Probabilistic evaluation of leak rates through multiple defects: the case of nuclear steam generators

摘要

A probabilistic approach aimed at predicting the probability of excessive leakage through flaws in tubes is proposed in the paper. The excessive leakage is assumed to occur when the sum of all individual leak rates through degraded tubes exceeds the predefined acceptable value. Additionally, a brief description of some estimators used in the field to obtain conservative estimates of the 95 th percentile (e.g., upper limit) of the total leak rate is given. The conservatism of those estimators is quantified through failure probability in the two numerical examples. The leak rates through the individual flaws were evaluated using models developed by EPRI for assessment of outside diameter stress corrosion cracks (ODSCC) in tubes of steam generators in nuclear power plants assuming postulated hypothetical accidental conditions. The distributions of flaw sizes are based on inspection data obtained from Slovene and Belgian plants with 3/4 inch tubes made of Alloy 600 Mill Annealed. The numerical examples analyse the behaviour of the model for both small and large flaw sizes. The results include: prediction and discussion of the probability of excessive leakage; the conservatism of approximate estimators; comments on the sensitivity of the probability of excessive leakage to the major uncertainties inherent to data and models used. As a conclusion, some suggestions to improve both the efficiency of the numerical estimators and models used to estimate the leak rates through individual defects are given.