A qualitative comparison of barium behaviour in the PHEBUS FPT0 test and analytical tests

Thermochemical considerations have been used to understand the difference between the barium releases observed in the PHEBUS FPT0 in-reactor severe accident test (<1%) and those observed in the ORNL and VERCORS analytical experiments (>40%). BaO is slightly soluble in UO2, and BaUO3 may also be formed as a separate phase, as may BaZrO3 and more complex solid-solution phases. Barium vapour pressure is generally a monotonic function of the mole fraction of BaO in the fuel; however, the Ba vapour pressure decreases in the region between the solidus and the liquidus, because of a simultaneous decrease of Ba concentration in both liquid and solid phases. Zirconium and iron can also reduce the Ba volatility. These phenomena may be used to rationalize the low release of Ba observed in the PHEBUS FPT0 experiment. (authors)