Session: DA-02-01 Design and Analysis of Piping and Components - 1

Paper Number: 81565

81565 - ASME Sec. III NB-3200-Based Environmental Fatigue Analysis of Safety Injection Piping for Determining Postulated Rupture Locations 

Appendix A of 10 CFR 50, General Design Criteria (GDC) 4, requires that safety-related structures, systems, and components (SSCs) of nuclear power plants (NPPs) should be designed to accommodate dynamic effects such as jet impingment and pipe whip caused by pipe ruptures. This requirement is to ensure the functional or structural integrity of SSCs required for safety shutdown of NPPs in the event of a loss of coolant accident (LOCA) by protecting safety-related SSCs from dynamic effets caused by pipe ruptures.

The guidelines for determining postulated pipe rupture locations are provided in Section 3.6.2 of NUREG-0800, Standard Review Plan, and its associated Branch Technical Position (BTP) 3-4. In general, actual pipe ruptures occur at locations where high stress or high fatigue is caused due to structural discontinuity such as terminal ends of a piping system like piping-connected nozzles of a component. Therefore, BTP 3-4 requires to determine the postulated rupture locations for piping where there are possibilities to have 1) plastic collapse due to high stress caused by primary loads, 2) excessive plastic deformation caused by primary plus secondary loads, and 3) high fatigue damage caused by cyclic loads.

For ASME B&PV Sec. III Class 1 piping, BTP 3-4 suggests only a NB-3600-based procedure to determine postulated pipe rupture locations by using the maximum stress and cumulative usage factor (CUF). But NB-3600-based piping analysis, which uses a simplified design-by-rule approach for piping design, may have excessively conservative values of piping stress and CUF due to conservatism of the applied stress indices and load combination methods. That means, postulated pipe rupture locations, whose probability of actual pipe rupture is somewhat low, may be selected because of conservative NB-3600-based analysis results. As a result, unnecessary pipe whip restraints and jet shields for preventing dynamic impacts of safety-related SSCs from pipe ruptures may need to be installed.

NB-3611.2, NB-3630, and NB-3653 of ASME B&PV Sec. III allow to use the detailed analysis method given in NB-3200, which provides design-by-analysis approach for piping design, when stresses determined by NB-3600-based simplified method exceed their limits.

In this paper, stress analyses for determining postulated rupture locations of a safety injection (SI) piping system have been performed based on NB-3200, which is a more detailed analysis method than NB-3600. The SI piping system is an important piping system for safety shutdown of NPPs because it is used to inject borated water into the hotleg pipe of the reactor coolant system following LOCA. Therefore, this SI piping system should be designed to accommodate dynamic effects caused by pipe ruptures.

In order to find out which part of the SI piping system is to be evaluated to NB-3200 for determining postulate rupture location, a NB-3600-based stress analysis was first performed. As a results of the NB-3600-based analysis, an elbow of the SI piping system was turned out to be the highest stress point as well as the postulated pipe rupture location. For the elbow, NB-3200-based detailed stress analyses were performed, and their results were compared with the NB-3600-based stress analysis results to evaluate how much conservatism the NB-3600-based stress analysis results have in determining postulated rupture location.

Recently, BTP 3-4 was revised as Rev.3 and provides two different CUF limits of 0.1 and 0.4 for Air-environments and Light Water Reactor (LWR) environments, respectively, for determining postulated pipe rupture locations. In this paper, fatigue analyses considering both the Air-environments and the LWR environments have been performed to evaluate the effect of the LWR environments on determining postulated rupture locations.

Presenting Author: Bonghee Lee KEPCO E&C

Presenting Author Biography: Bonghee Lee received B.S and M.S degrees from department of mechanical engineering, chungbuk national university in 2009 and 2011 respectively. He has worked for KEPCO engineering & construction company since 2011 and he is now pipe stress engineer. Mr. Lee research area includes pipe stress analysis, fatigue and environmental fatigue evaluation.

Authors:

Bonghee Lee KEPCO E&C
Ilkwun Nam KEPCO E&C
Wooseok Yang KEPCO-E&C
Chankyo Lee KEPCO E&C
Dongjae Lee KEPCO E&C