The last thing most people probably think of as a potential cause of a nuclear power plant shutdown is a jellyfish invasion. But that's exactly what brought Sweden's largest nuclear reactor to a halt this week. 

The incident occurred Sunday in Reactor 3 at the Oskarshamn power station, in the Baltic Sea, which is run by OKG, a subsidiary of the E.ON SE (OTCMKTS:EONGY), a German electricity company.

Literally tons of Aurelia aurita, or moon jellyfish, made their way into a cool-water intake of the light-water reactor, which has a 1,400MW output capacity. It took workers until early Tuesday to clear away the sea creatures.

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

Sweden has 10 operating nuclear power plants, which provide 5 percent of the country’s electricity.

Other seaside plants have also suffered from invasions of marine creatures. In April 2012, the Diablo Canyon 2 reactor in California shut down after jellyfish-like salp clogged circulating water screens, according to ABC News.