For high earthquake resistance and ease of installation, free standing racks which are not anchored to the pool floor or walls has been adopted in many countries.

Under the earthquake, the response of the free standing rack is highly nonlinear and involves a complex combination of motions (sliding, rocking, twisting, and turning) and impacts between the fuel assemblies and the fuel cell walls, rack-to-rack, and the pit floor and the rack pedestals. To obtain an accurate simulation of the free standing rack, the seismic analysis requires careful considerations of these complex phenomena (sliding, rocking, twisting, and turning), fluid coupling effects and frictional effects.

The important evaluation items while applying the free standing rack to the actual nuclear plants are maximum sliding displacement of the rack, maximum rocking displacement and maximum leg load under earthquake. When the sliding displacement increases, the rack may collide against the spent fuel pool wall. In addition, the free standing rack should not exhibit tilt sufficient to cause to the rack to overturn. The vibration tests were conducted in order to predict the rack behavior under earthquake, and the analysis method was validated by comparison to tests results. Furthermore, we developed the seismic design method to obtain the margin of safety for free standing rack.

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