Concept Design, Application and Risk Assessment of New Forced Safety Injection Tank for Station Blackout Accident Scenario

In the current fleet of nuclear power plants, engineered safety systems are designed to perform fundamental safety functions. These fundamental safety functions are crucial, and failure of any one of these may lead to devastating accidents like the Fukushima Daiichi accident. The lesson learned from the Fukushima accident led to the development of advanced passive safety systems for all the new reactor designs and safety enhancements to the current nuclear fleet, such as accident tolerant fuel and US diverse and flexible coping strategies (FLEX). Safety injection tanks are an essential part of the engineered safety features. They are designed to refill the core in the event of medium-large or large break LOCA accidents. The amount of coolant inventory inside the safety injection tanks can provide extended time to core damage if utilized in other accidents such as Station Blackout. This research presents the conceptual design of the new forced safety injection tanks (FSIT). FSIT is designed by introducing a piston-based assembly on the top of the existing safety injection tank. The principle of operation is that when this system is activated, the pressure of the FSIT is increased above the discharge pressure using the backpressure from the pressurizer or steam generator (SG) to drive inventory into the system to gain additional time to core damage. This time can be imperative in deploying FLEX to recover the lost safety system. FSIT can be driven by various sources but the focus of this paper is limited to operation through the steam from the main steam line header. In this research, an SBO case study is performed to demonstrate the applicability and operating principles of FSIT. A simple PRA model was utilized to perform the risk assessment and FSIT is found to reduce the risk of early failure of turbine-driven auxiliary feedwater pumps.