Corrosion issues in Pressurized water reactor steam generators

Traditional domestic commercial nuclear power plants were made with austenitic stainless steel generators for their outstanding corrosion resistance, weldability and other properties. A pressurized water reactor steam generator started in 1967, is made of Inconel 600 due to its outstanding resistance to chloride stress corrosion cracking. Although since its use, wastage and intergranular  cracking of alloy 600 has occurred in plants involving the use of sodium phosphate or volatile water treatment.

Intergranular corrosion has been attributed to the presence of free caustic, there have also been cases of intergranular stress corrosion cracking on the tube used in the surface in relatively pure water. Cracking of highly stressed Inconel resulted into steam generator tube failures that was due to corrosion.

In the pressurized water reactor steam generators, the heat released from the primary coolant is used to develop steam that passes through the turbine generators to the condenser where the waste heat is removed. The steam generator tubes made from Inconel 600 and tube shells made from carbon steel, an Inconel 600 cladding to which the tubes are welded. To limit the tubes vibration, a series of carbon steel supports are used for the straight parts of the tubes and for the u tube design, a series of retstraints as antivibration bars are used for the curved part of the tubes.

Both design features and environmental factors have had a determined effect on the integrity of steam generator tubing. Contaminants in the secondary water that are allowed to focus due to their thermal and hydraulic factors that result in regions of low flow, have resulted into degradation of steam generator. The main causes of contaminants are corrosion of materials in the turbine, condenser and feedwater pipes and inleakage of the tertiary coolant in the condenser.

Secondary water chemistry:

Secondary water chemistry focuses on reducing the corrosion and prevent scale development on the high-heat flux surface of the tubing by the contaminants. Types of water chemistries like in PWR secondary water systems- all volatile treatment is increased and the oxygen scavenged by volatile additives like hydrazine or morpholine, a phosphate treatment, in which sodium phosphates are added to the coolant to raise the pH and react with scale developing contaminants to develop harmless soft phosphate precipitates, combined with inclusions of morpholine or sodium sulphite to scavenge oxygen that uses full-flow condensate demineralization to eradicate caustic forming contaminants and scale forming solids from the steam generator.

Phosphate Composition and wastage cracking

Sodium phosphate treatment is commonly used for steam generators that eliminate precipitated or suspended solids by blowdown. Different steam generators with Inconel 600 tubes recieved stress corrosion cracking. The cracking was featured to free caustic that can be developed when sodium ratio exceeds the preferred limit of 2.6.`They accumulate as sludge on the tube sheet and tube supports at the main part of the tube bundle where limited water supply and high heat flux occurs.

With the use of high corrosion resistant nickel alloys, the problems of corrosion are significantly minimized that ensure the considerably longer life of service equipments.