Dissolution of Th$_{0.25}$U$_{0.75}$O$_2$ sintered pellets: Impact of nitrate ions and nitrous acid

In order to gain insights on the kinetics of the dissolution of Gen(IV) MOX fuels, sintered pellets of Th$_{0.25}$U$_{0.75}$O$_2$ were prepared as surrogates. Particularly, the impact of nitrogen species on the kinetics of dissolution of Th$_{1-x}$U x O$_2$ in nitric acid must be evaluated. In this work, various experiments were performed to determine the influence of nitrate ions and nitrous acid on the dissolution rate and mechanism of Th$_{0.25}$U$_{0.75}$O$_2$ in nitric acid. It appeared that the addition of 3 mol L$^{-1}$NH$_4$NO$_3$ to the 1 mol L$^{-1}$ HNO$_3$ solution led to a strong increase of the dissolution rate. Thus, it seemed that the greatest impact on the dissolution kinetics during the induction period was associated to undissociated HNO$_3$ instead of free nitrate ions. What is more, addition of a constant HNO$_2$ concentration of 2 × 10$^{-3}$ mol L$^{-1}$ to 1 mol L$^{-1}$ HNO$_3$ induced a strong increase of the dissolution rate. However, the dissolution kinetics remained much faster in 4 mol L$^{-1}$ HNO$_3$ , which was associated to the presence of an equivalent concentration of HNO$_2$ in the solution. It then appeared that HNO$_2$ species was not the prevailing species responsible for the catalysis of the dissolution kinetics of Th$_{0.25}$U$_{0.75}$O$_2$ . However, microstructural evolutions of the pellets dissolved in the presence of HNO$_2$ showed the existence of small intra-granular corrosion pits covering the entire surface of the grains. That confirmed the highly oxidizing nature of HNO$_2$ regarding to tetravalent uranium. Nevertheless, the presence of large cavities also suggested the existence of heterogeneous progress of the dissolution, associated to the existence of preferential dissolution zones consequently to the accumulation of reactive species in confined areas. Such reactive species might be HNO$_2$ , but more probably another unknown catalyst present at equilibrium with HNO$_2$ .