Jellyfish swarm shuts down Sweden’s largest nuclear reactor

Operators at the Oskarshamn nuclear power station in southeastern Sweden are now in the process of restarting the plant's largest reactor, after it had to be shut down after being inundated by a swarm of jellyfish.

It was on Sunday, only two days after the plant's number 3 reactor was scheduled to be brought back online from being down for maintenance, when it had to undergo an emergency shutdown when the pipes that deliver cooling water to the reactor's turbines became clogged by a massive swarm of moon jellyfish.

Moon jellyfish, which can grow up to 40 cm across, are typically found throughout the world's oceans, and are quite common along the east coast of Canada and the United States, and the coastlines of northern Europe. Blooms or swarms of these jellyfish have been known to cause problems for power plants before. Eight years ago, Oskarshamn's number 1 reactor had to be shut down for the same reason, and just last year, the Diablo Canyon power plant in California had to be shut down after their cooling pipes were clogged with gelatinous sea creatures called salps.

The staff of the plant spent the day on Monday very carefully clearing the pipes of Oskarshamn-3, by slowly running the cooling-water pumps to move the jellyfish out of the inlet pond and back into the sea.

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With cooling water now able to make it to the reactor's turbines, the plant will slowly be brought back online and should be running at full power by the end of the week.

Whether this kind of incident will become more common in the future is unknown. Without monitoring programs in place to keep track of jellyfish populations, there's no way to tell if their number are increasing or their habitat is changing.

However, according to Lene Moller, from the Swedish Institute for the Marine Environment: "It’s one of the species that can bloom in extreme areas that are overfished or have bad conditions."

"The moon jelly likes these types of waters," she said, according to CTV News. "They don’t care if there are algae blooms, they don’t care if the oxygen concentration is low. The fish leave and (the moon jelly) can really take over the ecosystem."

(Photo courtesy: Eric Gaillard/Reuters)

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