Effect of Thermal Conductivity Degradation for High-Burnup Fuel during a Postulated Control Element Assembly Ejection Accident

Abstract

To evaluate the effect of thermal conductivity degradation for high-burnup fuel, a postulated control element assembly (CEA) ejection accident is assessed with the SPACE (Safety and Performance Analysis CodE) code. The SPACE code, which is currently under development as a safety analysis code for nuclear power plants, can predict thermal-hydraulic responses of the nuclear fuel and nuclear steam supply system during design basis accidents with two-fluid, three-field governing equations. Fuel performance behaviors during the CEA ejection accident using six fuel conductivity models including the burnup-independent reference conductivity model, the Lyons model, are investigated and compared with results of the reference model within the range from 0 to 30 GWd/tonne U. The Oak Ridge National Laboratory model predicts the highest peak fuel centerline temperature of 4531°F at 0 GWd/tonne U, and the modified Nuclear Fuels Institute model shows the uppermost value of 4796°F, which is 364°F higher than the reference model at 30 GWd/tonne U. It is also observed that the peak fuel centerline temperature increases linearly with fuel burnup and that the maximum increase rate of the peak centerline temperature per fuel burnup is ˜11.6°F per GWd/tonne U. For all thermal conductivity models, the maximum radial average fuel enthalpies are <230 cal/g, and the rise in radial average fuel enthalpy during the CEA ejection accident still remains within the pellet-cladding-mechanical-interaction failure criterion.