A Swedish nuclear reactor was restarted on Wednesday following a three-day closure caused by a build-up of jellyfish in a cooling system, according to the operators.

The incident occured in reactor 3 at Oskarshamn power station on the Baltic Sea coast, which is run by OKG, a subsidiary of the German electricity company EON.

"It was a larger amount than we had ever seen. Every autumn we have to get rid of jellyfish, but not that many," OKG spokeswoman Emmy Davidsson told AFP.

The company announced on Sunday that the reactor -- Sweden's largest with a 1400 MW output and the world's largest boiling water reactor -- was "manually shut down due to a large amount of jellyfish present at the cooling water intake".

The closure did not lead to power outages.

On Wednesday the company said in a statement that the reactor was restarted once the jellyfish had been cleared from the system and the numbers of new arrivals had subsided.

"Furthermore we have reinforced our clearing system to deal with any future jellyfish invasions," wrote OKG.

The influx of jellyfish damaged the reactor's seawater filter mechanism, forcing OKG to replace parts and to clear cages with high pressure water, added spokeswoman Emmy Davidsson.

Similar incidents have occured in other coastal plants such as Torness on Scotland's east coast in 2011.

A number of factors are believed to have increased jellyfish numbers in the Baltic and other seas, including rising pollution levels, warmer waters and less biodiversity.