New insight on the simultaneous H2 and HNO2 production in concentrated HNO3 aqueous solutions under alpha radiation.

Knowledge of hydrogen and nitrous acid yields (G(H2) and G(HNO2)) from α radiolysis of nitric acid solutions is of critical importance for the technological aspects of reprocessing of spent nuclear fuel (SNF). This study provides critical information on the G values for external alpha irradiation of concentrated HNO3 solutions. An investigation-specifically developed experimental setup allows performing this investigation without encountering issues related to extreme high local doses. In situ monitoring of the UV-visible induced absorption in irradiated HNO3 solutions permitted quantification of HNO2 production, and mass spectrometry was used to quantify H2. The influence of the dose rate and HNO3 concentration was investigated, and the primary yields of these two species were determined. It was found that dose rate increase leads to diminished production of HNO2 and H2, while HNO3 concentration increase leads to increased HNO2 formation and reduced H2 production. The values of the primary yields of these two species were determined and compared to the literature reported values. While the determined values show similar trends as those reported, this study provides accurate radiolytic yields for H2 and HNO2 that are radioelement-independent compared to the α radiolysis using radioisotope/HNO3 mixtures and provides the basis for perfecting numerical codes used for simulating the radiolytic processes associated with SNF reprocessing.

Decoding the Three-Pronged Mechanism of NO3• Radical Formation in HNO3 Solutions at 22 and 80 °C Using Picosecond Pulse Radiolysis.

With nitric acid (HNO3) being at the core of nuclear technology through actinides separation and extraction processes, achieving a complete characterization of the complex processes involving concentrated HNO3 solutions under ionizing radiation equates bringing efficiency and safety into their operation. In this work, the three mechanisms contributing to the formation of nitrate radicals (NO3•) in concentrated nitric acid were investigated by measuring the radiolytic yield of NO3• in HNO3 solutions (0.5-23.5 M) at room (22.5 °C) and elevated (80 °C) temperatures on time scales spanning from picosecond to microsecond by pulse radiolysis measurements. We conclude that the formation yield of NO3•, just after the 7 ps electron pulse, is due to the direct effect and to the ultrafast electron transfer reaction between NO3- and the water cation radical, H2O•+. The absolute formation yield of NO3• radicals due to the direct effect, GNO3•dir, is found to be (3.4 ± 0.1) × 10-7 mol·J-1, irrespective of the concentration and temperature. On longer time scales, >1 ns, an additional contribution to NO3• formation from the reaction between •OH radicals and undissociated HNO3 is observed. The rate constant of this reaction, which is activation-controlled, was determined to be (5.3 ± 0.2) × 107 M-1·s-1 for 22.5 °C, reaching a value of (1.1 ± 0.2) × 108 M-1·s-1 at 80 °C.

Water reduction by photoexcited silica and alumina.

Excess electrons in solvent are amongst the most fascinating chemical species, at the very edge between physics and chemistry. In this contribution, we report the use of silica and alumina photochemistry to create and stabilize aqueous solutions of electron.