Kinetic and thermodynamic controls on CsI-Mo gas-phase reactions under varying oxygen potentials

Shiotsu, Hiroyuki  

The transport and release behavior of fission products (FPs) during nuclear power plant accidents is strongly influenced by their chemical forms, particularly gaseous species, which can lead to enhanced environmental release. For iodine, one of the most volatile FPs, condensable cesium iodide (CsI) has traditionally been regarded as the dominant chemical form in state-of-the-art source term evaluations. However, recent experiments have indicated that molybdenum (Mo), a semi-volatile FP, can promote the formation of gaseous iodine through gas-phase reactions with CsI. The key controlling factor of these reactions is the oxygen potential of the atmosphere. In the TeRRa experiments, CsI-Mo gas-phase reactions were observed at 1150 K under Ar-20%O-0.8%O conditions (-31.7 kJ/mol-O), whereas no reaction occurred under Ar-20%HO conditions (-149 kJ/mol-O). Nevertheless, the specific reactive conditions governing these reactions have not yet been fully clarified. In this study, the oxygen-potential dependence of gas-phase reactions between CsI and Mo vapors in the TeRRa-CsIMo series experiments was numerically investigated using chemical equilibrium, mass transport, and reaction kinetics analyses. Chemical equilibrium and transport analyses were performed using the VICTORIA code, while detailed kinetic analyses were conducted with the Cantera software and the ECUME database. The results demonstrate that although CsI-Mo gas-phase reactions are thermodynamically favorable under oxidizing conditions that stabilize Mo in the MoO form ( -158 kJ/mol-O), their contribution to gaseous iodine formation is strongly constrained by kinetic limitations under lower oxygen potential conditions, even at high temperatures around 1150 K. These findings suggest that both thermodynamic and kinetic effects must be considered for reliable evaluation of iodine source terms during severe accidents, particularly under oxygen-starved environments.