PREDICTION OF DYNAMIC BREAK-OPENING AREA UNDER BEYOND DESIGN BASIS SEISMIC LOADING

The thermal-hydraulics computer code, RELAP (Reactor Excursion and Leak Analysis Program) is used to analyze loss of coolant accidents (LOCAs) and system transients in PWRs and BWRs. However, RELAP requires the knowledge of break-opening area versus time history for Double-Ended Guillotine Break (DEGB) of a pipe fracture event as an input to calculate pressure drops at critical locations in the primary pipe loop. Previously authors conducted a detailed dynamic FE analyses to determine the condition for DEGB that provided moment versus rotation of the cracked-pipe and time histories for DEGB under beyond design basis seismic loading. In this paper, crack-opening area was calculated using the moment-rotation-time history obtained from dynamic FE analyses. In the LBB.ENG2 J-estimation scheme for circumferentially cracked pipe, the rotation at the cracked-pipe cross-sectional location (rotation due to the crack) is uniquely related to the total crack length and crack-opening displacement at the center of the crack. However, the relationship is only valid when the moment versus rotation from the FE analyses corresponds to the ductile tearing curve from the LBB.ENG2 ductile fracture analysis. During any unloading (and reloading) parts of the applied seismic history, the rotation can drop down from the upper-envelope for the tearing resistance of the cracked pipe in an elastic unloading manner from the seismic/cyclic unloading. During this part of the seismic time-history, the crack length remains constant but the center-crack-opening displacement decreases, i.e., there is crack closure with a constant crack length which needs to be included in predicting crack-opening area. Based on a number of past cyclic pipe fracture tests with large amounts of ductile tearing, a procedure was developed to predict the crack-opening area that included crack closure during cyclic loading of the seismic event. The resulting opening-area versus time history then becomes the input to the RELAP analysis for determination of emergency core cooling/safety processes.