While all of the four backup generators started, two of the converters needed for the uninterrupted power supply (UPS) did not work and two of the diesels did not start automatically. Consequently, two of four divisions in the 500 V internal grid were out for approximately 22 minutes before being manually switched, when the main internal grid regained power and the plant could go safely to a safe state.

Analysing the failure, Forsmark owner-operators Vattenfall state that the protection system in the 400 kV switchyard did not work as specified and as a result the magnitude of the electrical transient was higher the expected which affected the two UPS units. Furthermore, the generator frequency protection breakers failed due to a design error.

Despite concluding that operators handled the situation correctly in a preliminary investigation, SKI, the Swedish Nuclear Power Inspectorate, subsequently asked for safety analysis reports from the Ringhals and Oskarshamn plants to try to determine whether the same problem could occur there.

SKI has determined that Forsmark 3, Oskarshamn 3 and Ringhals units 1-4 can continue operation, but further investigations are required before Forsmark 1 can be restarted and as a result of the event Forsmark 2, Oskarshamn 1 and Oskarshamn 2 are currently shut down.

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