The shutdown happened automatically after a short circuit in a distribution plant outside the power station, around 130 kilometres north of Stockholm, at around 2pm on Thursday.
According to the Swedish Nuclear Power Inspectorate, two of four diesel generators that should start automatically during a shutdown failed to do so. The generators are supposed to power the station’s safety systems. The two were started manually, a process that took 20 minutes, news agency TT reported.
The failure also meant that a number of computers did not work as they should. Forsmark spokesman Anders Markgren said this did not cause important information to be lost.
“The reactor is now shut down and cooled down until the results of the investigation are known,” a statement on the inspectorate’s site said on Thursday.
The incident has been classed as a level 2 incident on the international INES scale. The INES scale gives nuclear power plant incidents a grade of between 0 and 7, with 7 being the most serious kind of accident. The Chernobyl disaster in 1986 was a level 7 accident. Thursday’s event was the first level 2 incident at the plant.
Anders Markgren said that the power station would be redesigned to make it better able to cope with electrical problems, in consultation with the Swedish Nuclear Power Inspectorate.
He said he didn’t know when the station would be generating electricity again.
“I don’t think it will be running again before the beginning of next week. There’s a lot of work to do, which will need to be approved by the authorities,” he said.