NuVision’s Plant Engineering group was awarded a project by the Tennessee Valley Authority (TVA) for the re-analysis of the Main Feedwater (MFW) piping due to a combined Safe Shutdown Earthquake (SSE) and MFW Check Valve Slam (CVS) event resulting from a break in the feedwater header for Watts Bar Unit 1. TVA wanted to ensure that loops 1 through 4 of the MFW piping would maintain structural integrity during combined SSE and CVS events.

Issues and  Work Scope

During the steam generator replacement (SGR), small sections of the MFW piping were removed and reinstalled in essentially the same location and configuration with the exception of some of the gaps at the PIPE Whip Restraint Devices (PDs), that weren’t shimmed to pre-SGR values.

The Analysis of Record (AOR) modeled PD gap conditions do not meet current configurations in the field. Thus it was concluded that the MFW system, while fully functional, was non-conforming in accordance with NRC Generic Letter 91-18. The purpose of the analysis was to reproduce and subsequently replace the AOR for the subject system, in order to regain conformance with GL 91-18 while minimizing or eliminating the need for plant physical modifications. The AOR for the MFW piping system is supported by a sophisticated elastic-plastic analysis using the ANSYS general purpose finite element code. The mathematical model required the use of both geometric (i.e. gaps, unidirectional supports, etc) as well as material non-linearities.

The analyses accounted for normal (i.e. deadweight thermal, etc.) as well as faulted loading (i.e. SSE and CVS). In addition, the acceptance criteria required the combination of normal and faulted loading, including phasing of the SSE and CVS events.