Lee K., Yoon K., Song K., Kang H., Song K.

A commercial nuclear fuel assembly for a Pressurized Water Reactor (PWR) consists of more than 250 fuel rods that are supported by small leaf springs at 6 points in the cells of more than 10 Spacer Grids along its length. Although the existence of various nonlinearities in this fuel assembly is highly expected, there are little accessible studies on this problem and no available information for the design use. Therefore, to understand the nonlinearities in the nuclear fuel assembly vibration is necessary to carry out mechanical performance tests and structural fuel design including analyses of its dynamic behavior. This paper presents a feasible way of identification and quantification of the nonlinearities on the vibration characteristics of the commercial nuclear fuel assembly. The test fuel assembly has same geometrical configurations with commercial nuclear fuel, but its fuel rods have Pb pellet instead of UO2 pellet for safety reason. KAERI-devised fuel assembly mechanical tester is used as an experimental test facility. Electro magnetic type shaker is used to apply random and sinusoidal shaking forces to the mid-grid of the fuel assembly and vibration responses by the linear gauge are measured at 12 locations. Nonlinear detection was performed by the classical method which is based on the frequency response function measured from the modal vibration test of random and sinusoidal excitation. Comparisons are made among different identification methods. Then nonlinear quantification factor was defined and estimated according to the degree of bending of frequency response curve. Possible direction of future studies on this issue for the vibration behaviors of the commercial nuclear fuel is discussed.