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

Abstract

ABSTRACT 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.