Study on the flow and mechanical properties of the control rod guide tube assembly

To investigate the impact of coolant flow within a 2×2 control rod guide tube (CRGT) assembly on the deformation and wear of control rods, this study employed a synergistic approach utilizing both experimental measurements and numerical simulations. The flow characteristics, along with control rod strain and deformation, were systematically analyzed from the upper nozzle to the continuous guide section outlet. Flow fields at critical internal locations were captured via Particle Image Velocimetry (PIV). To enhance predictive fidelity, the constants of the Realizable k-ϵ turbulence model were recalibrated using an Ensemble Kalman Filter (EnKF) algorithm. Subsequent fluid-structure interaction (FSI) simulations, grounded in these optimized constants, provided high-resolution data on unmeasured flow regions and mechanical responses. The experimental results indicated a highly non-uniform flow distribution within the CRGT. Among the flow holes on the core upper plate, the flow distribution ratio of the upper guide tube holes was 30% higher than that of the support column holes and 17% higher than that of the unobstructed flow holes. Pronounced transverse flow were identified near the hold-down springs and flange gap. After recalibration through data assimilation, the velocity error between the simulations and experiments at the gap between upper nozzle and guide tube was reduced from 10% to 4%. FSI analysis further revealed that the maximum principal strain was concentrated between the upper nozzle and the guide tube flange, where control rods were subjected to opposing fluid-induced excitation forces.