01 Overview
On July 25, 2006, a short circuit during a switchyard disturbance at the Forsmark-1 nuclear power plant caused the reactor to trip. Two of four backup diesel generators failed to start properly, meaning the plant had only 50% of its designed backup power available. For about 20 minutes, operators could not reliably determine the reactor's safety status.
02 Cause
A short circuit in the external grid caused two of the four redundant emergency power supply units to malfunction. A voltage transient damaged the uninterruptible power supplies that feed control room instrumentation, leaving operators without reliable reactor state information.
03 Impact
The reactor was successfully shut down and stabilised within 23 minutes. No radioactive release occurred. However, the Swedish Radiation Safety Authority concluded that if the situation had continued for another 30 minutes, core damage could have occurred.
04 Response
Plant operators followed emergency procedures and stabilised the reactor using the available backup power. The Swedish nuclear regulator (SSM) investigated and reported to the IAEA. Sweden's other nuclear plants were inspected for similar vulnerabilities.
05 Legacy
The Forsmark incident was a significant wake-up call about nuclear plant electrical system vulnerability. A former chief designer at ABB Atom stated the plant had been closer to disaster than initially admitted. The incident strengthened European discussions on nuclear safety margins.