Gamma Radiolysis of Hydroxylamine in Aqueous Nitric Acid Solutions

The radiolytic stability of hydroxylamine (NH₃OH⁺) in concentrated nitric acid (HNO₃) is directly relevant to plutonium finishing in used nuclear fuel reprocessing, where NH₃OH⁺ serves as both a plutonium reductant and nitrous acid (HNO₂) scavenger under high-radiation conditions. In this study, aqueous 1 M HNO₃ solutions containing NH₃OH⁺ were gamma irradiated to accumulated doses of up to 100 kGy. The radiolytic loss of NH₃OH⁺ was found to be independent of its initial concentration, pointing to its reaction with HNO₂ and hydroxyl (^•OH) and nitrate (NO₃^•) radicals as the primary degradation mechanisms. Nitrous oxide (N₂O) was the dominant degradation product, with smaller amounts of molecular nitrogen (N₂) and hydrogen (H₂)─the latter exclusively from water radiolysis. A multiscale computer model was developed, combining stochastic radiation track-structure simulations with a deterministic kinetic model, and validated against the complete experimental dataset. The model quantitatively reproduced the radiolytic loss of solute NH₃OH⁺ and solvent nitrate ions (NO₃^–), and the evolution of N₂ and H₂. The N₂O yields, however, are determined by reactions of the aminoxyl radical intermediate (NH₂O^•), whose rate coefficients under strongly acidic conditions remain experimentally uncharacterized, representing the principal source of model uncertainty. Nevertheless, the presented model provides robust, quantitatively accurate predictions of NH₃OH⁺ stability under conditions relevant to plutonium finishing operations. Because maintaining a sufficient NH₃OH⁺ inventory is critical for holding Pu(III) prior to oxalate precipitation, the validated multiscale model is a valuable predictive tool that could be leveraged for process optimization within reprocessing flowsheets.