E-Mail IMAGE: Pebble-bed reactors use passive natural circulation to cool down, making it theoretically impossible for a core meltdown to occur. view more Credit: Dr. Jean Ragusa and Dr. Mauricio Eduardo Tano Retamales/Texas A&M University Engineering When one of the largest modern earthquakes struck Japan on March 11, 2011, the nuclear reactors at Fukushima-Daiichi automatically shut down, as designed. The emergency systems, which would have helped maintain the necessary cooling of the core, were destroyed by the subsequent tsunami. Because the reactor could no longer cool itself, the core overheated, resulting in a severe nuclear meltdown, the likes of which haven't been seen since the Chernobyl disaster in 1986.