As all safety systems in the majority of existing nuclear power plants use the preferred power supply, any voltage surges in these systems could lead to common-cause failures. In the event of an unusual electrical system transient, it is essential that safety-related equipment be isolated or protected from the fault in order to ensure its ability to safely shut down the reactor and remove decay heat.

Based on the analysis of the voltage surges observed at Forsmark-1 in 2006 and Olkiluoto-1 in 2008, this technical opinion paper summarises the current state of knowledge of in-plant and external grid-related challenges to nuclear power plant safety-related electrical equipment. It will be of particular interest to nuclear safety regulators, nuclear power plant operators and grid system regulators and operators.