Of all the potential mishaps that can cause a nuclear plant shutdown, from an earthquake to operator error, the one that you might least expect is a swarm of jellyfish gumming up the works. (See related quiz: “What Do You Know About Nuclear Power?“)

But apparently, that’s exactly what happened in Oskarshamn, Sweden, on Sunday, when plant operators had to shut down Sweden’s biggest nuclear reactor after a huge swarm of Aurelia aurita, also known as moon jellyfish, swam into the cooling water inlet and blocked it. The individual creatures range between 2 and 15 inches in diameter.

“This situation is caused by a huge amount of jellyfish,” the plant’s press officer, Anders Österberg, told Bloomberg News in an email.

The number 3 unit at the Oskarshamn Nuclear Power Plant, a 1,400 megawatt unit that supplies about 5 percent of Sweden’s electricity needs, is now in the process of being restarted and will be back at full power this week. “The aim is to slowly start a couple of the cooling water pumps in order to drain the inlet pond of jellyfish and see that they are all distributed back to the sea,” Österberg told Bloomberg.  “When the amount of jellyfish is reduced to an acceptable level, we will be able to restart production.”

While a jellyfish-induced outage might seem like a freak event, the creatures have caused problems numerous times at nuclear plants before. Oskarshamn’s number 1 unit had to be shut down in August 2005 because of a similar outbreak of the aquatic creatures.

Jellyfish have caused plant shutdowns in this country as well, according to Nuclear Regulatory Commission documents. In October 2008, the Diablo Canyon 2 reactor in California had to be shut down, after a massive amount of jellyfish clogged the circulating water screens and caused a problem with water pressure. In August 2011, a “massive intrusion” of Aurelia aurita into a cooling canal at the St. Lucie Nuclear Plant in Florida caused a similarly massive fish kill, clogging the intake pipes with five tons of dead fish in the canal over a 24-hour period and triggering an unplanned shutdown.

Scott Burrell, an NRC spokesman, said an interview Tuesday that such incidents don’t pose a safety threat, but do interfere with the coastal plants’ operations. Normally, heat from the reactors turns water into steam, which then is used to turn the turbines that generate electricity. Afterward, the steam is cooled and converted back into liquid by running it through a device called a condenser, where the heat is drawn off by a flow of cool water from the ocean.

The channels through which that cooling water flows generally have debris screens, which can become clogged by jellyfish. When that cooling water stops flowing through the screens, the turbines can’t run properly and must be shut down. The reactors themselves usually are shut down as well—not because of any danger, but to avoid wasting fuel.

“I don’t know that I could quantify the number of jellyfish” required to shut down a plant, Burrell said. “But it is a situation that occurs from time to time.”

Burrell said there isn’t any way to prevent jellyfish incursions, but he said that the problem usually be fixed by running water in reverse through the system, which flushes out the screens. In some instances, divers may also go into the channels and clean the screens by hand.

The jellyfish-related shutdowns may be one of the consequences of what marine scientists warn is an ominous surge in the global jellyfish population. Thanks to overfishing, the gelatinous creatures have fewer natural predators, and unlike other aquatic species, they’re able to withstand increasing levels of acidity in the oceans, according to Lisa-Ann Gerswhin, an Australian marine researcher who is author of the 2013 book “Stung! On Jellyfish Blooms and the Future of the Oceans.”


  1. homayoun Sabbaghzadeh
    November 6, 2013, 3:22 am

    I solved this problem in near 5 years ago in one of our Non-Nuclear Power Plant and can help this attack to inlet of cooling system of Power Plant . In most cases the design and location of water inlet and outlet position near the sea is wrong . So I am so happy to help if need .

  2. Jim Steele
    Pacifica Ca
    October 7, 2013, 1:48 pm

    The most recent research shows that jellyfish around the world undergo approximate 20-year cycles of boom and bust, associated with Ocean Oscillations. In the Pacific there was a similar boom in jellyfish that clogged both fishermen’s nets and the intake pipes that cooled power plants during the Pacific Ocean Oscillation’s last cool phase. Past blooms raised such concerns that the American government passed the Jellyfish Control Act of 1966 to “control or eliminate” troublesome jellyfish. Dr. Robert H. Condon, Marine Environmental Sciences Consortium recently wrote “The realization that jellyfish populations have been pulsing globally at decadal scales should lead to a broadening of the search for the drivers of change.”

  3. Patty Parfait
    Chula Vista, Ca
    October 2, 2013, 4:11 pm

    let Californians know how that works for you, San Onofre needs dismantling and storage. I understand that all that has been paid during the facility off-line and as consumers see when consumers can expect when the rebate checks maybe will be applied to WHAT account?

  4. Cy
    October 2, 2013, 1:47 pm

    Even jelly fish joined greenpeace!

  5. EliB
    October 2, 2013, 9:29 am

    it has begun

  6. New Yorker
    New York, USA
    October 1, 2013, 4:03 pm

    Nuclear material is creepy–supposedly no one really knows all of the particles emitted from many of the different varities, or really how it works. Jelly fish are not creepy–they really appear as more peaceful floaters…

  7. Louie in Medford
    October 1, 2013, 4:03 pm

    Dateline 2013: Jellyfish in Sweden. Right-o. Global warming must be a myth.