Behavior of cesium molybdate, Cs₂MoO₄, in severe accident conditions. (1) Partitioning of Cs and Mo among gaseous species

In order to better understand the behavior of cesium in severe accident of Light Water Reactor (LWR), the high-temperature chemistry of Cs₂MoO₄ in H₂O + H₂ gas was studied. The pseudo-binary system, Cs₂MoO₄-MoO₃, was thermochemically modeled with Redlich-Kister formulation to form a basis to analyze the high-temperature behavior of Cs₂MoO₄. The model prediction was compared with the thermogravimetric measurements of Cs₂MoO₄ in dry and humid argon, which revealed that the mass-loss rate was enhanced in humid atmosphere. The thermochemical model was further applied to predict the partitioning of cesium and molybdenum among gaseous species in the boiling water reactor-core degradation condition typical of short-term station blackout. Effects of the total pressure (3.5-75 bar) as well as the H₂/H₂O ratio (1/4000-2) were examined. CsOH(g) is the predominant cesium species, when the damaged fuel temperature is higher than 2000 K at higher steam pressures, but Cs₂MoO₄(g) would become more important as the steam cools toward the steam dome. The condensation of Cs₂MoO₄ occurs below ∼1900 and ∼1550 K at 75 and 3.5 bar, respectively. Besides, the ideal mixing of complex component model has also been examined for its simplicity. The latter gave satisfactory prediction as far as the condensed phase composition is concerned. (author)