Keywords: core damage probability, seismic fragility, cumulative absolute velocity, fatigue, p-box, random stress amplitude, stress intensity factor range.

Seismic probabilistic safety assessment of nuclear power plants recently became of high importance. One of the basic issues of seismic probabilistic safety assessment (PSA) is the definition of component and plant fragilities. Sparse statistical information exists on the behaviour of complex structures and, or machines subject to earthquake loads. One possible way for the seismic PSA improvements might be the utilization of a bounding approach, the use of the interval and p-box description of uncertainties as outlined in the paper. A bounding approach to risk analysis extends and complements traditional probabilistic analyses when analysts cannot specify precise parameter values for input distributions or point estimates in the model, precise probability distributions for some or all of the variables, etc. Upper and lower bounds on parametric values can be provided, typically from expert elicitation.

In the seismic PSA practice, the component fragility development is based on the design information anchored into peak ground acceleration (PGA). Other representations of load, for example using the cumulative absolute velocity (CAV) as a load parameter may improve the calculation of the probability of failure. As it is shown in the paper, the CAV-value correlated to the failure can be used as the failure load parameter. It is also shown in the paper, that the CAV is an adequate damage indicator since it is reflecting the main parameters of damaging processes, e.g. the CAV is proportional to load cycles causing low-cycle fatigue type damage. In the paper the dependence of the CAV on the strong motion duration, number of load cycles, median frequency, and amplitude of the alternating load (the ground motion) is demonstrated.

Based on the interpretation of the CAV of the equipment given in the paper, the CAV can be correlated to the failure criteria for fatigue. The CAV can also be linked to the failure criteria of the random amplitude, frequency-domain fatigue analysis, as well as to the low-cycle fatigue failure criteria. A correlation between the CAV and the stress intensity factor range can also been established.

Having this correlation, one can assess the condition of the equipment if an earthquake happens, which may contribute to the rapid assessment of the plant condition after an earthquake.

There is an advantage in the use of the CAV for damage indicator, since the CAV is calculated nearly real-time form the easy to measure free-field acceleration signal. We may define for the critical equipment the correlation between fatigue failure and the CAV for failure. Having this correlation, one can assess the condition of the equipment if an earthquake occurs. The presentation of the CAV given above allows the correlation of the CAV to the theories of frequency-domain fatigue analysis taking into account the narrow-band character of the seismic excitation.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £130 +P&P)