Containment fan cooler heat transfer calculation during main steam line break for Maanshan PWR plant

A thermal analysis has been performed for the Containment Fan Cooler Unit (FCU) during Main Steam Line Break (MSLB) accident, concurrent with loss of offsite power, for Maanshan PWR plant. The analysis is performed in order to address the waterhammer and two-phase flow issues discussed in USNRCʹs Generic Letter 96-06 (GL 96-06). Maanshan plant is a twin-unit Westinghouse 3-loop PWR currently operated at rated core thermal power of 2822 MWt for each unit. The design basis for containment temperature is Main Steam Line Break (MSLB) accident at power of 2830.5 MWt, which results in peak vapor temperature of 387.6 °F. The design is such that when MSLB occurs concurrent with loss of offsite power (MSLB/LOOP), both the coolant pump on the secondary side and the fan on the air side of the FCU loose power and coast down. The pump has little inertia and coasts down in 2–3 s, while the FCU fan coasts down over much longer period. Before the pump is restored through emergency diesel generator, there is potential for boiling the coolant in the cooling coils by the high-temperature air/steam mixture entering the FCU. The time to boiling depends on the operating pressure of the coolant before the pump is restored. The prediction of the time to boiling is important because it determines whether there is potential for waterhammer or two-phase flow to occur before the pump is restored. If boiling occurs then there exists steam region in the pipe, which may cause the so called condensation induced waterhammer or column closure waterhammer. In either case, a great amount of effort has to be spent to evaluate the waterhammer pressure and even the structural integrity. With respect to this, GOTHIC computer code is used to simulate the thermal response of FCU during MSLB/LOOP. The transient response of containment vapor temperature and vapor saturation temperature presented in FSAR are used as boundary conditions of GOTHIC to drive the heat transfer across the cooling coils given a predetermined condensation heat transfer coefficient and convective heat transfer coefficient. The coolant temperature response has been calculated and compared with the saturation temperature corresponding to the coolant operating pressure to determine the time to boiling. The information is then used as the basis to determine whether the current system design has a sufficient margin to prevent boiling from occurring or whether a betterment engineering and design is required to increase the operating pressure at the coolant pump suction.