TOKYO Jellyfish sounded the retreat on Friday after blocking an all-important seawater cooling pipe at a western Japanese nuclear power plant, the plant operator said.
Several Japanese utilities are struggling to meet peak summer demand due to safety concerns after a March 11 earthquake and tsunami triggered a nuclear crisis at Tokyo Electric Power Co's plant in Fukushima, north of Tokyo.
The jellyfish managed to block the cooling system at one reactor at the Shimane plant on Thursday, prompting the operator to lower its generation capacity by 6 percent.
The Hiroshima-based operator, Chugoku Electric Power Co, said on Friday the jellyfish, common in Japanese waters during the summer, had backed off and operation was back to normal.
It was the first time jellyfish, about 20-30 cm long (8-12 inches), had interrupted operations at the Shimane plant since 1997, a company spokesman said.
The March tsunami crippled the Fukushima cooling systems, leading to fuel rod meltdowns.
The nuclear disaster has prompted the government the rethink its nuclear energy policy "from scratch" as routine maintenance and public concern leave only 19 of Japan's 54 reactors functioning.
Not resuming operations at the reactors would lead to power shortages around Japan, while replacing all energy generated by nuclear reactors with thermal power generators would raise costs for industry and consumers.
(Reporting by Osamu Tsukimori Writing by Nick Macfie; Editing by Alex Richardson)