1000MW PWR Nuclear Island Steam Generator Tube Sheet Overlay Welding Technology

Literature Overview

This 2011 paper by Li Shuangyan from Shanghai Electric Nuclear Equipment Co., Ltd. provides a comprehensive account of the overlay welding technology applied to steam generator tube sheets in 1000 MW-class pressurized water reactor (PWR) nuclear power plants. Published in the journal Welding (2011, No. 10, pp. 36-41), the paper details the critical role of tube sheet overlay welding in ensuring the corrosion resistance, tube-to-tubesheet weld quality, and overall manufacturing schedule of steam generators, which are among the most critical components in a nuclear power plant.

Core Technical Content

The steam generator tube sheet is a massive component, typically made from carbon steel or low-alloy steel, that supports thousands of U-tubes connecting the primary and secondary sides of the steam generator. The tube sheet is exposed to the secondary side (feedwater and steam), which requires excellent corrosion resistance to prevent tube degradation and maintain the integrity of the primary coolant circuit. Overlay welding a corrosion-resistant alloy layer onto the tube sheet surface is therefore a critical manufacturing step.

Overlay Welding Process Configuration

Technical Analysis

The selection of strip electrode electroslag welding (SES) for the flat surface of the tube sheet is driven by several factors:

1. High deposition rate. SES provides deposition rates of 5-15 kg/h, significantly higher than manual processes, which is essential for covering the large surface area of the tube sheet within the manufacturing schedule.
2. Excellent weld quality. The slag coverage provides a protective atmosphere that eliminates atmospheric contamination, resulting in clean, defect-free weld metal. The deep penetration and narrow weld width minimize dilution from the base metal.
3. Consistency and repeatability. The automatic nature of SES ensures consistent weld parameters throughout the entire overlay, reducing the variability associated with manual welding.
4. Low residual stress. The cyclic heating and cooling in SES produces lower residual stresses compared to other processes, which is important for maintaining the dimensional accuracy of the tube sheet.

The use of SMAW for the tube sheet edge is a practical choice, as the edge geometry is difficult to access with automatic equipment. The edge overlay must be carefully blended with the flat surface overlay to ensure a continuous, uniform corrosion-resistant layer.

Quality Assurance Requirements

The quality requirements for nuclear-grade overlay welding are extremely stringent:

• Chemical composition control. The overlay weld metal must meet the specified chemical composition, particularly the carbon content (typically < 0.03% for 308L) to prevent sensitization and intergranular corrosion.
• Microstructural requirements. The overlay weld metal should be fully austenitic with minimal delta ferrite (typically < 5%) to ensure good corrosion resistance and avoid solidification cracking.
• Non-destructive testing. The overlay layer must be inspected using magnetic particle testing (MT) or penetrant testing (PT) to detect surface and near-surface defects. Ultrasonic testing (UT) may also be required to detect subsurface defects.
• Hardness testing. The overlay weld metal hardness should be within the specified range to ensure good weldability for subsequent tube-to-tubesheet welding.

Engineering Practice Implications

For engineers involved in nuclear component manufacturing, this paper provides several important lessons:

1. Process selection must be driven by quality and productivity requirements. The combination of SES for the large flat area and SMAW for the edges represents an optimal balance between quality, productivity, and practicality.
2. Automated welding should be preferred wherever possible. The paper explicitly recommends automatic overlay welding to ensure consistent quality, and this recommendation is well-founded given the critical nature of nuclear applications.
3. Repair procedures must be carefully controlled. The use of GTAW for repair is appropriate, as it provides excellent control over heat input and weld quality. However, repairs must be limited in size and number, and each repair must be thoroughly inspected.
4. Pre-overlay testing is essential. The paper emphasizes the importance of extensive testing before production, including weld procedure qualification, consumable qualification, and inspection method validation. This systematic approach is critical for ensuring that the production process consistently produces acceptable results.
5. Overlay quality directly impacts tube-to-tubesheet weld quality. The overlay layer serves as the base for the tube-to-tubesheet weld, and any defects or property variations in the overlay will propagate into the tube weld. This makes the overlay welding process a critical control point in the overall manufacturing process.

Study Insights and Reflections

This paper is a valuable resource for engineers working in nuclear component manufacturing, as it provides practical details on the overlay welding of a critical component. The emphasis on quality assurance, process qualification, and inspection is consistent with the regulatory requirements for nuclear applications and serves as a model for other high-integrity welding applications.

The integration of multiple welding processes (SES, SMAW, GTAW) for different zones of the same component demonstrates the importance of process flexibility in complex manufacturing. Engineers must be familiar with the characteristics of multiple welding processes and be able to select the most appropriate process for each specific application zone.

The paper also highlights the importance of the overlay welding step in the overall manufacturing sequence. The overlay is not an isolated operation but is intimately connected to subsequent manufacturing steps, particularly the tube-to-tubesheet welding. This systems thinking is essential for ensuring the overall quality and reliability of nuclear components.