Over time in nuclear power plant operation, corrosion products, such as copper and iron from piping and feedwater systems, deposit on the tube sheet, tube bundle, and in crevices in the secondary side of steam generators. These deposits can be harmful to the steam generator and steam generator tubes by inducing tube degradation such as stress corrosion cracking (SCC), as well as a phenomenon known as tube denting. To remove corrosion deposits that have grown over a number of cycles and to mitigate further tube damage, a chemical cleaning may be designed and applied to remove bulk deposit loading with targeted regions of cleaning. The design and performance of a chemical cleaning system and application is tailored to the individual utility, since deposit loadings and characteristics, steam generator designs, and utility objectives differ. A successful chemical cleaning requires much planning and design with consideration of safety, environmental, and performance factors. These three factors drive system modifications and design improvements as utilities seek to decrease costs with improved efficiency while performing a safe chemical cleaning that has a low-impact on schedule.
Corrosion products depositing in the secondary side of steam generators is detrimental to both the steam generator and steam generator tubes as it can cause tube denting, pitting and other corrosion mechanisms, loss of heat transfer within the steam generator, tube withdrawal, and overall steam generator degradation. Tube denting is where forces from corrosion deposit buildup between the tube and tube support structure place additional stress on the steam generator tube wall causing localized constrictions, referred to as dents, in the tubes. These dents can cause tube withdrawal, tube cracking, and increased steam generator tube inspections requirements. However, dented tubes may necessitate plugging if inspections can not be performed. Steam generator tube inspections are typically performed every 18 to 24 months. As utilities seek extensions in their operation cycles, these inspections will be performed with less frequency; however, deposits will continue to accumulate on the secondary side. For example, a feedwater iron concentration of 1 ppb Fe will result in 100-200 lbs (45-91 kg) of deposit per cycle depending upon the secondary system design and cycle duration. For those utilities that have exhibited increased deposit loading, the frequency of planned inspections may increase as the unit ages. This cumulative sludge loading may damage the tubes; however, utilities do have a variety of options to avoid these possible outcomes of corrosion and deposits. These options include steam generator replacement, improving or altering chemistry control, sludge lancing or other mechanical deposit removal techniques, a chemical cleaning, or continual plant operation and scheduled maintenance until decommissioning. The action performed to mitigate corrosion and remove deposits depends on the utility, severity of the corrosion degradation and deposit loading, system design, and operation objectives.
A significant amount of planning and design is required when performing a chemical cleaning with the intent to remove as much deposit as possible in a safe and environmentally friendly approach. Safety, environmental, and performance issues are the core factors that are considered when designing a chemical cleaning system and process sequence that is cost and time effective while being safe and efficient. Each one of these factors are thoroughly processed and discussed individually to provide a successful chemical cleaning.
CORE FACTORS IN DESIGN
Safety
Safety factors play a major role in de
