Plant operator E.On shut down Oskarshamn-3, a unit of Sweden's most powerful nuclear power plant, after an intake port for cooling water became choked with jellyfish swimming in the waters along the Baltic coast.
Oskarshamn-1 had to shut down in August 2005 for the same reason. Plant operators said it takes a "huge amount" of jellyfish to cause a problem like this.
The species of jellyfish causing the problem are called moon jellyfish. They have pale, translucent bodies and live along coastlines.
Oskarshamn-3 was halted on September 8 for maintenance and was scheduled to resume output September 27.
The plant is jointly owned by E.On and Finland-based utility company Fortum.