Evaluation of cumulative damage failure of a reactor feed vertical pump under seismic excitation

In a nuclear power plant a vertical pump is used as an emergency pump against earthquake excitation. This type of pump has a merit that the settled area of the pump is small, on the other hand shaft length of the rotor is long and the structure is soft. Thus natural frequency of the shaft becomes low and deflection of the shaft becomes large. When the pump is subjected to a strong seismic excitation, impeller may be touched to a wearing. If the excitation is large, the water film between the impeller shroud and the wearing breaks. In this case, torsional vibration of the shaft is excited by the friction. If the national frequency of the shaft coincides with the predominant frequency of the seismic excitation, the torsional vibration becomes large by resonance. Thus cumulative damage due to shear stress increases and finally the shaft may be broken.