Since the accident at the Fukushima Daiichi power plant, in addition to “design to prevent accidents”, “measures to mitigate the severe accident” have come to be emphasized. Therefore, it is necessary to evaluate the actual failure mode under beyond design basis events (BDBEs). In this study, authors consider the failure mode of piping in nuclear power plants under excessive earthquake. The piping design of nuclear power plants has conventionally been conservative assuming that seismic load acts as load controlled-stress and the maximum acceleration induces collapse. However, the test conducted by Electric Power Research Institute (EPRI) confirmed that when excessive vibration load was applied to the piping with the elbow, ratchet deformation occurred with time and eventually collapsed. If the pipe collapses, a great deal of damage is expected, such as stopping the cooling function and leaking contaminated water. Unfortunately, this failure mechanism is not clear, so it is highly important to consider the actual failure mode, ratchet leading to collapse. The mechanism of ratchet deformation has been clarified by experiments and analyses of inputting acceleration to a beam simulating piping. Hereafter, the ratio of the frequency of vibration load to the natural frequency of beams is referred as “frequency ratio”. According to these results, it is identified that ratchet deformation is likely to occur when the vibration load whose frequency ratio is lower than 1 is applied, and is difficult to occur on the high frequency side. This tendency can be explained from the viewpoint of input energy to the beam and amplitude magnification. In this study, a half-cycle vibration load was applied to the beam, and the frequency dependence of the collapse phenomenon was investigated. In addition, authors aimed to consider the frequency dependence from the energy aspect and to identify the mechanism. As a result, it was found that collapse is likely to occur when a vibration load of lower frequency ratio applied to as in the case of ratchet deformation. Considering this from the viewpoint of energy, the phase difference between the input acceleration and the response velocity was smaller when the frequency ratio was low, and it was easier for energy to transfer. As a lesson learned from the result, we should take care of the fact that installing a large number of supports in the piping design increases the natural frequency of the piping itself, which may cause ratchet deformation leading collapse easier.


Frequency Dependency of Beam Collapse Due to Vibration Loads

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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