Steam generation corrosion control

Primary water stress corrosion cracking (PWSCC) has been observed in PWR steam generator tubing, control rod drive mechanism penetrations, and bolting materials. Methods to evaluate susceptibility of components, mechanisms, and methods for qualifying alternate materials for resistance to this specific form of IGSCC are required. 

Boron, in the form of boric acid, is used in the PWR primary system water to compensate for fuel consumption and to control reactor power (3). The concentration is varied over the fuel cycle. Small amounts of the isotope lithium-7 are added in the form of lithium hydroxide to increase pH and to reduce corrosion rates of primary system materials (4). Primary-side corrosion problems are much less than those encountered on the secondary side of the steam generators. 

Water Treatment. Water and steam chemistry must be rigorously controlled to prevent deposition of impurities and corrosion of the steam cycle. Deposition on boiler tubing walls reduces heat transfer and can lead to overheating, creep, and eventual failure. Additionally, corrosion can develop under the deposits and lead to failure. If steam is used for chemical processes or as a heat-transfer medium for food and pharmaceutical preparation there are limitations on the additives that may be used. Steam purity requirements set the allowable impurity concentrations for the rest of most cycles. Once contaminants enter the steam, there is no practical way to remove them. Thus all purification must be carried out in the boiler or preboiler part of the cycle. The principal exception is in the case of nuclear steam generators, which require very pure water. These tend to provide steam that is considerably lower in most impurities than the turbine requires. A variety of water treatments are summarized in Table 5. Although the subtieties of water treatment in steam systems are beyond the scope of this article, uses of various additives maybe summarized as follows ... 

Pressure vessels and appurtenances should be constructed of stainless steel or other corrosion-resistant materials. Ideally, these steam generators should receive hot demineralized FW to minimize chemical treatment requirements. Alternatively, where a main boiler plant is installed, 100% steam condensate provides a good source of FW. In practice, it is very difficult to accurately control the correct amount of chemical feed. Chemicals are typically restricted to potable grade, deposit control agents such as polyacrylates, and other materials listed under the Code of Federal Regulations, CFR 21 173.310, or National Sanitary Foundation (NSF International) approval system. These boilers may be electrically heated or gas-fired. 

Power plant boilers are either of the once-through or dmm-type design. Once-through boilers operate under supercritical conditions and have no wastewater streams directly associated with their operation. Drum-type boilers operate under subcritical conditions where steam generated in the drum-type units is in equilibrium with the boiler water. Boiler water impurities are concentrated in the liquid phase. Boiler blowdown serves to maintain concentrations of dissolved and suspended solids at acceptable levels for boiler operation. The sources of impurities in the blowdown are the intake water, internal corrosion of the boiler, and chemicals added to the boiler. Phosphate is added to the boiler to control solids deposition. 

Recent variants have attempted steam generation (as LP steam) or getting hot water from the hot acid circulating in the IPAT and FAT. Generation of LP steam needs very careful process control for the acid concentration to minimize the corrosion of equipment and piping. 

R. Garnsey [1979] Boiler Corrosion and the Requirement for Feed- and Boiler-Water Chemistry Control in Nuclear Steam Generators, Nucl. Energy 18, 117. 

The chemistry of the feedwater to the steam generators is precisely controlled by demineralization, de-aeration, oxygen scavenging, and pH control. A blowdown system is provided for each steam generator that allows impurities collected in the steam generators to be removed to prevent their accumulation and possible long-term corrosive effects. In some reactors, the blowdown is collected and recirculated. 

FIG. 3—Three components of corrosion control in steam generating cycles. 

Corrosion control in steam-generating systems is by a combination of design, material selection, water treatment, and... 

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