Transverse vibration of beam with beats, induced by seismic excitation

One of the problems of the earthquake resistance of the primary circuit of nuclear power plants is the transverse vibration of fuel elements with the possibility of the occurrence of beats. In WWER-440 reactors, fuel rod bundles may thus vibrate relative to the hexagonal covering, and entire fuel elements relative to the core basket. A mathematical model of a beam of constant cross-section was therefore constructed substituting for the said parts of the reactor. The continuous beam was substituted with a five-mass mechanical model for the purposes of the calculation. The motion system was modelled on an analog computer. The results show that the maximum transverse displacement of the fuel element is 0.8 mm and that the fuel element will not impact onto the core basket. Inside the fuel element 150 beats will occur during a seismic event lasting 6 seconds, with the centre of the fuel rod bundle having the most frequent impacts. (Z.M.)