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.