The Swedish Forsmark 1 nuclear reactor shut down automatically on 25 July 2006 due to a short circuit that cut off the plant’s external electric power supply. An anomaly in an electric power supply system (inverter/rectifier) prevented two of the four emergency generator units starting up during the incident. The generator units are responsible for taking over the external electric power supply and this failure caused the reactor safeguard systems to come on line. The defective generator units could nevertheless be restarted manually 23 minutes later. The reactor is currently shut down and the incident has had no adverse environmental impact.
The Swedish Safety Authority (SKI) carried out an inspection on the site and its first analysis concluded that the operators had dealt with the incident correctly.
The exact causes of the incident have not yet been fully explained. It could be linked to a design fault in the inverter/rectifier assembly which has already shown up in Germany. SKI has concluded that the fault on this system was a common-mode fault* and has classified the incident as an INES Level 2 event.
Whilst awaiting the conclusions of the investigations now in progress, the Swedish Safety Authority has decided to shut down Swedish reactors of identical design and to advise the Finnish authorities operating a reactor with the same type of inverter/rectifier on their territory.
Forsmark 1 is a boiling water reactor, unlike the pressurised water reactors installed in France. In addition, the inverter/rectifier systems in the French reactors are of different manufacture.
Ten nuclear power plants produce electricity in Sweden, distributed over three sites: three at Forsmark, three at Oskarshamn and four at Ringhals. Seven are boiling water reactors and three (at Ringhals) are pressurised water reactors. These ten reactors produce approximately half the country’s electrical power.
IRSN is examining this incident carefully and will produce an in-depth analysis in conjunction with SKI in particular as soon as more detailed information is available. It will draw all possible lessons for the French facilities.
* fault that may occur simultaneously on redundant equipment.