Impact of fuel thermal conductivity degradation on Doppler feedback during rod ejection accident

The power pulse, the time evolution of average fuel temperature, and the peak enthalpy rise during the transient were examined. It was confirmed that the impact of the fuel thermal conductivity degradation is more significant when the control rod is ejected at hot full power conditions. If the fuel conductivity degradation was not taken into account, less conservative CTF/TORT-TD predictions for the transient power response were obtained. For the selected 4 × 4 pin array, the coupled code calculated 13 MW higher power pulse when modeling degradation effects on fuel conductivity. The difference in the power response is due to the less negative prompt fuel temperature (Doppler) coefficient at elevated temperatures. Lower thermal conductivity will lead to higher fuel pellet temperatures, and subsequently to a less negative Doppler coefficient, which will result in a stronger power pulse. The maximum fuel enthalpy rise during the hot full power rod ejection accident was found to be 60 cal/g (251,208 J/kg).