It happened in Israel and Japan!
"A nuclear reactor in Japan was forced to shut down due to infiltration of enormous swarms of jellyfish near the power plant. A similar incident was also reported recently in Israel, when millions of jellyfish clogged the sea-water cooling system of a power plant."
http://hardware.slashdot.org/story/11/07/10/0234250/Millions-of-Jellyfish-Invade-Nuclear-Reactors
"A nuclear reactor in Japan was forced to shut down due to infiltration of enormous swarms of jellyfish near the power plant. A similar incident was also reported recently in Israel, when millions of jellyfish clogged the sea-water cooling system of a power plant."
http://hardware.slashdot.org/story/11/07/10/0234250/Millions-of-Jellyfish-Invade-Nuclear-Reactors