Statistical Analysis of Pit Growth in CANDU Steam Generator Tubes

Abstract

During the outage of some CANDU steam generators, eddy current inspection detected volumetric indications in the tubes in the top-of-tubesheet region (TTS). The majority of the indications were found to be pit-type defects based on a signal-to-signal analysis of previous inspection data. This investigation studies the pit growth by collecting individual pit size change from data collected during two successive outages and calculating the growth rates and their confidence intervals using statistical methods. The pits at the TTS are divided into separate populations according to their locations and statistical test of homogeneity. The mean growth rate, 95% upper bound growth rate, and their confidence intervals of each population are evaluated using the Bootstrap resampling method. A method is also proposed to lower the 95% upper bound growth rate by adjusting for the effect of the eddy current measurement error with a Gamma growth model.