The largest nuclear reactor of it's kind, Oskarshamn nuclear plant in southeastern Sweden, had to shut down a nuclear reactor after a horde of jellyfish started clogging up it's system. Oskarshamn uses Baltic Sea water to cool down it's turbines and the invading jellyfish made their way into the cooling pipes, forcing operators to shutdown to clean the system out. Had they not shut it down, the reactor would have shut itself down because of low coolant supply.
This problem has occurred before, examples can be found in California last year and in Israel two years ago.
Lene Moller, a researcher at the Swedish Institute for the Marine Environment, indicates that this is a problem that is likely to occur more often and states "There seems to be more and more of these extreme cases of blooming jellyfish.