Extended blackout mitigation strategy for PWR

Abstract

The accident at the Fukushima Daiichi nuclear power plant demonstrated the vulnerability of the plants on the loss of electrical power and loss of the ultimate heat sink events.A set of measures are proposed and currently implemented in response of the accident at the Fukushima Daiichi nuclear power plant. Those measures include diverse and flexible mitigation strategies that increase the defence-in-depth for beyond-design-basis scenarios. Mitigation strategies are based on the utilization of the portable equipment to provide power and water to the nuclear power plants in order to maintain or restore key safety functions. The verification of the proposed measures with the plant specific safety analyses is endorsed in the mitigation strategies.This paper investigates utilization of the turbine driven auxiliary feedwater pump (TD-AFW) or portable water injection pump for the mitigation of the event of loss of all alternate current sources and batteries (extended station blackout). Methodology for assessment of the required pump injection flow rate with the application of the standard deterministic safety analysis code is developed and presented. The required injection rate to the steam generators is calculated from the cumulative water mass injected by the turbine driven auxiliary feedwater pump in the analysed scenarios, when desired normal level is maintained automatically. The developed methodology allows assessment of the required injections rates of pump, TD-AFW or portable, for different plant configurations and number of flow rate changes.The methodology is applied to a pressurized water reactor on reference model. Several case scenarios are developed and analysed considering different sources of reactor coolant system inventory loss and procedures to limit this loss.The obtained results show effectiveness of the proposed strategy for mitigation of the event of loss of all electric power sources at the nuclear power plant. The obtained results demonstrate the need for assessment of the pump injection flow rates before the utilization of the pump for mitigation of the event. The applicability of the developed method on operational power plant is validated.