PWR heat exchanger tube defects: Trends, signatures and diagnostic techniques

In pressurized water reactor, one of the most important barriers to the release of radioactive material to the environment is the steam generator heat exchanger tubes. Degradation of steam generator tubes could result in increased primary-to-secondary leakage and delayed diagnosis of the event could eventually result in through-wall tube rupture. To prevent through-wall tube rupture, an effective trend monitor is essential. Moreover, to increase the cost-effectiveness and reliability of energy production from nuclear plants, reduction in maintenance and repair downtime caused by steam generator tube defects is crucial. This paper reviews the state-of-the-art diagnostic techniques and condition monitoring methods for heat exchanger tubes. In particular, it discusses steam generator tube degradation and integrity issues, the physical phenomena, and the analysis of theoretical and experimental research conducted in the past few decades on the inspection of steam generator tubes. In addition, plants' monitoring systems are categorized and the predictive models utilized for the monitoring and evaluation of water chemistry is discussed. The major contribution is the presentation of critical parameter trends, deviations and empirical signatures observed when steam generator tube rupture occurs in a fully functional CNP1000 pressurized water reactor, and the analysis of machine learning approach for rupture event diagnosis. Furthermore, the operator response to thermal-hydraulic parameter trend indicating cracks and incipient leaks in steam generator tubes, as well as comparative advantages and demerits of water chemistry monitors are also presented. Finally, possible future research focus and likely challenges are discussed and a technique appropriate for effective online diagnosis of increased primary-to-secondary leakage event is recommended.