Releases of Cesium and Poorly Volatile Elements from UO sub(2) and MOX Fuels under Severe Accident Conditions

Radionuclide release from fuel under severe accident conditions has been investigated in the VEGA program at the Japan Atomic Energy Agency. In this program, three types of fuel, two UO sub(2) fuels irradiated at PWR and BWR and a MOX fuel irradiated at the ATR Fugen, were heated up to about 3130 K in helium atmosphere at 0.1 MPa. Comparison of experimental data and evaluation with computer code analyses showed that Cs release is essentially identical among the three fuels. The Cs release from fuel may differ below about 1770 K due to a difference in migration to grain boundaries during irradiation. The difference was not also observed for releases of poorly volatile elements, namely, U, Pu, Sr and Mo between UO sub(2) and MOX fuels. The release rate of Pu became slightly higher than that of U at 3130 K. The release rate of Sr increased at 3130 K, while that of Mo was quite low at temperatures above 2310 K.