The purpose of this study is to investigate dynamic response behaviors of fuel assemblies in a boiling water reactor (BWR) under seismic loading. The core of BWR consists of several hundreds of fuel assemblies. They are supported with both top guide and fuel support and are surrounded by coolant water. It is important to grasp their dynamic response behaviors under seismic loading for securing the structural integrity of the fuel assembly itself as well as for assessing control rod scrammability. In this study, we employ two different numerical simulation methods of acoustic fluid-structure interaction (AFSI) developed by the present authors independently. The one is a three-dimensional parallel finite element method for AFSI problems with solid elements based on a partitioned coupling approach, while the other is a finite element method of beam elements for fuel assemblies combining added mass matrix, which represents coupled inertia effects caused by coolant water.

Both methods are first applied to a problem of 36 fuel assemblies for numerical verification, and then applied to a problem of 368 fuel assemblies for validation. The latter problem was set up based on the demonstration test performed by the NUPEC (Nuclear Power Engineering Corporation) in 1986. Both simulation results agreed well with each other in all cases, and the simulated results also agreed well with the experimental ones. In addition, we have precisely discussed seismic response behaviors of the fuel assemblies, which were not shown in the demonstration test. Accordingly, we conclude that the both developed simulation methods are powerful tools to grasp the precise behavior of fuel assemblies of BWR under seismic loading and to improve the seismic safety design of BWR core.

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