Theoretical insights into the reduction mechanism of neptunyl nitrate by hydrazine derivatives

Abstract

Abstract In the advanced spent fuel cycle, the control and adjustment of neptunium valence state is greatly important for the highly efficient separation of neptunium. Hydrazine and its derivatives as salt-free reagents can selectively reduce Np(VI) to Np(V), but their reduction mechanisms are still unclear. We explored the reduction of [NpVIO2(H2O)2(NO3)2] by N2H4 and its two derivatives HOC2H4N2H3 and CHON2H3 using scalar relativistic density functional theory. The thermodynamic energy of the reactions [NpVIO2(H2O)2(NO3)2] with three reductants are sensitive to the substitution group, HOC2H4N2H3 enhances thermodynamic ability of the reaction and CHON2H3 shows contrary result. Both HOC2H4N2H3 and CHON2H3 have lower energy barrier compared to N2H4 based on the potential energy profiles (PEPs), which probably attributes to the intramolecular hydrogen bond of hydrazine derivatives. The nature of these redox reactions is that the hydrogen atom of reductants is gradually transferred to the axis oxygen atom of neptunyl, which accompanies the N–H bond dissociation and Oax–H bond formation. The reduction of Np(VI) with HOC2H4N2H3 is the most favorable reaction based on the thermodynamic and kinetic results. This work provides theoretical perspective into the reduction of Np(VI) to Np(V), which is beneficial to the development of more effective free-salt reductants for the separation of neptunium from uranium and plutonium in spent fuel reprocessing.