Risk analysis with dynamic safety measure for pressurized water reactors with advanced safety features

Abstract

After the Fukushima Daiichi Accident, the US Department of Energy initiated the LightWater Sustainability Program to investigate safety options for existing nuclear fleets, such as accident-tolerant fuel and US diverse and flexible coping strategies. As these safety options are relatively new, developing a framework that can assess their risk and benefits effectively is essential. A dynamic probabilistic risk assessment framework is suggested to analyze the risk and safety of these options during abnormal deviations or accidents. The framework can explicitly highlight the risk and safety benefits of the existing and new safety systems, perform sensitivity analysis of physical parameters and optimize the value of design variables of the system and components. The risk and sensitivity analysis has been demonstrated by leveraging the suggested framework for analyzing two diverse accident scenarios: Station blackout and medium break loss of coolant accident. A CDF- based benefit index and included in the framework can assess the risk margin offered by the existing or the new safety options and provide a mathematical way of comparing those. Additionally, a new concept design for forced safety injection tanks has been suggested. The feasibility of this system has been demonstrated by modeling and integrating this new system into the nuclear power plant model and analyzing the risk benefits. The presented DPRA framework is further leveraged to optimize the FSIT's design parameters, such as actuation set-point or the delays between the series operation of FSITs. Since time is crucial during an accident evolution, a time-based dynamic event importance index is introduced that supports the operator in deciding how much resources he should invest for each component to recover the lost safety function. This measure provides reliability and timebased component importance and ranks them for the operator to allocate resources to restore those components. This measure is compared with the conventional measures. Then efforts are made to estimate the cost savings from re-categorizing the safety components from RISC1 and RISC2 safety classification to RISC3 and RISC4, respectively.