Probabilistic assessment of excessive leakage through steam generator tubes degraded by secondary side corrosion Original Research Article

A probabilistic approach aimed at predicting the probability of excessive leakage through degraded steam generator (SG) tubes is proposed in the paper. The excessive leakage is assumed to occur during postulated hypothetical accidental conditions when the sum of all individual leak rates through degraded tubes exceeds the predefined acceptable value. The leak rates through the individual defects were evaluated using models developed by EPRI for assessment of outside diameter stress corrosion cracking (ODSCC) at the tube support plate intersections. Additionally, a brief description of the procedures which are used in the field to obtain conservative estimates of the total leak rate under accident conditions is given. The conservativeness of those methods is quantified through failure probability in the numerical examples. Two numerical examples are provided. They are based on inspection data obtained from Slovene and Belgian plants with 3/4 in. tubes made of Inconel 600 MA. The numerical examples analyze the behavior of the model for both small (<3 V) and large defect sizes (>10 V). The discussion of the results includes: (i) prediction and discussion of the probability of excessive leakage; (ii) the conservativeness of the approximate summation procedures used in the field to obtain conservative estimates of the total leak rate under accident conditions; (iii) some comments on the sensitivity of the probability of excessive leakage to the major uncertainties inherent in the data and models used. As a conclusion, some suggestions to improve both the efficiency of the numerical procedures and models used to estimate the leak rates through individual defects are given.