Crack Propagation Analysis of Cladding Structure on Pressurizer Nozzle Safe End in Nuclear Power Plants

Literature Overview

This paper by Luo Jiacheng, Yu Li, Zhang Yong, and Li Pengzhou from the China Nuclear Power Research and Design Institute, published in Nuclear Power Engineering in 2019, presents a rigorous crack propagation analysis of the overlay welding (cladding) structure on the safe end of a pressurizer nozzle in a nuclear power plant. The study applies fracture mechanics methods to evaluate fatigue and stress corrosion cracking (SCC) induced crack growth under design loading conditions, providing a safety assessment basis for the cladding structure design and qualification.

Technical Context and Safety Significance

The pressurizer is a critical component in a Pressurized Water Reactor (PWR) nuclear power plant, serving to maintain reactor coolant system pressure stability. The pressurizer nozzle connections are subject to severe cyclic thermal and pressure loading during normal operations, including load following, startup/shutdown transients, and emergency depressurization events. The safe end of the pressurizer nozzle, which interfaces with the reactor coolant system piping, often incorporates overlay welding (cladding) to provide corrosion resistance in the highly corrosive reactor coolant environment.

The overlay welding creates a dissimilar metal weld (DMW) zone that is susceptible to:

• Fatigue cracking due to cyclic thermal and pressure loading
• Stress corrosion cracking (SCC) in the corrosive reactor coolant environment
• Creep-fatigue interaction at elevated temperatures
• Thermal mismatch stresses between dissimilar materials

Analytical Methodology

The analysis employed linear elastic fracture mechanics (LEFM) methods in accordance with applicable nuclear industry standards:

The assumed initial crack sizes and crack growth rates were evaluated against the applicable code limits to determine whether the cladding structure satisfies safety requirements throughout the design life.

Crack Propagation Results

Dissimilar Metal Weld Zone (DMW)

Stainless Steel Weld Zone

The results indicate that all crack propagation quantities are well within acceptable limits, confirming the safety and adequacy of the cladding structure design.

Critical Technical Observations

Crack Orientation Asymmetry

The markedly different crack propagation rates in the circumferential versus axial directions reflect the anisotropic stress state at the nozzle connection:

• Axial cracks in DMW zone: The 23.6×10^-3 mm axial crack growth in the DMW zone is significantly larger than the circumferential growth (0.4×10^-3 mm), indicating that the primary driving force for crack growth is the hoop stress, which drives axial crack extension.
• Circumferential cracks in SS weld zone: The 12.4×10^-3 mm circumferential crack growth in the stainless steel weld zone, with zero axial growth, suggests that the axial stress component is compressive or insufficient to drive axial crack propagation in this region.

This asymmetry is consistent with the expected stress state at a nozzle connection, where hoop stresses are typically the dominant stress component driving fatigue crack growth.

DMW Zone Vulnerability

The DMW zone shows the highest axial crack growth rate, which is expected given:

• Thermal mismatch between dissimilar materials creating residual stresses
• Potential for strain localization at the weld boundary
• Possible microstructural heterogeneity at the DMW interface
• Susceptibility to both fatigue and SCC mechanisms

Safety Margin Assessment

Design and Qualification Implications

The analysis provides several important insights for cladding structure design in nuclear applications:

1. Crack orientation sensitivity: The dominant crack orientation varies between the DMW zone and the stainless steel weld zone, requiring separate evaluation for each crack direction in each material region.
2. DMW zone criticality: The DMW zone is the most vulnerable location for crack initiation and propagation, particularly for axial cracks driven by hoop stress. This reinforces the importance of DMW qualification and inspection programs in nuclear power plants.
3. Load spectrum adequacy: The design load spectrum used in the analysis produces crack growth rates well below code limits, providing significant safety margins for the cladding structure.
4. Combined loading effects: The simultaneous consideration of fatigue and SCC mechanisms provides a conservative assessment that accounts for the synergistic interaction between cyclic loading and corrosive environment.

Engineering Practice and Inspection Recommendations

Based on the analysis results, the following inspection and monitoring recommendations are appropriate:

The inspection program should be designed to detect cracks at sizes significantly smaller than those predicted by the analysis, providing early warning capability before crack growth reaches critical dimensions.

Study Insights and Reflections

This paper exemplifies the rigorous analytical approach required for nuclear component qualification. The crack propagation analysis demonstrates that the cladding structure on the pressurizer nozzle safe end provides adequate safety margins under the full design load spectrum, which is essential for regulatory approval and long-term operational confidence.

The finding that the DMW zone exhibits the highest crack growth rate is consistent with industry experience and underscores the well-known vulnerability of dissimilar metal welds in nuclear applications. The DMW zone is subject to multiple degradation mechanisms including thermal mismatch fatigue, creep, and stress corrosion cracking, and its qualification requires careful consideration of material selection, welding procedure, and post-weld treatment.

The zero axial crack growth in the stainless steel weld zone is noteworthy and suggests that the stress state in this region is favorable for crack resistance. This may be attributed to the compressive axial stress component in the stainless steel weld zone, which effectively closes any axial crack and prevents propagation. This observation has practical implications for inspection strategy, as the stainless steel weld zone may require less intensive axial crack monitoring than the DMW zone.

From a regulatory perspective, the analysis methodology and results should be evaluated against applicable nuclear codes and standards including ASME Section XI for in-service inspection, ASME Section III for design, and RCC-MR or similar nuclear codes for fracture mechanics assessment. The conservative assumptions employed in the analysis, including initial crack size selection and crack growth rate parameters, provide additional confidence in the safety assessment.

The study also highlights the importance of integrating fracture mechanics analysis with inspection planning. The predicted crack growth rates can be used to establish inspection intervals that provide adequate detection capability while minimizing unnecessary inspection costs. This risk-informed approach to inspection planning is increasingly recognized as a best practice in nuclear plant maintenance.

In conclusion, this study provides a comprehensive fracture mechanics assessment of the pressurizer nozzle safe end cladding structure that confirms design adequacy with significant safety margins. The analysis methodology, results, and conclusions are directly applicable to similar cladding structure designs in nuclear power plants and serve as a valuable reference for future design qualification and safety assessment activities.