Nickel-Based Alloy Surfacing of Nuclear Steam Generator Tube Sheets

Literature Overview

The paper by Liu Mingyu, Xu Wenjing, Zhang Maolong, and Sun Zhiyuan, published in Welding (2011, Issue 1, pp. 28–33), addresses one of the most critical welding operations in nuclear power plant manufacturing: the surfacing of nickel-based alloys on steam generator (SG) tube sheets. The authors, representing CGN Engineering and Shanghai Electric Nuclear Power Equipment, present a comparative analysis of two distinct surfacing processes employed in the CPR1000 and EPR nuclear reactor programmes. This work is of considerable practical importance, as the SG tube sheet surfacing quality directly affects the integrity of the primary coolant boundary and the long-term reliability of the nuclear island.

Background and Significance

The steam generator tube sheet is a thick-walled component (typically 150–250 mm thick) that separates the primary and secondary coolant circuits. The tube sheet material is typically a low-carbon steel or low-alloy steel, while the tube-to-tube-sheet joint requires a nickel-based alloy overlay (commonly Alloy 690 or Alloy 800H) to provide corrosion resistance against reactor coolant water. The surfacing layer must be free of cracks, pores, and inclusions, and must meet stringent radiographic and ultrasonic acceptance criteria. Any defect in the surfacing layer can propagate into the tube sheet base material and compromise the pressure boundary.

Comparative Process Analysis

CPR1000 Programme: Strip Electrode Electroslag Surfacing (SEES)

In the CPR1000 project, the SG tube sheet was surfaced using strip electrode electroslag surfacing. This process uses a continuous strip of nickel-based alloy as the consumable electrode, with a molten slag pool providing thermal insulation and protection. The arc burns beneath the slag, creating a deep, narrow weld pool that allows high deposition rates.

Process Parameter	Typical Range
Arc current	400–700 A
Arc voltage	35–50 V
Travel speed	100–200 mm/min
Strip electrode width	20–40 mm
Strip electrode thickness	1.5–3 mm
Deposition rate	5–15 kg/h
Heat input	High, 60–100 kJ/mm
Dilution with base metal	5–15%
Number of passes	2–4 for typical overlay thickness

The advantages of SEES include high deposition efficiency, low dilution (due to the slag pool acting as a thermal barrier), and the ability to build thick overlays in few passes. The process is well-suited to large-area surfacing on flat or slightly curved surfaces.

EPR Programme: Hot Wire TIG Surfacing (HWS)

In the EPR project, the SG tube sheet was surfaced using hot wire TIG surfacing, which combines a TIG arc with a separately heated consumable wire fed into the arc. The hot wire is preheated by an independent electric current before entering the arc, increasing the deposition rate while maintaining the low heat input characteristics of TIG welding.

Process Parameter	Typical Range
TIG arc current	150–250 A
Hot wire current	100–200 A
Travel speed	80–150 mm/min
Wire feed rate	100–200 mm/min
Wire diameter	1.6–2.4 mm
Shielding gas	Argon or Ar/He mixture
Deposition rate	2–5 kg/h
Heat input	Moderate, 20–40 kJ/mm
Dilution with base metal	3–10%
Number of passes	3–6 for typical overlay thickness

The advantages of HWS include lower heat input, finer control over the weld pool, and better surface finish. The process is more flexible for complex geometries and is less prone to excessive dilution.

Defect Analysis and Countermeasures

Both processes are susceptible to specific defect modes that must be controlled through process parameter optimisation and rigorous inspection.

Defect Type	SEES	HWS	Root Cause	Countermeasure
Hot cracks	Moderate risk	Low risk	Low melting point impurities at grain boundaries	Control S and P content in filler; preheat base metal
Cold cracks	Low risk	Moderate risk	Hydrogen embrittlement in HAZ	Preheat and post-heat; use low-hydrogen consumables
Porosity	Moderate risk	Low risk	Gas entrapment in slag pool	Ensure proper slag composition; clean strip surface
Incomplete fusion	Low risk	Moderate risk	Insufficient penetration at pass boundaries	Increase heat input; optimise travel speed
Undercut	Low risk	Moderate risk	Excessive arc energy at edges	Use proper torch angle; control current
Excessive dilution	Moderate risk	Low risk	Deep weld pool in SEES	Reduce arc current; increase travel speed

Quality Control Requirements

Nuclear SG tube sheet surfacing is governed by stringent quality requirements, typically specified by ASME Section III, Division NB, and the relevant national nuclear regulatory codes. The surfacing layer must be 100% radiographically inspected (RT) and ultrasonically inspected (UT), with acceptance criteria that are far more restrictive than those for conventional pressure vessels. The surfacing layer thickness must be uniform within specified tolerances, and the transition zone between the surfacing layer and the base metal must be free of any cracks or inclusions.

In practice, the quality assurance programme includes:

Welder qualification and continuous performance monitoring under the specific code requirements.
Process parameter validation through test welds on representative base material with the same thickness and chemistry.
100% RT and UT inspection of the completed surfacing layer, with no less than 100% coverage.
Metallographic examination of witness coupons to verify microstructure, hardness profile, and absence of cracking.
Hardness survey across the surfacing layer and HAZ to confirm that the hardness does not exceed the code-specified limit (typically 350 HV for the base material and a controlled gradient in the overlay).

Engineering Insights

The comparison between SEES and HWS for SG tube sheet surfacing illustrates a fundamental trade-off in welding process selection: deposition efficiency versus process flexibility and quality control. SEES is superior for large, flat areas where high deposition rate is essential, while HWS offers better control and is more suitable for complex geometries or where lower heat input is critical. The choice between the two processes in the CPR1000 and EPR programmes reflects not only technical considerations but also the different design philosophies and regulatory frameworks of the Chinese and European nuclear programmes.

In my own assessment, the HWS approach offers a more favourable risk profile for nuclear applications due to its lower heat input and better control over dilution. However, the higher cost and lower deposition rate of HWS must be balanced against the programme schedule and the cost of inspection and repair. The key lesson is that process selection for nuclear SG tube sheet surfacing must be driven by a holistic assessment of the entire quality assurance programme, not merely by deposition efficiency.