Role of passive safety systems in prevention and mitigation of severe accidents at nuclear power plants

Abstract

Fukushima Daiichi accidents led to the worldwide update of the NPP safety requirements. Particular attention was paid to severe accidents both during the design of new NPP units and during the safety reassessment of existing ones. Performed safety reassessment of operating units was focused on development and implementation of technical means and strategies directed to prevention and/or mitigation of severe accidents. Technical solutions that are used in modern designs of reactor facilities involve several options for providing of heat removal from the core and containment. Such options include heat removal systems from the reactor core and from the containment of reactor facility. Most of such systems are based on passive principals of operation and do not require or require a minimum action of operating personnel. At the same time, reactor facilities that are operated in Ukraine are based on old VVER designs, which do not include passive safety systems for long-term heat removal (especially for the case of total station blackout, which was in the focus during safety reassessments, so-called “stress tests”). Thus, the necessity of such systems at Ukrainian NPPs with VVER-1000 and VVER-440 was discussed together with analysis of existing types of passive systems and their prevalence. Based on the results of performed analysis the feasibility of implementation at Ukrainian NPPs of a passive heat removal system from the core for VVER-1000 and from the confinement for the VVER-440 was identified. The possibility of implementation of such systems at Ukrainian NPPs requires a separate analysis. In addition, a general information on current state of passive systems reliability assessment was described. It should be noted that despite the reliability of passive safety systems is one of the important issues it is still poorly studied and require additional analyses.