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Ab initio molecular orbital calculations have been performed to develop an elementary reaction mechanism for the autocatalytic and scavenging reactions of hydroxylamine in an aqueous nitric acid medium. An improved understanding of the titled reactions is needed to determine the "stability boundary of hydroxylamine" for safe operations of the plutonium-uranium reduction extraction (PUREX) process. Under the operating conditions of the PUREX process, namely, 6 M nitric acid, the reactive forms of hydroxylamine are NH2OH, NH3OH+, and the complex NH3OH.NO3, and those of nitrous acid are NO+, H2ONO+, N2O4, N2O3, NO2, and NO. High-level CBSQB3/IEFPCM and CBSQB3/COSMO calculations were performed using GAUSSIAN03 to investigate the energy landscape and to explore a large number of possible ion-ion, ion-radical, ion-molecule, radical-radical, radical-molecule, and molecule-molecule pathways available to the reactive forms of the reactants in solution. It was found that in solution the autocatalytic generation of nitrous acid proceeds through free radical pathways at low-hydroxylamine concentrations from unprotonated NH2OH via hydrogen abstraction. At high [NH3OH+], we suggest a possible involvement of the NH3ONO+ intermediate via the reaction NH2ONO + NO2 --> HNO + HONO + NO. The NH3ONO+ intermediate, in turn, is formed favorably via the ion-ion reactions of NH3OH+ with NO+ and/or the reaction between NO+ and hydroxylammonium nitrate (HAN). The intermediates involved in the scavenging reaction of nitrous acid by hydroxylamine are NH3ONO+, NH2ONO, NH2(NO)O, NH(NO)OH, and HONNOH and the rate-determining step is the 1,2-NO migration in NH2ONO leading to NH2(NO)O. Reactions NH2ONO --> NH2(NO)O and NH2(NO)O --> NH(NO)OH were studied with two explicit water molecules and the results are discussed in the context of the importance of the explicit treatment of solvent in the determination of the energetics and mechanism of these processes. The rate constants for the reactions were estimated using transition-state theory and other traditional techniques. The kinetic parameters obtained at the B3LYP/CBSB7/IEFPCM level are in reasonable agreement with the limited experimental value. IEFPCM results on free energy of undelocalized polar ions such as NO3-, NO2-, and NH3OH+ are not very accurate and have difficulties in predicting the right direction of acid dissociation equilibrium of HONO2, HONO, and NH3OH+. Explicit incorporation of a solvation shell to these ions improves the theoretical descriptions of acid ionization equilibria as it captures some of the nonlocal effects of these ions. Additional work is needed to correctly describe the solvation shell and to introduce consistency in the theoretical treatment involving explicit solvent molecules. Nevertheless, this systematic exploration of reactions in solution and mechanism development for a solution phase process based on self-consistent reaction field (SCRF) results is likely to be one of the first of its kind.

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We propose a new modeling approach for scatter estimation and descattering in polyenergetic X-ray computed tomography (CT) based on fitting models to local neighborhoods of a training set. X-ray CT is widely used in medical and industrial applications. X-ray scatter, if not accounted for during reconstruction, creates a loss of contrast in CT reconstructions and introduces severe artifacts including cupping, shading, and streaks. Even when these qualitative artifacts are not apparent, scatter can pose a major obstacle in obtaining quantitatively accurate reconstructions. Our approach to estimating scatter is, first, to generate a training set of 2D radiographs with and without scatter using particle transport simulation software. To estimate scatter for a new radiograph, we adaptively fit a scatter model to a small subset of the training data containing the radiographs most similar to it. We compared local and global (fit on full data sets) versions of several X-ray scatter models, including two from the recent literature, as well as a recent deep learning-based scatter model, in the context of descattering and quantitative density reconstruction of simulated, spherically symmetrical, single-material objects comprising shells of various densities. Our results show that, when applied locally, even simple models provide state-of-the-art descattering, reducing the error in density reconstruction due to scatter by more than half.