The incident, which the agency said at no time presented a danger to the public, occurred at the Oskarshamn 3 reactor in southeastern Sweden, which has since been shut down. SKI has not yet given the green light for it to re-start.

On the international INES scale of nuclear accidents that ranges from 1 (anomaly) to 7 (major accident), SKI ranked the September 23 incident as a 1.

However, a report in Swedish daily Sydsvenska Dagbladet said SKI had in a preliminary report sharply criticized Oskarshamn 3 management for its handling of the incident over which employees did not have full control.

SKI would only say that it was currently investigating whether staff and management acted correctly and whether the reactor was damaged during the blackout, in which a chain of events led to a thermal shock in the reactor's tank.

"When operators began to restart the reactor after the quick stoppage and loss of electricity, an unusual but not unknown situation occurred, where 265 degree Celcius (509 degree Fahrenheit) water was in most of the reactor tank and much colder water, 135 degrees (275 degrees Fahrenheit), was in the bottom of the tank," SKI said in a statement.

"During the restart process, the reactor tank's pumps quickly pulled the hot water down toward the cold bottom of the reactor tank. The limit for how quickly temperature changes can occur was thereby exceeded," it said.

The reactor was subsequently stopped, but the situation in the control room during the incident "was stressed", SKI said.

SKI is now investigating whether operators should have refrained from restarting the reactor given the vast temperature difference, which can damage the reactor.

The September 23 blackout, which left four million people in Sweden and Denmark without power for several hours, was caused by valve failure at the Oskarshamn reactor which shut down the power station.

Another nuclear station which should have taken over the power supply, was also shut down following the breakdown of its link to the electricity grid.

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