In an episode that evokes B-grade sci-fi movie plots from the 1950s, but actually reflects a continuing global problem, nuclear engineers in southeastern Sweden have been wrestling with a giant swarm of jellyfish that forced the shutdown of the world’s largest boiling-water reactor.

The plant’s operator said that over the weekend, a huge cluster of moon jellyfish clogged the cooling water intake pipes at the Oskarshamn nuclear power plant on the Baltic Sea coast, forcing the complex’s 1,400-megawatt Unit 3 to shut down.

But Anders Osterberg, a spokesman for the operator, Oskarshamns Kraftgrupp AB, said Tuesday that the jellyfish had been cleared and that engineers were preparing to restart the reactor. The outlook was uncertain, he said.

“We hope we have solved the problem regarding the jellyfish, but we are not sure because they can come back,” Mr. Osterberg said by phone. The species is known as the common moon jellyfish, a resilient type that can sting, but it is generally not dangerous to humans.