Eccentric drop of a control rod in the guide tube with annular gap flow

The drop time of control rod is essential for safety analysis of nuclear power plant. The motion of control rod can be characterized by an annular gap flow with large blockage ratio. In this study, we studied the effects of the eccentricity between the control rod and the guide tube on the annular gap flow, and further on the drop time. By considering the added mass and the upward flow of fluid in the annular gap, we developed a theoretical model for computing the drop time of eccentric rod. In the proposed model, particularly, the annular gap flow velocity was obtained from full-scale computational fluid simulations with respect to blockage ratio and eccentricity. The simulation and experiment results agree well on the drop time, the histories of velocity and acceleration, and the hydrodynamic characteristics. Results show that the drop time increases drastically when blockage ratio alpha increases from 0.83 to 0.91, and the eccentricity effect on drop time is negligible when the blockage ratio is small (e.g., alpha < 0.83). Nevertheless, for large blockage ratio (alpha >= 0.83), the drop time increases significantly for eccentric rod drop, especially for a lighter control rod. The present study provides a general method for analysis of control rod drop, which is critical for the nuclear reactor design.