Electrochemical Behaviour of Neptunium in Hot and Concentrated Nitric Acid Media: Thermodynamic and Kinetic Modeling

The initial step in spent nuclear fuel reprocessing involves the dissolution of actinides and fission products in concentrated hot nitric acid. Neptunium (Np) is an element requiring specific attention due to its long half-life (t₁/₂ = 2.14 × 10⁶ years). In acidic aqueous environments, Np can exist in multiple oxidation states (range from +III to +VI), depending on the surrounding conditions [1]. However, characterizing the speciation of neptunium in such environments remains challenging, particularly for gaining a deeper understanding of the corrosion mechanisms in the dissolution medium. The electrochemical behaviour of Np in nitric acid has been investigated under varying acid concentrations and thermal conditions using cyclic voltammetry and linear sweep voltammetry on a rotating disk electrode. Initially, four types of inert electrodes (Platine (Pt), Gold (Au), Glassy Carbon (GC), and boron-doped diamond (BDD)) were used to study the behaviour of Np in 1 mol.L -1 nitric acid at room temperature. It has been found that the choice of inert material is crucial for the study of redox species. In nitric acid, inert metals form oxides that overlap and influence the redox peaks of Np. In contrast, glassy carbon and BDD do not form oxides but are less resistant in aggressive environments. By selecting the most relevant electrode from previous tests, the electrochemical behaviour of Np in different nitric acid solutions and Np concentrations has been investigated, and the thermodynamic and kinetic constants (diffusion, coefficient, formal potential of an electrode, standard heterogeneous rate constant,...) have been determined at acid concentrations higher than 1 mol.L -1 and temperatures above 25 °C. Keywords: Neptunium, Redox reaction, Nitric acid, Diffusion coefficient, Cyclic voltammetry [1] S. Ahrland, in: J.J. Katz, G.T. Seaborg, L.R. Morss (Eds.), The Chemistry of the Actinide Elements, second ed., vol. 2, Chapman and Hall, London, 1986, pp. 1480–1546.