An improved system for cleaning the upper bundle region of pressurized water reactor steam generators is now available. This system employs high pressure water jets to remove sludge and scale from tube and support plate surfaces and from the broach holes throughout the upper bundle region of steam generators. Benefits of sludge removal can include mitigation of the corrosion process, improvement of plant performance and increased operational stability. The cleaning equipment is installed through existing handholes in the tubesheet region and accesses the upper bundle through flow slots in the tube support plates. The system includes a Bulk Cleaning Head which directs high pressure water jets into the tube bundle, a Vertical Deployment System which delivers and positions the head at the desired locations in the upper bundle region, and a high pressure water pumping and sludge processing system. This paper describes the system, several key components, overall system operation, safety issues, and field use chronology. Specific performance issues that are discussed include the cleaning coverage achieved, the time required to clean a steam generator, and cleaning effectiveness. The Upper Bundle Hydraulic Cleaning System described in this paper is available for use in several types of steam generators.
Maintaining steam generators is a major challenge for pressurized water reactor operators. These steam generators are vulnerable to a variety of damage mechanisms, many of which are a result of deposit buildup on the secondary side. Fouling of flow holes in support plates causes operational instability. Fouling of outside tube surfaces may decrease output pressure and reduce electrical generation capacity. Stress corrosion cracking (XC) may require tube plugging, tube sleeving and, in extreme cases, steam generator replacement. Outside Diameter Stress Corrosion Cracking (ODSCC), aggravated by secondary side deposit buildup, has become a leading cause of tube plugging. Of total tubes plugged, the percentage of steam generator tubes plugged due to ODSCC in Westinghouse steam generators rose from 40 percent in 1989 to 71 percent in 1991.
Sludge lancing removes deposits from the tubesheet area of the secondary side. It is estimated, however, that only about one-tenth of the secondary side deposits are in the tubesheet region. The balance is located in the upper region of the steam generator, which is inaccessible to existing sludge lancing systems. Chemical cleaning is effective in this upper bundle region, but has several disadvantages, including high cost and the generation of a large quantity of potentially hazardous waste, Another significant disadvantage of chemical cleaning can be the relatively long duration, especially in today?s shorter outages.
Developed as a viable alternative to chemical cleaning, the UBHC System provides a safe, cost-effective means of removing secondary side deposits from the upper bundle region. By combining the Brooks? patented Vertical Deployment System, Foster-Miller?s Bulk Cleaning Head (BCH) and Westinghouse?s sludge processing system, deposits can now be hydraulically removed from the entire tube bundle.
The new Upper Bundle Hydraulic Cleaning System (UBHC) is installed through the secondary handhole at the tubesheet level and extends
