A postulated loss-of-residual heat removal (RHR) event for a reactor coolant system (RCS) in a midloop condition is analyzed for the Palo Verde Nuclear Generating Station (PVNGS) using the RETRAN-2/MOD4 computer code. The PVNGS is a Combustion Engineering, two-loop, 3800-MW(thermal) pressurized water reactor (PWR). This analysis was prompted by the U.S. Nuclear Regulatory Commission generic letter 88-17, which requires PWR licensees to perform analyses of loss-of-RHR events for their facilities. Such analyses yield a detailed understanding of loss-of-RHR events and provide a basis for emergency procedures and recovery actions. Simulations of events following a loss of RHR are used to determine the time for RCS coolant to reach boiling, evaluate the potential for a rapid core uncovery due to pressurization of the reactor ves sel head with a corresponding liquid ejection from a cold-leg breach, and evaluate options for maintaining the RCS inventory above the core. Key features of the PVNGS midloop condition RETRAN model include isolating both steam generators from the RCS by nozzle dams, setting the initial RCS inventory at midloop, venting the RCS to containment by removing the pressurizer safety relief valves, defining a maintenance breach on one cold leg, and providing options to evaluate makeup water injection to a hot or cold leg by either pump or gravity feed. The RETRAN model provides a satisfactory method for dynamically evaluating loss-of-RHR events from a reduced RCS inventory condition, and for evaluating alternative recovery actions for PVNGS.
