A new strategy to select axial power distribution for thermal hydraulic analysis

An appropriate method for selecting the axial power distribution that results in the most conservative thermal margins was developed in this study. Nuclear physics calculations provided axial power distributions with various shapes based on the core states and cycles. A total of 17 cycles were required to reach the equilibrium core from the initial core. The axial power distributions were sorted based on the power peaking and sum-to-peak (STP) values. In the present study, three different power peaking criteria were used to sort the axial power distributions: the upper 50%, upper 25%, and upper 10%. After the first sorting based on the power peaking, the axial power distributions were grouped based on the power peak location in the axial direction. In each group, the axial power distribution with the highest STP was chosen for thermal margin analyses. In addition, the axial power distributions in the hottest channel and fuel assembly from the initial to the equilibrium core were obtained. Since the axial power distributions in the hottest channel and fuel assembly were bounded by the axial power distribution with the upper 10% F Q, the thermal hydraulic analysis results gave the more conservative thermal margins using the selection method presented in this study. The Onset of Nucleate Boiling (ONB) temperature margin with the upper 10% F Q was 22.5 degree C, but it was 25.0 degree C for the hottest channel. Therefore, the axial power distribution used for thermal hydraulic and safety analyses should be selected using the appropriate method presented in this study.