Reevaluation of Neptunium-Nitric Acid Radiation Chemistry by Multiscale Modeling

Multiscale modeling has been used to quantitatively reevaluate the radiation chemistry of neptunium in a range of aerated nitric acid solutions (0.1-6.0 mol dm^-3). Exact calculation of initial radiolytic yields accounting for changes in radiation track chemistry was found to be crucial for reproducing experimental data. The γ irradiation induces changes in the Np(VI)/Np(V) oxidation-state distribution, predominantly driven by reactions involving HNO₂, H₂O₂, NO₂^•, and NO₃^• from the radiolysis of aqueous nitric acid. Oxidation of Np(V) by NO₃^• (k = 8.1 × 10⁸ dm³ mol^-1 s^-1) provides the initial increase in Np(VI) concentration, while also delaying net reduction of Np(VI) by consuming HNO₂. Reduction of Np(VI) is dominated by thermal reactions with HNO₂ (k = 0.7-73 dm³ mol^-1 s^-1) and H₂O₂ (k = 1.9 dm³ mol^-1 s^-1). A steady state is eventually established once the concentration of Np(V) is sufficiently high to be oxidized by NO₂^• (k = 2.4 × 10²-3.1 × 10⁴ dm³ mol^-1 s^-1). An additional thermal oxidation reaction between Np(V) and HNO₃ (k = 2.0 × 10³ dm³ mol^-1 s^-1) is required for nitric acid concentrations >4.0 mol dm^-3. For 0.1 mol dm^-3 HNO₃, the rate of Np(VI) reduction is in excess of that which can be accounted for by radiolytic product mass balance, suggesting the existence of a catalytic-acid-dependent reduction process.