Estimation of the Number of Physical Flaws from Periodic ISI Data of SG Tubes Using Effective POD

摘要

It is necessary to know the number of flaws and their size distribution in order to calculate the probability of failure or to estimate the amount of leakage through the tube wall of steam generators. But in-service inspection (ISI) flaw data is different from the physical flaw data. In case of a single inspection, it is easy to estimate the number of physical flaws using the POD curve. However, we may be faced with some difficulties in obtaining the number of physical flaws from the periodic in-service inspection data. In this study a simple method for estimating the number of physical flaws from periodic in-service inspection data is proposed. In order to obtain the flaw growth history, the flaw growth is simulated using the Monte Carlo method and the flaw size and the corresponding POD value are obtained for each flaw at each periodic inspection time. The flaw growth rate used in the simulation is statistically calculated from the in-service inspection data. By repeating the simulation numerous flaw growth data can be generated and the effective POD curve can be obtained as a function of flaw size. From the effective POD curve the number of physical flaws and their size distributions are obtained. The effective POD value is affected by flaw growth rate and POD of the used inspection method. The usefulness and convenience of the proposed method is evaluated from several applications and satisfactory results are obtained.