Reevaluation of Neptunium-Nitric Acid Radiation Chemistry by Multiscale Modeling

Area of Science:

• Radiochemistry
• Nuclear Chemistry
• Chemical Kinetics

Background:

• Understanding the radiation chemistry of neptunium is critical for nuclear fuel reprocessing and waste management.
• Previous models often simplified the complex interplay of radiolytic products in nitric acid solutions.

Purpose of the Study:

• To quantitatively reevaluate the radiation chemistry of neptunium in aerated nitric acid solutions using multiscale modeling.
• To identify the key radiolytic species and reactions governing neptunium oxidation-state changes.
• To investigate the influence of nitric acid concentration on neptunium redox behavior.

Main Methods:

• Multiscale modeling was employed to simulate the radiation chemistry.
• Initial radiolytic yields were calculated with precise accounting for radiation track chemistry.
• Experimental data for neptunium oxidation-state distribution in nitric acid solutions (0.1-6.0 mol dm⁻³) were used for validation.

Main Results:

• Gamma irradiation significantly alters the neptunium(VI)/neptunium(V) oxidation-state distribution.
• Reactions involving nitrous acid (HNO₂), hydrogen peroxide (H₂O₂), nitrite radicals (NO₂•), and nitrate radicals (NO₃•) are dominant.
• Oxidation of Np(V) by NO₃• and reduction of Np(VI) by HNO₂ and H₂O₂ were quantified; an additional Np(V)-HNO₃ oxidation pathway is needed for high acid concentrations (>4.0 mol dm⁻³).
• At low acid concentration (0.1 mol dm⁻³), an unaccounted catalytic-acid-dependent reduction process is suggested.

Conclusions:

• Accurate calculation of initial radiolytic yields is crucial for reproducing experimental observations.
• The established model successfully describes neptunium radiation chemistry across a range of nitric acid concentrations.
• The findings highlight the complexity of neptunium redox behavior in acidic media and identify key reaction pathways.