Ab Initio Investigation of the Competitive Adsorption of RuO4 and NOX 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 NO_X 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 NO_X 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 NO_X partial pressure and decreases with temperature. The results of the study will also be of interest in other fields like heterogeneous catalysis, where RuO_x surfaces are involved.