Speaker
Description
For nuclear safety of spent fuel reprocessing plant, special attention must be paid to the volatile ruthenium species, particularly in case of the High-Level Liquid Waste (HLLW) storage tank accident scenario. The behavior of gaseous ruthenium therefore requires a detailed understanding of the successive stages of ruthenium volatilization and its chemical speciation, from liquid solutions up to residues. Up to now, it has been assumed that ruthenium tetroxide is the predominant volatile species; however, additional poorly characterized compounds, including ruthenium hydroxides (Ru-OH), oxo-hydroxides (Ru-(O)OH), and nitrates (Ru-NO3), have also been experimentally reported. A key unresolved question is whether RuO₄ is directly released from solution as the primary volatile species, or whether it is formed secondarily through the decomposition or oxidation of other dissolved precursors, such as nitrosyl–ruthenium complexes (Ru(NO)(NO3)3), which may instead constitute the initially and congruently released species. The present work aims to address this question by investigating, from a kinetic standpoint, whether such a gas-phase transformation is feasible under conditions relevant to accident scenarios in HLLW storage tanks.
Density Functional Theory (DFT) calculations were carried out using the TPSS hybrid functional in conjunction with augmented correlation-consistent basis sets (aug-cc-pVnZ, n = T, Q, and 5). Complete basis set (CBS) extrapolations were performed to obtain reliable energetic profiles. Starting from ruthenium nitrosyl complexes, the energetics of ligand dissociation were investigated in the gas-phase environments. The results indicate that the decomposition of nitrosyl complexes, as well as that of their primary by-products, does not lead to the formation of RuO₄ due to the presence of kinetically inaccessible or highly unstable intermediate species along the reaction pathway, even though the overall reaction is thermodynamically favorable.
These findings imply that, if RuO₄ is experimentally observed in the gaseous phase during HLLW processing, it must correspond to a species released congruently and directly from the liquid solution rather than being formed through secondary gas-phase transformations.