Effects of fuel oxidation and dissolution on volatile fission product release under severe accident conditions

Kudo, Tamotsu ; Kida, Mitsuko; Nakamura, Takehiko ; Nagase, Fumihisa ; Fuketa, Toyoshi

Release of volatile fission product from high burnup UO fuel was examined in a steam atmosphere under severe accident conditions as a part of VEGA program. Effects of fuel oxidation and dissolution were totally evaluated, by comparing the results with those from previous inert, hydrogen and steam atmosphere tests. It was shown that oxidation of UO to UO by steam generally enhances Cs and Kr release. However, the enhancement becomes smaller above the melting temperature of Zircaloy, about 2030 K, likely due to reduction of UO by molten Zircaloy. Quick dissolution of the fuel by molten Zircaloy and consequent burst release of Cs occur above about 2300 K in a hydrogen atmosphere, while the release rate does not increase so significantly for the examined temperature range (2800 K) in the steam atmosphere. This corresponds to decrease in fuel dissolution rate by a factor of about 1/1000.