  Quantitative predictions of the release of volatile radiocontaminants of
ruthenium (Ru) in the environment from either nuclear power plants (NPP) or
fuel recycling accidents present significant uncertainties while estimated by
severe accidents nuclear analysis codes. Observations of Ru from either
experimental or modeling works suggest that the main limitations relate to the
poor evaluation of the kinetics of gaseous Ru in the form of RuO$_3$ and
RuO$_4$. This work presents relativistic correlated quantum chemical
calculations performed to determine the possible reactions pathways leading to
the formation of gaseous Ru oxides under NPP severe accident conditions, as a
result of reactions of RuO$_2$ gaseous with air radiolysis products, namely
nitrous and nitrogen oxides. The geometries of the relevant species were
optimized with the TPSSh-5%HF functional of the density, while the total
electronic energies were computed at the CCSD(T) level with extrapolations to
the complete basis set CBS limit. The reaction pathways were fully
characterized by localizing the transition states and all intermediate
structures using the internal coordinate reaction algorithm (IRC). The rate
constants were determined over the temperature range 250-2500 K. It is revealed
that the less kinetically limiting pathway to form Ru gaseous fraction is the
oxidation of Ru by nitrogen oxide, corroborating experimental observations.
