Development and Assessment of Improved Spacer Grid Model in  Rod Bundle Subchannel

In order to improve the prediction accuracy of two-phase local parameters in the rod bundle subchannel, based on the distributed resistance method, an improved spacer grid model is established in this paper, with the appropriate expression of the friction forces and a detailed model of mixing vane. The Carlucci turbulent mixing model is used to calculate the turbulent mixing rate, and the blocking factor is introduced to consider the turbulent mixing effect caused by the spacer grid. Coupled with subchannel analysis code ATHAS, the improved spacer grid model is used to calculate and analyze the PWR subchannel and bundle tests(PSBT) data. The results indicate that the improved spacer grid model has higher accuracy in predicting the distribution of the void fraction and outlet temperature in rod bundle subchannel. It lays a foundation for the accurate calculation of local parameters and CHF prediction in the rod bundle subchannel.