Reevaluation of Neptunium–Nitric Acid Radiation Chemistry by Multiscale Modeling

Journal Article · Mon Nov 14 23:00:00 EST 2016 · Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry

DOI:https://doi.org/10.1021/acs.jpcb.6b09683· OSTI ID:1470313

Horne, Gregory P. ^[1]; Grimes, Travis S. ^[2]; Mincher, Bruce J. ^[2]; Mezyk, Stephen P. ^[3]

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1. California State Univ. at Long Beach, Long Beach, CA (United States); Univ. of Notre Dame, Notre Dame, IN (United States)
2. Idaho National Lab. (INL), Idaho Falls, ID (United States)
3. California State Univ. at Long Beach, Long Beach, CA (United States)

Here, multi-scale modelling 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 gamma 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, whilst 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 enough 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.

• Research Organization: Idaho National Laboratory (INL), Idaho Falls, ID (United States)
• Sponsoring Organization: USDOE Office of Nuclear Energy (NE)
• Grant/Contract Number: AC07-05ID14517
• OSTI ID: 1470313
• Report Number(s): INL/JOU--16-40072-Rev000
• Journal Information: Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry, Journal Name: Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry Journal Issue: 49 Vol. 120; ISSN 1520-6106
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

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