This paper describes an investigation on damping characteristics of large-bore LMFBR piping. To clarify the characteristics, two kinds of seismic tests were conducted in a laboratory. The one was snapback test for a 22-inch insulated straight pipe, and the other was sinusoidal excitation test using a hydraulic actuator for an insulated 22-inch three-dimensional piping supported by several seismic devices. The piping components, insulation and support devices used in these tests were similar to typical LMFBR coolant piping. In consequence, following conclusions have been obtained. (1) Though the damping effects due to the insulation structure itself were very small, the damping ratios of the insulated LMFBR piping were shown to be twice larger than the design value. (2) The damping ratios of the insulated LMFBR piping can be evaluated properly by the same manner in LWR's.
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