Decoding the Three-Pronged Mechanism of NO Radical Formation in HNO Solutions at 22 and 80 °C Using Picosecond Pulse Radiolysis.

With nitric acid (HNO) being at the core of nuclear technology through actinides separation and extraction processes, achieving a complete characterization of the complex processes involving concentrated HNO 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 (NO) in concentrated nitric acid were investigated by measuring the radiolytic yield of NO in HNO 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 NO, just after the 7 ps electron pulse, is due to the direct effect and to the ultrafast electron transfer reaction between NO and the water cation radical, HO. The absolute formation yield of NO radicals due to the direct effect, G, is found to be (3.4 ± 0.1) × 10 mol·J, irrespective of the concentration and temperature. On longer time scales, >1 ns, an additional contribution to NO formation from the reaction between OH radicals and undissociated HNO is observed. The rate constant of this reaction, which is activation-controlled, was determined to be (5.3 ± 0.2) × 10 M·s for 22.5 °C, reaching a value of (1.1 ± 0.2) × 10 M·s at 80 °C.