Study on the PWR Steam Generator Behavior with improved steam-driven aux feedwater system under prolonged SBO Accident

The only available passive decay heat removal system of current PWRs is a turbine-driven auxiliary steam generator (SG) feedwater (TD-AFW) system. If a SG water level becomes too high, however, turbine blades could be damaged due to a large amount of becomes too high, however, turbine blades could be damaged due to a large amount of moisture in steam and the SG cooling capability would not be maintained any longer. Therefore, the SG water level should be controlled to prevent the turbine from being damaged during a Station Black-Out (SBO) accident. In this paper, an improved design feature is proposed to provide electric power for controlling SG water level when both off-site power and the emergency diesel generators are not available. There are additional SG level gauges and valve controllers to control the steam flow into the auxiliary turbine in an improved TD-AFW system. Electric power for this control system is provided by a small additional generator which is connected to the existing auxiliary turbine shaft. Using this new feature, decay heat cooling is available for 29 hours with only 1 condensate storage tank which is the water source of the AFW. Eventually, it is concluded that the improved TD-AFW system with an additional SG level controller and generator can avoid an early SG full level and continue long term cooling during a prolonged SBO accident