The contribution to safety of a diverse backup system for digital safety I&C systems in Nuclear Power Plants, a probabilistic approach

NRG performed a research project on the influence on safety of diverse backup systems next to the existing digital I&C safety systems in Nuclear Power Plants (NPPs). As part of this research project a probabilistic approach has been used to evaluate the basic options to connect a diverse backup system logically to the existing digital I&C systems. One can distinguish four different basic design options: (1) no backup system is used; (2) the backup system is used only if the digital system has failed, the switch-over to the backup system is automatic; (3) the backup system is used only if the digital system has failed, the switch-over to the backup system is manual; (4) the backup system is in continuous operation, with an equal vote as the digital system. Design (2) and (4) have been modeled and compared with the situation without backup system (design 1). This paper will discuss the model and the results, including the sensitivity analyses, in order to reflect on the probabilistic impact of a diverse backup system.