Development and Testing of an Improved Westinghouse Containment Design Basis Analysis Methodology

Westinghouse utilizes separate computer models for the loss-of-coolant accident (LOCA) mass and energy (M&E) release and containment response calculations for the containment design basis accident (DBA) analysis. The current LOCA M&E release model uses very conservative assumptions to calculate the rate of steam generator (SG) energy transfer to the reactor coolant system (RCS). Westinghouse is upgrading the containment DBA analysis to use WCOBRA/TRAC (WC/T) for the LOCA M&E release calculation and GOTHIC for the containment response calculation. Furthermore, the formerly separate calculations can now be performed concurrently to allow communication between the RCS and containment calculation models. WC/T utilizes more representative models for calculating the rate of SG energy transfer to the RCS than the currently approved LOCA M&E release calculation model. The full length emergency core heat transfer separate-effects and system-effects test (FLECHT-SEASET) data was used to validate the WC/T SG model for the post-blowdown LOCA M&E release calculation. The WC/T SG model originally calculated a higher SG energy transfer rate than the test data, which is conservative for containment analysis. The WC/T code was updated to include recommended changes to model the SG tube quench front. Following implementation of the recommended software changes, WC/T calculates a more realistic, yet still conservative, SG energy transfer rate. A WC/T plant model was developed to perform a LOCA M&E release calculation, and the WC/T results were compared to results from the currently approved LOCA M&E release calculation model. WC/T calculated a lower post-blowdown release rate than the currently approved LOCA M&E release calculation model. The lower post-blowdown release rate will provide additional peak pressure margin for plants that rely on passive cooling, containment spray, and/or fan coolers to control the containment peak pressure, and is also expected to extend the time of ice melt-out for ice condenser containments. The upgraded containment DBA analysis model that utilizes WC/T and GOTHIC concurrently will establish a conservative, but more representative long-term containment pressure and temperature response. Results based upon the GOTHIC containment model running with the WC/T plant model have shown a pressure reduction of approximately 10 psi (70 kPa) in the post-blowdown pressure transient.