Thermodynamic Properties of Gaseous Ruthenium Species Faoulat Miradji, †,‡,§,∥ Sidi Souvi, †,∥ Laurent Cantrel, †,∥ Florent Louis, ‡,∥ and Vale ́ rie Vallet* ,§ † Institut de Radioprotection et de Sû rete ́ Nucle ́ aire (IRSN), PSN-RES, Cadarache, St Paul Lez Durance 13115, France ‡ PhysicoChimie des Processus de Combustion et de l’Atmosphe ̀ re (PC2A), UMR 8522 CNRS/Lille1, Universite ́ Lille 1 Sciences et Technologies, Cite ́ Scientifique, Bâ t C11/C5, F-59655 Villeneuve d’Ascq, Cedex, France § Physique des Lasers Atomes et Molé cules (PhLAM), UMR 8523 CNRS/Lille1, Universite ́ de Lille, F-59655 Villeneuve d’Ascq, Cedex 59655, France ∥ Laboratoire de Recherche Commun IRSN-CNRS-Lille1 “Cine ́ tique Chimique, Combustion, Ré activite ́ ” (C3R), Cadarache, Saint Paul Lez Durance 13115, France * S Supporting Information ABSTRACT: The review of thermodynamic data of ruthenium oxides reveals large uncertainties in some of the standard enthalpies of formation, motivating the use of high-level relativistic correlated quantum chemical methods to reduce the level of discrepancies. The reaction energies leading to the formation of ruthenium oxides RuO, RuO 2 , RuO 3 , and RuO 4 have been calculated for a series of reactions. The combination of different quantum chemical methods has been investigated [DFT, CASSCF, MRCI, CASPT2, CCSD(T)] in order to predict the geometrical parameters, the energetics including electronic correlation and spin−orbit coupling. The most suitable method for ruthenium compounds is the use of TPSSh-5%HF for geometry optimization, followed by CCSD(T) with complete basis set (CBS) extrapolations for the calculation of the total electronic energies. SO-CASSCF seems to be accurate enough to estimate spin−orbit coupling contributions to the ground-state electronic energies. This methodology yields very accurate standard enthalpies of formations of all species, which are either in excellent agreement with the most reliable experimental data or provide an improved estimate for the others. These new data will be implemented in the thermodynamical databases that are used by the ASTEC code (accident source term evaluation code) to build models of ruthenium chemistry behavior in severe nuclear accident conditions. The paper also discusses the nature of the chemical bonds both from molecular orbital and topological view points. ■ INTRODUCTION During a severe accident occurring in a pressurized water reactor (PWR), fission products (FPs) are released from the nuclear fuel and may reach the nuclear reactor containment building. Among the FPs, ruthenium is of particular interest due to its ability to form volatile oxide compounds in highly oxidizing conditions, and its potential high contribution to the dose in case of outside releases, in the short term, via isotope 103 Ru (half-life of 39.3 days), and in the midterm, via isotope 106 Ru (half-life of 372.6 days). A literature survey 1 was carried out about ruthenium behavior in severe accident conditions based on quite old works. It was concluded that additional data are required in order to make a reliable assessment of the ruthenium outside releases in case of severe accident. Since this literature survey in 1986, many works were performed on this topic 2−12 to fill the gap in the prediction of ruthenium behavior. Ruthenium is usually considered as a low volatile fission product, with less than 5% releases, but in some conditions involving ruthenium oxides formation, as shown during the VERCORS HT2 tests 9 dealing with the fission product release from UO 2 fuel with a moderate burn-up [48 (GWd/tU)], in pure steam, the ruthenium released amounted to 60% of the fuel inventory and a relevant part (12%) was transported as the oxide form and not deposited in the experimental line, even at quite a low temperature (150 °C). Some modelings were performed 7,10 to predict ruthenium release from the fuel. Concerning ruthenium transport in the reactor coolant system, recent experimental works 4,8,11,12 were devoted to better characterize deposits and gas phase of ruthenium oxides along a thermal gradient. All data indicate that a fraction of gaseous ruthenium is measured at the outlet, at quite a low temperature, about 150 °C, which may result from a direct formation in gas phase as well as some revaporization from Ru deposits onto surfaces. This gaseous fraction is attributed to RuO 4 ; the decomposition process of RuO 4 to RuO 2 seems to not be fast enough to reach the thermochemical equilibrium state as the rate of decomposition slows down with decreasing temperature. This behavior tends to indicate that a fraction of ruthenium can reach the nuclear containment building either under gaseous form or under Received: February 17, 2015 Revised: April 23, 2015 Published: April 23, 2015 Article pubs.acs.org/JPCA © 2015 American Chemical Society 4961 DOI: 10.1021/acs.jpca.5b01645 J. Phys. Chem. A 2015, 119, 4961−4971