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.