Theoretical perspectives on the reduction of Pu(IV) and Np(VI) by methylhydrazine in HNO3 solution: Implications for Np/Pu separation

Effective adjustment and control of the oxidation state of plutonium (Pu) and neptunium (Np) is an indispensable component of Np/Pu separation in spent nuclear fuel reprocessing. Some hydrazine derivatives including methylhydrazine (CH3N2H3) effectively achieves the reduction of Np(VI) to Np(V) without reducing Pu(IV). Herein, we explored the reduction mechanisms of Pu(IV) and Np(VI) by CH3N2H3 in HNO3 solution using scalar-relativistic density functional theory. We elucidated the difference in the reduction mechanism between Np(VI) and Pu(IV) ions by CH3N2H3. The energy barrier for the reduction of [NpVIO2(H2O)5]2+ and [NpVIO2(NO3)(H2O)3]+ by CH3N2H3 is largely different due to the coordination of nitrate ion. Moreover, the energy barrier of the reduction of [NpVIO2(H2O)5]2+ is apparently lower than that of [PuIV(NO3)2(H2O)7]2+, which is in line with the experimental observations. The results of Mayer bond order and localized molecular orbitals clarify the structural evolution of the reaction pathways. Analysis of the spin density demonstrates that the first Np(VI) and Pu(IV) reduction belongs to the outer-sphere electron transfer and the second Np(VI) and Pu(IV) reduction is the hydrogen transfer. This study explains theoretically why CH3N2H3 reduces Np(VI) but not Pu(IV), and helps to design promising reductants for the Np/Pu separation in spent nuclear fuel reprocessing.