Re-assessment of station blackout accident in VVER-1000 NPP with additional measures following Fukushima accident using Relap/Mod3.2

The Fukushima accident indicated that the safety systems of nuclear power plants should be re-assessed against site-specific extreme natural hazards using additional BDBA management equipment and organizational measures. The occurrence of loss of power supply (SBO) accident in VVER-1000 NPP, which is the most contributor to the total core damage frequency, is the focus of this study. The SBO accident analysis of the reference plant with and without operator actions are investigated by Relap5 code to identify the characteristics of additional measures, available times for operator actions (critical time points) as well as scenarios to manage the accident consequences. The additional BDBA management equipment includes robust and reliable portable additional electrical power backup sources and water supplies into the primary and secondary cooling system to remove the residual heat of the core. The main purpose of this study is an evaluation of efficiency and sufficiency of additional equipment and organizational measures at NPP site based on deterministic safety analysis during SBO accident. This analysis is also expected to prepare a useful guideline for the SBO management in VVER-1000/V-446 NPP. A validated Relap5 model of VVER-1000 is employed to implement the proposed SBO accident scenarios consisting of residual heat removal through primary and secondary cooling systems as well as additional measures to mitigate this accident. The main thermal-hydraulic parameters of NPP in response to SBO accident are compared with specified acceptance criteria. The deterministic safety analysis demonstrates the provided accident scenarios with specified additional BDBA management equipment ensures the long-term reliable cooling of the reactor core under loss of power supply condition. The result also illustrates that a high degree of confidence in the robustness of the VVER-1000 NPP could be achieved against SBO accident with the inclusion of additional measures. (C) 2019 Elsevier Ltd. All rights reserved.