What RCC-M Section II Nuclear Tube Solutions Are Available for Nuclear Steam Generator Replacement?

What RCC-M Section II Nuclear Tube Solutions Are Available for Nuclear Steam Generator Replacement?

Replacing a nuclear steam generator is one of the most demanding projects a pressurized water reactor (PWR) operator will ever plan. The steam generator tube bundle sits on the boundary between the primary and secondary coolant circuits, so the tubes installed in a replacement unit must hold up for decades under high temperature, high pressure, and radiation while keeping the two loops perfectly separated. In France and in the many markets that follow the French nuclear code, the material rules for those tubes come from RCC-M Section II. This article explains the RCC-M Section II nuclear tube solutions available for steam generator replacement, what each material is used for, and how to specify them correctly.

Why Nuclear Steam Generators Get Replaced

Many of the steam generators still running in the world's PWR fleet were originally built with Alloy 600 (Inconel 600) tubes. After years of operation, Alloy 600 proved vulnerable to primary water stress corrosion cracking (PWSCC), particularly in the U-bend region and at the tube-to-tubesheet expansion joints. As tube degradation accumulates, operators face a choice: keep plugging defective tubes and lose capacity, or ｒｅｐｌａｃｅ the steam generator outright and restore the unit to full output.

Replacement steam generators are almost always fitted with Alloy 690 thermally treated (690 TT) tubes. The alloy carries a much higher chromium content than Alloy 600, typically 27% to 31% against 14% to 17%, combined with a tightly controlled low cobalt level, which makes it far more resistant to PWSCC and to stress corrosion in both primary and secondary water. This is why Alloy 690 TT has become the accepted standard for new and replacement steam generator tubing worldwide.

What RCC-M Section II Covers

RCC-M, the Design and Construction Rules for Mechanical Components of PWR Nuclear Islands, is published by AFCEN. Section II of the code covers material specifications and is the governing document for the procurement, manufacturing, and inspection of materials used in PWR nuclear components. It defines the ordering information a buyer must provide, the material codes and classification system, the acceptable ranges for chemical composition and mechanical properties, the mandatory inspection and testing requirements, and the certification and traceability documentation that must accompany every batch.

Material codification follows the AFCEN classification, where M-series codes identify base metals and S-series codes identify filler metals. For a steam generator replacement project, the codes that matter most are the nickel-based alloys used for the tube bundle, with stainless steels, carbon steels, and copper-nickel alloys covering the internals, shells, and auxiliary circuits.

Available RCC-M Section II Tube Solutions

The table below summarizes the main RCC-M Section II material codes relevant to steam generator replacement and where each one is typically applied.

Nickel-Based Alloys: The Core of the Tube Bundle

For the heat transfer tubes themselves, the nickel-based alloys are the primary RCC-M Section II solution. Alloy 690, coded M4102, is the first choice for replacement steam generator tubes because its high chromium content gives it outstanding resistance to PWSCC and to chloride and caustic stress corrosion on the secondary side. It is supplied in the thermally treated condition, which develops the grain-boundary microstructure that resists cracking over the long term.

Alloy 600, coded M4101, still appears in older units and in some replacement internals, but it is rarely selected for new tube bundles given its known PWSCC history. Alloy 718, coded M4104, is a high-strength grade used for fasteners and load-bearing components rather than for heat transfer tubes.

Stainless, Carbon, and Copper-Nickel Grades for the Rest of the Unit

A steam generator replacement is not only about the tube bundle. Stainless steel grades such as Z2CN18-10 and Z2CND17-12, coded M3201 and M3204, are used for internals, support plates, and auxiliary piping where corrosion resistance and weldability are required. Carbon and low-alloy steels, including the M1903 tube grades and the manganese-molybdenum steels coded M2131 and M2132, go into shells, tube sheets, and other pressure-retaining parts. On the secondary side, copper-nickel alloys such as CuNi30Mn1Fe, coded M6201, serve in auxiliary cooling and seawater circuits where resistance to flowing seawater is essential.

Manufacturing and Testing Requirements

Steam generator tubes are typically supplied as U bend tubes with precise bend radii, because the U-bend region is where ovalization and residual stress concentrate during bending. Bend quality, stress relief, and the final heat treatment all affect the tube's long-term resistance to cracking, so the manufacturing route matters as much as the alloy chemistry.

RCC-M Section II mandates inspection and testing according to the material class. For nuclear-grade tubes this normally includes ultrasonic testing for internal flaws, eddy current testing for surface defects, hydrostatic testing for pressure integrity, and helium leak testing on the U-bend sections. Eddy current testing is often performed at several stages of production, on the straight tube, before bending, and again after bending, to catch defects at the point where they are introduced. Every batch must be accompanied by material certificates complying with EN 10204 Type 3.1 or 3.2, with full traceability from heat number through melting process and manufacturing route.

How to Choose the Right Solution

Selecting the right RCC-M Section II nuclear tube solution for a steam generator replacement comes down to a few practical decisions:

• Confirm the governing code edition and the material class required for each component before ordering.
• Match the alloy to the service: Alloy 690 TT for the tube bundle, stainless grades for internals, carbon steel for shells, and copper-nickel for auxiliary cooling circuits.
• Specify custom dimensions, including tube outside diameter, wall thickness, length, and U-bend radii, so the tubes match the replacement steam generator design exactly.
• Verify the supplier's testing and documentation capability. Full non-destructive testing, EN 10204 certificates, and complete traceability are non-negotiable for nuclear service.
• Work with a manufacturer that can produce custom heat exchanger tube and U-bend configurations to your drawing, with the mill test certificates to back them up.

Conclusion

Steam generator replacement is a once-in-a-generation investment, and the tube bundle is the component that determines whether the new unit will serve its full design life. The RCC-M Section II nuclear tube solutions available today, led by Alloy 690 TT for the tube bundle and supported by stainless, carbon, and copper-nickel grades for the rest of the unit, give operators a proven path to restore capacity and extend plant life. The key is to specify the right material class, demand complete testing and documentation, and work with a supplier that understands nuclear-grade requirements from ordering to final certification.