Main Steam Line Constant Spring Support Failures

Several constant spring supports attached to the main steam lines (MSLs) at Palo Verde Nuclear Generating Station (PVNGS) failed over a period of several years. The continued failures presented a need to instrument the MSL piping and supports, such that data could be collected in support of a cause determination and problem resolution effort. Extensive monitoring and data acquisition was performed over an 18-month period in all three PVNGS operating units, encompassing several revisions to the instrumentation configuration. This document presents an account of the project, with a timeline of events, detailed information on the instrumentation and data analysis, and other engineering efforts. Ultimately, no single source of failure was identified, as several potential contributors were uncovered. Modifications were implemented to mitigate the hanger failures, and the instrumentation was used to verify their effectiveness. No further failures have occurred after implementation of the modifications.