Study on the Nonlinear Dynamic Model of a Single Fuel Assembly Based on the Bouc-Wen Hysteresis Model

To simulate the nonlinear phenomena during lateral vibration of a single fuel assembly, this study introduces a Bouc-Wen hysteresis model with two-stage stiffness degradation based on the linear single-beam model. Using the finite element discretization method, we established a nonlinear dynamic model for lateral vibration of the single fuel assembly. Furthermore, combined with partial mechanical property test data, the multi-objective genetic optimization algorithm (NSGA-Ⅱ) was employed to identify unknown parameters of the model. The comparison between computational results and experimental data reveals the following: The root mean square error (RMSE) of normalized displacement during the lateral loading-unloading process is 0.027. Under five different initial conditions, the relative errors of free vibration response frequencies are less than 6%, and the relative errors of damping ratios are less than 3%. These results demonstrate that the nonlinear dynamic model effectively captures key phenomena observed in fuel assemblies, including stiffness degradation, frequency reduction, and damping ratio increase with growing lateral displacement. This study provides a methodological reference for the accurate simulation of lateral mechanical response characteristics in fuel assemblies. Furthermore, the developed nonlinear model exhibits broader applicability and may facilitate the exploration of design margins for fuel assemblies under large-deformation scenarios.