Mathematical modeling of the radiation-chemical behavior of Np(V) in HNO3

A mathematical model of the radiation-chemical behavior of Np(V) in HNO3 solutions is presented, based on the reaction set and rate constants obtained previously in modeling of the radiolytic behavior of Np(IV) and Np(VI). It is shown that, irrespective of the concentrations of nitric acid and neptunium and dose rate, Np(V) is initially oxidized to Np(VI) at a rate increasing with increasing HNO3 concentration. Calculations, as well as the supporting experimental data, show that the oxidation rate tends to decrease with increasing time of irradiation; with [HNO3] ≤ 3 M, the quasi-equilibrium is rapidly established between Np(V) and Np(VI), whereupon both are reduced to Np(IV). The rate of Np(V) reduction is significantly lower than that of its oxidation and tends to increase with increasing [HNO3]. Kinetic equations are derived for the rates of the radiolytic oxidation and reduction of Np(V). Also, reactions responsible for oxidation and reduction of Np(V) under actual experimental conditions are elucidated