Free Vibration Characteristics of Multi-constrained Fuel Rod

In order to develop flow-induced vibration calculation program for nuclear fuel design, a theoretical method for analyzing multi-constrained fuel rod vibration is established based on beam theory and potential flow theory. Firstly, the vibration control equations in air and the overall stiffness matrix and mass matrix in dry modal are obtained through the multi-span continuous beam theory. Then the additional mass matrix in the wet modal is presented by considering the effect of axial flow and boundary conditions through the potential flow theory. Finally, Finally, the pressurized water reactor (PWR) fuel rod is used as an example. The theoretical analysis results about its natural frequencies and modes are obtained and the effects of spring stiffness and added mass coefficient on the natural frequency are explored. The results show that the theoretical analysis results are consistent with those calculated by the ANSYS. As the fuel rods are in bundle in the core and are surrounded by high-speed flow, its vibration frequency and mode are affected by axial fluid flow and rod boundary, but due to multi-constraints the vibration mode is seldom affected. The larger the tension and torsion spring stiffness, the higher the vibration frequencies of the fuel rod. The first natural frequency can be increased by 79.1% with the torsion spring increasing. The higher the additional mass coefficient, the lower the vibration frequencies of the fuel rod. The first natural frequency can be reduced by 18.2% as the additional mass coefficient increases. The ideal vibration characteristics can be obtained by optimizing the stiffness, which provides reference for the design of the grid.