Modelling of the nitric acid reduction process: Application to materials behavior in reprocessing plants

In France, the recycling process of nuclear waste fuels involves the use of hot concentrated nitric acid. The understanding and the prediction of the structural materials (mainly austenitic stainless steels) behaviour requires the determination of the nitric acid reduction process. Nitric acid is indirectly reduced by an autocatalytic mechanism depending on the cathodic overpotential and the acid concentration. This mechanism has been widely studied. All the authors agree on its autocatalytic nature, characterized by the predominant role of the reduction products. It is also generally admitted that nitric acid or the nitrate ion are not the electro-active species. However, uncertainties remain concerning the nature of the electro-active species, the place where the catalytic species regenerates and the thermodynamic and kinetic behaviour of the reaction intermediates. The aim of this study is to clarify some of these uncertainties by performing an electrochemical investigation of the 4 mol.L -1 nitric acid reduction process at 40 deg. C occurring on an inert electrode (platinum or gold). An inert electrode was chosen as a working electrode in a first step in order to avoid its oxidation and focus the research on the reduction mechanism. This experimental work enabled to suggest a coherent sequence ofmore » electrochemical and chemical reactions. Then, a kinetic modelling of this sequence was carried out for a gold rotating disk system. In this objective, a thermodynamic study at 25 deg. C led to the evaluation of the composition of liquid and gaseous phases for nitric acid solutions from 0.5 to 22 mol.L -1. The kinetics of the reduction process of nitric acid 4 mol.L -1 was investigated by cyclic voltammetry and chrono-amperometry on an inert electrode at 40 deg. C. A coupling of chrono-amperometry and FTIR in gaseous phase led to the identification of the gaseous reduction products as a function of the cathodic overpotential. These different results showed that for potentials between 0.6 V/NHE and 1.15 V/NHE, the reduction process is autocatalytic. The regeneration of the electro-active species may take place at the surface of the electrode for lower over-potentials, and otherwise in solution. When the potential is lower than 0.6 V/ENH, the fast reduction of nitrogen oxide may lead to the rupture of the autocatalytic cycle. (authors)« less