Stochastic Modeling For The Degradation Of Steam Generator Tubes Due To Pitting Corrosion

In a pressurized water reactor, steam generators are heat exchangers which convert the heat in primary water into steam in the secondary loop to propel turbine generator to produce electricity, playing an important role in the safe and stable operation of nuclear power plant. Leakage of steam generator tubes will lead to not only the release of radioactivity into environment, but also abnormal reactor shutdown, resulting in significant loss in economy and safety. One of the most malicious failure modes that will eventually lead to the leakage of steam generator tubes is pitting corrosion. Based on experimental results, this research proposed a complete framework for the degradation simulation and reliability evaluation of alloy 690 steam generator tubes, including experimental method, pitting degradation modeling method and simulation method.Firstly, characteristics of pit initiation and propagation of alloy 690 have been investigated thoroughly via accelerated corrosion test. Non-homogeneous Poisson Process model and non-stationary gamma process model have been developed to depict these two phases respectively. Parameters of these models were estimated via maximum likelihood method using experimental data. Results of data fitting showed that these two stochastic processes can effectively describe the actual process of pitting.Secondly, this research derived the accelerated corrosion factors of non-homogeneous Poisson Process and non-stationary gamma process to relate the pitting process under accelerated corrosion conditions to that under actual operating conditions. This research also identified and analyzed the impacts of important material and environmental factors on pitting behavior, and established the relationship between these factors and the parameters of the two stochastic processes models derived in the previous step, strengthened the physical meaning of these models.Finally, this research developed a Monte Carlo simulation method for the modeling of degradation of steam generator tubes due to pitting corrosion by integrating multi-pit initiation and pit propagation processes. The simulation results returned the lifetime distribution of steam generator tubes with respect to time, with the confidence intervals estimated by pointwise bootstrap method.