QUANTITATIVE ASSESSMENT OF CORROSION-FATIGUE IN NUCLEAR POWER PLANT COMPONENTS USING SIMULATION AND EXPERIMENTAL DATA

Corrosion-fatigue (CF) is widely recognized as one of the most complex and deleterious degradation mechanisms threatening the structural integrity and long-term operation (LTO) of primary circuit components in Nuclear Power Plants (NPPs). As global nuclear fleets age and transition toward Subsequent License Renewal (SLR) periods of 80 years, the limitations of traditional, empirical fatigue models—which often rely on conservative, air-based design curves—have become increasingly apparent. This research presents a comprehensive quantitative assessment framework that bridges the gap between theoretical materials science and practical structural health monitoring by integrating high-fidelity numerical simulations with advanced experimental datasets. The study focuses on quantifying the synergistic effects of cyclic mechanical loading and the aggressive electrochemical environment of high-temperature, high-pressure reactor coolant, specifically targeting the vulnerabilities of 316L stainless steel and Alloy 690.The methodology utilizes a dual-pillar approach: first, conducting rigorous fatigue tests in a high-pressure recirculating autoclave loop to establish empirical crack initiation and growth rates ( ) under simulated reactor water chemistry. These experiments employ Direct Current Potential Drop (DCPD) sensors for sub-millimeter precision in real-time crack monitoring. Second, these physical results are synthesized into a Multiphysics Finite Element Analysis (FEA) framework that incorporates the Slip-Dissolution/Oxidation Model to simulate the localized electrochemical conditions at the crack tip. A key finding of this research is the identification of a critical "threshold" in the loading waveform, where tensile rise times exceeding 45 seconds were found to accelerate environmental damage by a factor of 15 compared to fast-transient cycles.Furthermore, the study quantifies the impact of Manganese Sulfide (MnS) inclusions, demonstrating that 85% of cracks initiate at micro-pits with a critical depth-to-width ratio of 0.8. By applying Monte Carlo simulations, the research establishes a probabilistic distribution for the Remaining Useful Life (RUL) of critical components, identifying that fluctuations in local strain rates account for 45% of total predictive variance. The ultimate output of this study is a calibrated "Digital Twin" framework that achieved a 98% accuracy in real-time crack growth prediction. These results provide a technically defensible foundation for risk-informed maintenance, suggesting that proactive, data-driven monitoring can safely extend the operational life of nuclear assets while maintaining the highest margins of structural safety and regulatory compliance.