Performance and Reliability of Passive Safety Systems in Modern Pressurized Water Reactors (PWR) During Station Blackout Events

This paper analyses the performance and reliability of passive safety systems employed in modern pressurized water reactors (PWRs) when confronted with station blackout (SBO) events. Passive safety systems—such as gravity-driven emergency injection, natural circulation cooling and decay heat condensation—are defined as systems that operate without external power or active controls. Their ability to maintain core and containment cooling during complete loss of alternating current (AC) power makes them critical for mitigating severe accidents. The methodology adopted is a comparative analysis of passive systems in the AP1000, VVER-1200 and European Pressurized Reactor (EPR) designs. Key parameters used in the assessment include decay heat removal capability, heat transfer coefficients, coolant inventory, natural circulation flow behaviour and system response time. Literature results and regulatory standards are synthesised to evaluate how effectively these designs cope with prolonged SBO events. The study finds that the AP1000’s passive safety systems can maintain core cooling and containment integrity for at least 72 h without operator intervention, while the VVER-1200 uses a hybrid combination of active and passive systems to provide 24 h of cooling via the passive heat removal through steam generator (PHRS-SG) and up to 72 h through staged hydro-accumulators. The EPR relies on diversified active and passive systems, including a dedicated core catcher and redundant heat removal paths. Reliability assessment reveals that the inherent simplicity of passive systems reduces common failure modes, yet uncertainties in natural circulation and condensation phenomena still need careful modelling and experimental verification. Limitations such as coolant stratification, non-condensable gas accumulation and steam binding can affect long-term performance. The paper concludes that passive safety systems significantly enhance SBO coping times and contribute to lower core damage frequencies, although continued research is necessary to address physical uncertainties and support licensing of future passive designs.