Modeling of Fuel Assemblies in a Boiling Water Reactor for Seismic Analysis

The purpose of this study is to develop a seismic analysis model of a group of fuel assemblies in a boiling water reactor and to confirm the validity of the developed model. Each fuel assembly was modeled as a beam on the basis of the finite element method. The mass matrix of the model includes an added mass matrix, which represents the coupled inertia effect caused by the coolant water, in order to simulate the coupled vibration of fuel assemblies. The added mass matrix was obtained by calculating the coefficient matrix of the acceleration vector and fluid force vector under the condition that each fuel assembly moves at unit acceleration. The validity of the model was confirmed by comparing the calculated results with experimental ones. The compared specimens for the experiments were full-scale mock-ups. The vibration characteristics of fuel assemblies in each case of 4 bodies and 368 bodies were compared. As a result of the comparison, the calculations of the frequency response were in agreement with the experimental results. Particularly, the calculation results on the resonance frequency were in good agreement, with an error of less than 2 percent, with the experimental ones. Furthermore, the calculated vibration characteristics of 368 fuel assemblies in the case of an earthquake, such as the excited vibration mode and phase characteristics, were in agreement with the experimental ones. We concluded that the developed model of fuel assemblies was applicable to seismic analysis of a boiling water core.