Degraded Power Supplies
Authors: Lundbäck Maja , Karlsson Mattias 
Published: October 2016

The electrical disturbance in Forsmark 2006 [3] led to increased attention being paid to the power supplies of nuclear power plants and their role in safety system reliability, both nationally and internationally. Since then numerous disturbances similar in nature have occurred in the electrical power supply which raises questions whether best available technology (BAT) has been utilised in the design and analysis of the electrical power supply of the safety functions of nuclear facilities. On repeated occasions this type of disturbances has had an impact on redundant parts of several safety systems due to functional dependencies between these. The frequency of these occurrences has been unexpected.
The Swedish Radiation Safety Authority (SSM) has decided to write this document to clarify the regulators position on this issue. The document is also intended to support SSM:s assessments and evaluations of the Licensees efforts regarding degraded power supplies.
An assessment of nuclear power plant electrical power systems is necessary in the light of the past years’ operational experience [1–7], where disturbances in the electrical power supply on repeated occasions have caused a power supply with degradation severe enough to challenge plant safety. As the potential consequences of such a degraded power supply can be severe it must be proven that the frequency of such occurrences is tolerably low. Furthermore, it is important to consider experiences from known situations with degraded power supplies, to enable a reasonable approach to identify and take counter-measures based on the root-cause and ensure utilisation of best available technology.
A sufficient approach to enable prevention, protection and mitigation against this type of disturbances has been difficult to identify. Actual events and conditions causing a degraded power supply have often been complex in nature and difficult to anticipate, wherefore events and conditions which has not yet occurred are difficult to foresee. For this reason it is deemed most effective to identify and implement proportional measures that enhances the independence of the power supplies, such that a degraded power supply with a higher reliability is prevented from propagating to multiple parts of the safety systems.
In this memorandum, SSM describes a state-based approach to analysing electrical power system functionality in different states of degraded power supply. The approach is intended to identify potential design weaknesses and measures to enhance robustness. Such an approach is viewed as more favourable in facilitating the identification of such measures, which may otherwise be neglected due to an estimated low frequency of occurrence, or missed due to incomplete identification of possible events and conditions.
Furthermore this document describes how an assessment of electrical power system design can be performed, where the lowest common denominator from operational experience e.g. [1–7] is identified and counteracted. Actual occurrences of degraded power supplies, which all have been “unknown during the event identification process” but “well-known electrical phenomena”, can be described as unidentified degrading conductive disturbances.