Development of an optimal mitigation strategy with a new safety system for enhancing the safety response of a nuclear power plant

Abstract

The functional modeling method is utilized to identify a reduced number of accident scenarios that cause core damage systematically and develop all possible mitigation procedures of the new safety system through causal inference analysis. In the accident scenario identification process, an improved reasoning method and the conversion process are proposed. The mitigation success domain is applied to determine an available time range for the recovery actions with the given accident conditions. Time-dependent recovery probability is also handled using this domain. As a case study, the optimal mitigation strategy of a hybrid safety injection system (HSIT) which is a new passive safety feature is developed by utilizing the proposed methodology. The optimal value of operating parameters of the HSIT that are needed to develop mitigation strategy is determined by analytical study and sensitivity study with thermal-hydraulic systems code. The test case results validate the minimum plant risk with the HSIT under a loss of coolant accident.

Among various risk measurements, conditional core damage probability (CCDP) turns out to be appropriate risk measurement as 1) it allows to calculate the risk magnitude for a particular event, and 2) the results could be summed for an operational period of time. However, there are challenges in determining optimal mitigation strategy by estimating CCDP: 1) there are the infinite number of accident scenarios in given conditions that can cause core damage, and 2) the time-dependent recovery probability and various available time of component recovery according to plant conditions give complexity on calculating CCDP. To overcome these challenges, we propose the use of a functional modeling method and a concept of the mitigation success domain to reduce the complexity based on a limited number of accident scenarios, thereby quantifying plant risk more practical. Based on those approaches, a novel methodology is proposed to determine the optimal mitigation strategy of the new system on the basis of plant risk.

In the wake of the Fukushima accident, several applications of new safety features have been highlighted to prevent severe accidents. As the feasibility of the newly applied systems may vary depending on how they are used, the optimal strategy of the new mitigation system should be developed to complement the vulnerabilities in the NPPs. Optimization of emergency operating strategies necessitates the risk quantification of mitigation action. The measurement of the risk, however, is challenging in view to the fact that 1) it changes over time as components are taken out of service or repaired by operators, and 2) plant safety depends on complex combinations of diverse active and passive safety functions.