Introduction: The Backbone of Safe Energy

In a pressurized water reactor (PWR), steam generators function as a critical barrier between the nuclear core and the secondary turbine system. These massive heat exchangers transform primary circuit heat into steam to drive electricity production—while simultaneously acting as a pressure vessel and radiation shield.

Among their vital components are tubesheets and tie rods—machined assemblies that hold thousands of heat transfer tubes in place. In this context, material failure is not just costly—it’s a nuclear safety issue.

That’s why Alloy 690 rods have become a cornerstone in modern nuclear engineering. Offering superior corrosion resistance, mechanical strength, and long-term reliability, they are replacing older alloys in the heart of nuclear infrastructure.

The Corrosion Challenge in Nuclear Steam Generators

PWR steam generators operate under:

• High-pressure, high-temperature conditions (up to 15 MPa and 320°C)

• Ultra-pure water chemistry to minimize impurities

• Aggressive flow-accelerated corrosion, particularly near crevices

• Risk of Primary Water Stress Corrosion Cracking (PWSCC), especially in legacy materials

Stress corrosion cracking is triggered when:

1. Tensile stress is present in the material

2. Corrosive agents, even in small amounts, infiltrate protective films

3. Susceptible microstructure (sensitization or phase segregation) exists

Earlier steam generators used Alloy 600 rods and supports, which suffered from:

• Cracking at welds and heat-affected zones

• Crevice corrosion at tight geometries

• Long-term thermal degradation

Introducing Alloy 690: Designed for the Nuclear Century

Alloy 690 (UNS N06690) is a nickel-chromium-iron alloy developed specifically to overcome the weaknesses of Alloy 600 in nuclear environments.

Composition:

• Nickel (Ni): ~58%

• Chromium (Cr): 27–31%

• Iron (Fe): 7–11%

• Trace: Ti, Si, Mn

Key Strengths:

• High chromium content provides excellent resistance to PWSCC

• Stable gamma-phase matrix, limiting phase transformation at high temp

• Low carbon content prevents sensitization at grain boundaries

• Superior thermal stability and mechanical strength

Today, Alloy 690 rods are standard in replacement steam generators (RSGs) globally, and are also used in:

• Tubesheet tie rods

• Lateral tube support plates

• Lower support grid rods

• Dowel pins and load-bearing connectors

Case Study: South Korean Nuclear Power Plant

A major nuclear operator in South Korea retrofitted a PWR unit’s steam generator structure, replacing Alloy 600 rods with Alloy 690 rods and tie components. The goal was to:

• Eliminate premature cracking

• Reduce inspection frequency

• Extend operating license without component replacement

Results over 10 years:

• No evidence of PWSCC or surface attack

• Mechanical inspections showed <2% dimensional deviation

• Regulatory approval for extending refueling outage intervals

• Enabled plant to move to 90-day inspection intervals instead of 60

Plant manager noted:

"Alloy 690 gave us the confidence to push inspection boundaries without compromising safety."

PWSCC and Crevice Resistance: How Alloy 690 Excels

Stress corrosion in Alloy 600 originates at:

• Grain boundary carbides

• Local depletion zones of Cr and Ni

• Heat-affected zones after poor welds

Alloy 690 resists this by:

• Using low carbon (<0.02%) to prevent intergranular sensitization

• Maintaining single-phase gamma microstructure during welding

• Forming a dense, stable chromium oxide film even under primary coolant chemistry

In thermal cycling and crevice exposure:

• Retains corrosion resistance up to 350°C

• Withstands 10⁵+ stress cycles without crack initiation

• Avoids localized thinning at support contact points

Fabrication and QA in Nuclear Applications

Alloy 690 rods must meet nuclear-grade quality assurance protocols:

• Solution annealed and stress-relieved to ensure grain stability

• Precision-machined with tight tolerance threads and surface finishes (Ra < 0.4 μm)

• Welded using GTAW (TIG) or electron beam welding with Alloy 52 filler metals

• Verified with ultrasonic, dye penetrant, and eddy current NDT techniques

• Certified per ASME III, RCC-M, and KEPIC nuclear codes

Comparison with Other Tube Support Alloys

Expanding Use in Advanced Nuclear Systems

As nuclear power evolves toward modular and advanced reactors, Alloy 690’s role expands:

• Small Modular Reactors (SMRs): Compact steam generators, integral supports

• High-Temperature Gas Reactors (HTGRs): Oxidizing coolant compatibility

• Molten Salt Reactors: Alloy 690’s Cr content offers base resistance to halide-induced corrosion

• Fusion reactor blankets: Alloy 690 is under review for tie rods in helium-cooled support systems

Its versatility and proven record make it a first-choice material for high-integrity nuclear assemblies.

Conclusion: Nuclear Trust, Materialized

In nuclear power, the smallest crack can lead to the biggest consequences. The rods and structural elements inside steam generators are not just support—they are barriers to failure.

Alloy 690 rods stand as silent guardians: engineered to withstand decades of thermal stress, radiation exposure, and chemical attack without compromise.

For the next generation of reactors—and the next generation of engineers—Alloy 690 means safety, strength, and stability.