by R. Musat, J. L. Marignier, C. Le Naour, S. Denisov, L. Venault, Ph. Moisy and M. Mostafavi
Concentrated nitric acid solutions subjected to radiation produce radicals of extreme importance in the reprocessing of spent nuclear fuel. Knowledge of the different rate constants of the reactions involved in this chemistry is needed to improve the efficiency of the process and to define safe operating practices. Pulse radiolysis measurements are performed to find the rate constant of the reaction between NO3˙ radicals and U(IV) in highly concentrated nitrate solution. The optimal stabilization conditions toward thermal oxidation are defined for the considered solutions at room temperature and at 45 °C by adding anti-nitrous agents such as hydrazinium nitrate (HN) and hydroxyl ammonium nitrate (HAN). The decay of the NO3˙ radical is monitored and its reaction rates with HN, HAN and U(IV) are found to be 1.3 × 105, 1.5 × 107 and 1.6 × 106 M−1 s−1 at room temperature. The latter value is more than 10 times lower than the one currently used in numerical codes for simulation of the long-term radiolytic degradation associated with the reprocessing and storage of spent nuclear waste. At 45 °C, conditions similar to the reprocessing of spent fuel, the values of the rate constants of NO3˙ radical toward HN, HAN and U(IV) increase and are found to be 2.6 × 105, 2.9 × 107 and 9.3 × 106 M−1 s−1.
by Raluca Musat, Sergey A. Denisov, Jean-Louis Marignier and Mehran Mostafavi
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.