Zhuojin Pipe Fitting Co., LtdResidual Stress Analysis of DMW Overlay Repair on Qinshan Nuclear Pressurizer Nozzle
Literature Overview
This study by Zhang Shiwei, Chen Xuede, Zhang Yong, Sun Lei, and Li Xihua from the China Nuclear Power Research and Design Institute, published in Nuclear Power Engineering (2016, Vol. 37, S2, pp. 4-6), presents a finite element analysis of residual stress distribution during overlay repair of dissimilar metal welds (DMW) on a pressurizer spray nozzle at the Qinshan Phase I nuclear power plant. The work addresses a critical safety issue in nuclear power plant operations — primary water stress corrosion cracking (PWSCC) of Alloy 81/182 DMW welds and the use of overlay repair as a mitigation strategy.
Core Technical Content
The pressurizer spray nozzle in a nuclear power plant's primary coolant system is a critical component that controls reactor pressure through steam injection. The nozzle-to-pipe weld is a dissimilar metal weld (DMW) joining Alloy 81 (low-carbon austenitic stainless steel, UNS N08811) to Alloy 182 (nickel-based fill metal, UNS N06822). These DMW welds are susceptible to PWSCC, which manifests as intergranular cracking in the weld metal and heat-affected zone (HAZ) under the combined action of primary coolant water (containing boric acid and lithium hydroxide) and tensile residual stresses.
Finite Element Model Configuration
| Parameter | Specification |
|---|---|
| Geometry | Axisymmetric model based on actual pressurizer spray nozzle dimensions |
| Simulation method | Birth and death element technique for welding sequence simulation |
| Thermal analysis | Transient thermal analysis with welding heat input |
| Stress analysis | Thermal-mechanical coupled analysis |
| Material | Alloy 81 base metal, Alloy 182 weld metal, overlay material |
| Boundary conditions | Symmetry conditions, thermal boundary conditions |
The birth and death element technique is a widely used approach in welding finite element analysis. In this method, elements representing the weld metal are initially "killed" (deactivated) and are "born" (activated) at the appropriate time during the welding sequence. As each element is born, it is subjected to a thermal load corresponding to the local temperature rise, followed by cooling to the ambient temperature. This approach captures the progressive nature of welding — each pass adds material and induces thermal-mechanical effects on the existing structure.
PWSCC Mechanism and Overlay Repair Strategy
PWSCC is a well-documented degradation mechanism in nuclear power plant primary coolant systems. The susceptibility of DMW welds to PWSCC is influenced by several factors:
- Microstructure: The coarse-grained HAZ of Alloy 81 is particularly susceptible to intergranular cracking.
- Residual stress: Tensile residual stresses in the weld region provide the driving force for crack initiation and propagation.
- Environment: The primary coolant water composition (boric acid concentration, lithium hydroxide pH, temperature) affects the cracking susceptibility.
- Time: PWSCC is a time-dependent process, with cracking susceptibility increasing with irradiation dose and exposure time.
Overlay repair involves depositing a layer of compatible material (typically Alloy 81 or Alloy 182) on the inner surface of the DMW weld to isolate the susceptible region from the primary coolant. The overlay material acts as a diffusion barrier, preventing the corrosive coolant from contacting the PWSCC-susceptible weld metal and HAZ.
Residual Stress Distribution and PWSCC Control
The finite element analysis revealed that after overlay repair is completed, the residual stress in the inner wall region of the DMW weld is converted from tensile to compressive. This compressive residual stress is critical because:
- Crack initiation resistance: Compressive stresses oppose crack opening, significantly reducing the driving force for PWSCC initiation.
- Crack propagation resistance: Even if micro-cracks exist, compressive stresses at the crack tip reduce the stress intensity factor, slowing crack growth.
- Fatigue life extension: Compressive residual stresses improve the fatigue life of the weld under cyclic pressure loading.
The axisymmetric model is appropriate for this application because the pressurizer spray nozzle is a rotationally symmetric component, and the overlay repair is applied uniformly around the circumference. This simplification reduces computational cost while maintaining accuracy for the stress state in the weld region.
Process Considerations for Overlay Repair
Overlay repair of nuclear-grade DMW welds is subject to stringent quality requirements. The welding procedure must be qualified in accordance with applicable codes (such as ASME BPV Section IX, or national nuclear codes such as RSE-M or GB/T). Key process considerations include:
| Consideration | Requirement |
|---|---|
| Welding process | GTAW (TIG) preferred for low heat input and precise control |
| Pre-heat | Controlled to prevent cold cracking and minimize HAZ grain growth |
| Interpass temperature | Typically limited to 150–250°C to control cooling rate |
| Heat input | Low to moderate, to minimize dilution and HAZ susceptibility |
| Post-weld heat treatment | Solution annealing to relieve residual stresses and homogenize microstructure |
| NDT requirements | UT, PT, and RT per nuclear code requirements |
The finite element analysis provides valuable input for optimizing the overlay welding sequence. By understanding the residual stress evolution during the repair process, engineers can design welding sequences that minimize the peak residual stresses and ensure that the final stress state is compressive in the critical inner wall region.
Key Questions and Reflections
A significant question that arises from this work is the long-term stability of the compressive residual stress in the overlay layer. During reactor operation, the component is subjected to thermal cycling, pressure cycling, and irradiation. These factors can relax residual stresses over time, potentially reducing the PWSCC mitigation benefit. The analysis should ideally include a long-term stress relaxation model that accounts for creep and thermal cycling effects.
Another important consideration is the integrity of the overlay layer itself. The overlay weld is also susceptible to degradation mechanisms — intergranular corrosion, sensitization, and even PWSCC if the overlay material is not properly qualified. The finite element model should ideally include the overlay weld HAZ as a separate region with its own material properties and susceptibility characteristics.
Study Insights and Implications
This work demonstrates the value of finite element analysis in nuclear component repair decision-making. By quantifying the residual stress state before and after overlay repair, the analysis provides objective evidence that the repair is effective in mitigating PWSCC risk. This type of analysis is increasingly important as nuclear power plants extend their operating lives beyond their original design life, and as more DMW welds approach the end of their PWSCC service life.
The axisymmetric modeling approach is efficient and appropriate for this specific application, but future work should consider three-dimensional models that account for geometric discontinuities, nozzle-to-pipe misalignment, and the actual welding sequence with multiple passes. The integration of material property databases with irradiation effects and long-term stress relaxation models will further enhance the predictive capability of such analyses.