Ab Initio Investigation of the Competitive Adsorption of RuO₄ and NO_X on a RuO₂ Surface: Assessment of RuO₄ Release in a Severe Nuclear Accident

Nuclear safety has recently become a central issue in the context of the current energy and geopolitical crises. Accurate estimates of the nature and extent of radioactive species potentially released from nuclear facilities are essential for assessing and optimizing safety plans. Computational techniques appear as a powerful tool to understand the formation and transport of such species and to quantify their amount. This study focuses on the decomposition of gaseous ruthenium tetroxide (RuO4), a fission product likely to be released into the emergency ventilation ducts in the event of an accident at a fuel reprocessing plant. RuO4(g) is expected to decompose during its transport on the available surfaces into solid RuO2 (c) and amorphous O2(g). However, other species like NOX present in the environment might interact with the surfaces of the growing deposit (RuO2), poisoning the reactive sites vis-à-vis the RuO4 adsorption/decomposition and favoring its transport in the gas phase to the environment. To understand this poisoning effect on RuO2 surfaces, we carried out periodic density functional theory calculations at the PBE level. The impact of dispersion corrections (PBE+D level) is presented in the Supporting Information. RuO4, NO, and NO2 molecules at different coverages are computed on the RuO2 (110) surface. We provide adsorption Gibbs free energies, isotherms, and the poisoning effect in the conditions of an accident. Our results show that the NOX seems to have a noticeable poisoning impact on the RuO2 (110) surface, reducing the adsorption of RuO4(g), the first step of the condensation mechanism. Our calculations show that this impact increases with NOX partial pressure and decreases with temperature. The results of the study will also be of interest in other fields like heterogeneous catalysis, where RuOx surfaces are involved.