Reevaluation of Neptunium–Nitric Acid Radiation Chemistry by Multiscale Modeling

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• 作者：G. P. HorneT. S. Grimes ; B. J. Mincher ; S. P. Mezyk
• 刊名：Journal of Physical Chemistry B
• 出版年：2016
• 出版时间：December 15, 2016
• 年：2016
• 卷：120
• 期：49
• 页码：12643-12649
• 全文大小：383K
• ISSN：1520-5207

文摘

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