2020

Hänninen, Seppo; Astero, Poria; Lehtonen, Matti; Rosenström, Miro

This report studies the safety assessment of electric systems for nuclear power plants. Term "safety case" is deliberately used here instead of safety analysis report to emphasize the safety demonstration aspect of the safety assessment. The work covers the definition of overall safety case for electric systems by investigating faults and disturbances related to electric systems. <br/><br/>The work bases on the literature review of the internal and external electrical systems of the nuclear power plant. The literature focuses on electrical system reliability, electrical disturbances and simulation of electrical systems. The literature survey does not attempt to analyse incident reports directly. Instead, it reviews various reports that have already analysed and categorised entries from operational experience databases. Based on the literature review and an analysis of these conditions, this report describes the current state of research on these topics. <br/><br/>The proposed safety case includes typical fault based studies of electrical systems, like lightning overvoltages, short circuit, ground and open phase faults. Also other types of events are included in the safety case, which are related to the safe operation of the nuclear power plant like subsynchronous oscillation, loss of AC power, operation of the switch-over automation, connections to the external grid and flexible operation of the nuclear power plant. The aim is not to provide a complete compilation of possible events for the safety case related to electric systems, but to show important illustrative examples. Some of them will be selected later for the analyses by simulation using the COSI simulation platform. <br/><br/>The report covers also the role of electric systems in probabilistic risk assessment (PRA). The main initiating events connected electric systems in PRA are the loss of external power (LOOP) and also in some countries the open phase conditions (open phase condition).