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 HNO 2, H 2 O 2, NO 2 •, and NO 3 • from the radiolysis of aqueous nitric acid. Oxidation of Np(V) by NO 3 • ( k = 8.1 × 10 8 dm 3 mol –1 s –1 ) provides the initial increase in Np(VI) concentration, while also delaying net reduction of Np(VI) by consuming HNO 2 . Reduction of Np(VI) is dominated by thermal reactions with HNO 2 ( k = 0.7–73 dm 3 mol –1 s –1 ) and H 2 O 2 ( k = 1.9 dm 3 mol –1 s –1 ). A steady state is eventually established once the concentration of Np(V) is sufficiently high to be oxidized by NO 2 • ( k = 2.4 × 10 2 –3.1 × 10 4 dm 3 mol –1 s –1 ). An additional thermal oxidation reaction between Np(V) and HNO 3 ( k = 2.0 × 10 3 dm 3 mol –1 s –1 ) is required for nitric acid concentrations >4.0 mol dm –3 . For 0.1 mol dm –3 HNO 3, 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.