Zhuojin Pipe Fitting Co., LtdCrack Propagation Analysis of Overlay Weld Structure at Pressurizer Nozzle Safe End
Literature Overview
The paper by Luo Jiacheng, Yu Li, Zhang Yong, and Li Pengzhou from the China Institute of Atomic Energy (CIAE), published in Nuclear Power Engineering (2019, Vol. 40, No. A1, pp. 110–113), addresses a critical safety-related issue in pressurized water reactor (PWR) nuclear power plants: the integrity of overlay welded structures at the safe end of the pressurizer nozzle. The pressurizer is a key component of the reactor coolant system (RCS), and its nozzle connections are subject to severe thermal cycling, pressure loading, and potential stress corrosion cracking (SCC) environments. This study applies fracture mechanics principles to evaluate crack growth under both fatigue and stress corrosion conditions in the overlay weld region.
Core Technical Approach
The authors assume initial cracks at the safe-end weld zone and perform crack propagation calculations under pressurizer design loads. The analysis follows the fracture mechanics framework established in ASME Section XI, which governs the in-service inspection and safety evaluation of nuclear components. The two primary crack growth mechanisms considered are:
- Fatigue crack propagation — driven by cyclic thermal and pressure loads during normal reactor operation and transient events.
- Stress corrosion cracking (SCC) — driven by the corrosive RCS environment, particularly in the presence of chloride ions and high temperatures.
The analysis distinguishes between two weld regions: the dissimilar metal weld (DMW) zone and the stainless steel weld zone. This distinction is critical because the DMW region, typically involving a carbon steel or low-alloy steel base metal joined to austenitic stainless steel cladding, is inherently more susceptible to cracking due to thermal mismatch, residual stress concentration, and susceptibility to intergranular SCC.
Key Results and Interpretation
| Crack Orientation | Dissimilar Metal Weld Zone | Stainless Steel Weld Zone |
|---|---|---|
| Maximum circumferential crack depth growth at end of cycle | 0.4 × 10⁻³ mm | 12.4 × 10⁻³ mm |
| Maximum axial crack depth growth at end of cycle | 23.6 × 10⁻³ mm | 0 |
The results demonstrate that crack growth is highly anisotropic depending on the weld region and crack orientation. The dissimilar metal weld zone exhibits the most significant axial crack growth (23.6 × 10⁻³ mm), which aligns with the expectation that the DMW region experiences the highest residual stresses and is most susceptible to both fatigue and SCC mechanisms. The stainless steel weld zone shows circumferential crack growth (12.4 × 10⁻³ mm) but no axial growth, suggesting that the axial stress state in this region is insufficient to drive crack propagation.
Engineering Practice Integration
From a practical standpoint, this analysis has several important implications for nuclear power plant maintenance and inspection planning:
- In-service inspection (ISI) interval justification: The calculated crack growth rates provide quantitative support for ISI intervals, ensuring that crack detection occurs before critical crack lengths are reached.
- Leak-before-break (LBB) assessment: The relatively small crack growth magnitudes suggest that the overlay weld structure maintains leak-before-break integrity under design conditions, which is a key safety philosophy in PWR design.
- Weld overlay design optimization: The results inform future design decisions regarding overlay thickness, weld procedure selection, and post-weld heat treatment (PWHT) parameters to minimize residual stress.
Key Questions and Reflections
A significant question that arises from this study is the conservatism inherent in the assumed initial crack sizes. The ASME Section XI methodology typically uses conservative initial flaw sizes based on inspection sensitivity limits. If the actual initial crack sizes are smaller than assumed, the calculated growth would be even less, providing additional margin. Conversely, if the RCS environment is more aggressive than modeled (e.g., due to boric acid concentration variations or coolant chemistry excursions), the SCC growth rate could be underestimated.
Another important consideration is the interaction between fatigue and SCC mechanisms. The study appears to treat them separately, but in reality, fatigue can accelerate SCC by opening and closing cracks during cyclic loading, exposing fresh crack surfaces to the corrosive environment. This synergistic effect may lead to crack growth rates exceeding the sum of individual mechanisms, which is a concern for future research.
Study Insights and Implications
This paper provides a valuable quantitative safety assessment methodology for nuclear component overlay welds. The application of fracture mechanics to overlay weld structures is particularly important because these welds are inherently difficult to inspect due to their location and geometry. The results confirm that the current overlay weld design at the pressurizer nozzle safe end is adequate under design loads, but emphasize the need for continued monitoring through ISI and careful maintenance of RCS chemistry to prevent SCC initiation and growth. The study also highlights the importance of understanding the metallurgical differences between DMW and homogeneous stainless steel weld zones when evaluating weld integrity in nuclear service.