added scavenging agents (methylurea, acetohydroxamic acid).