Mechanism of Nitric Acid Reduction and Kinetic Modelling

Abstract

International audienceIn France, the recycling of nuclear waste fuels involves theuse of hot concentrated nitric acid. The understanding andprediction of the behaviour of the structural materials(mainly austenitic stainless steels) requires the determinationand modelling of the nitric acid reduction process. Nitric acidis indirectly reduced by an autocatalytic mechanism dependingon the cathodic overpotential and acid concentration.This mechanism has been widely studied. All theauthors agree on its autocatalytic nature, characterized bythe predominant role of the reduction products. It is also generallyadmitted that neither nitric acid nor the nitrate ion isthe electroactive species. However, the nature of the electroactivespecies, the place where the catalytic species regeneratesand the thermodynamic and kinetic behaviour ofthe reaction intermediates remain uncertain. The aim of thisstudy was to clarify some of these uncertainties by performingan electrochemical investigation of the reduction of4 M nitric acid at 40 °C at an inert electrode (platinum orgold). An inert electrode was chosen as the working electrodein a first step to avoid its oxidation and focus the research on the reduction mechanism. This experimental workenabled us to suggest a coherent sequence of electrochemicaland chemical reactions. Kinetic modelling of this sequencewas then carried out for a gold rotating disk electrode.A thermodynamic study at 25 °C allowed the compositionof the liquid and gaseous phases of nitric acid solutionsin the concentration range 0.5–22 M to be evaluated. The kineticsof the reduction of 4 M nitric acid was investigated bycyclic voltammetry and chronoamperometry at an inert electrodeat 40 °C. The coupling of chronoamperometry and FTIRspectroscopy in the gaseous phase led to the identification ofthe gaseous reduction products as a function of the cathodicoverpotential. The results showed that the reduction processis autocatalytic for potentials between 0.6 and 1.15 V/NHE.The electroactive species may be regenerated at the surfaceof the electrode for lower potentials, otherwise this regenerationprocess occurs in solution by a homogeneous chemicalreaction. When the potential is less than 0.6 V/NHE, the fastreduction of nitrogen oxide may lead to rupture of the autocatalyticcycle

Formalization of the kinetics for autocatalytic dissolutions. Focus on the dissolution of uranium dioxide in nitric medium

“…Lefers and Sicsic [2,10] also mentioned equilibrium (3) for low acidities. As NO (aq) free enthalpy does not exist in literature, the thermodynamic constant cannot be calculated here [2].…”

“…Sicsic et al [2], Schwartz and White [3,4] gave an overview of the numerous equilibrium, and their thermodynamics, linked to nitric acid. The species that need to be considered in order to describe the nitric medium [2] are reported in À , and ½U IV ðH 2 OÞ X ðNO 3 Þ 5 À , whose prevalence depends on nitric acid concentrations [5]. In our conditions, with 2-7 mol·l À1 of nitric acid, ½U IV O 2 ðH 2 OÞ 2 ðNO 3 Þ 2 is the main species [5].…”

“…The second hypothesis was defined by Sakurai [6], who suggested that NO is the only dissolution product and that NO 2 is observed because of the equilibrium presented at equation (2). K is the thermodynamic constant of the reaction.…”

“…As NO (aq) free enthalpy does not exist in literature, the thermodynamic constant cannot be calculated here [2].…”

“…These phenomena could be explained by a change in nitric medium equilibrium with temperature. For example, Sicsic et al [2] mentioned that nitric acid dissociation is less favored at high temperature. Taylor suggested that the nitrates, more than the protons, were the important reactants in UO 2 dissolution [15].…”