The March 11 , 2011 , Tōhoku earthquake and tsunami caused various nuclear accidents , the worst of which was the Fukushima Daiichi nuclear disaster . At an estimated eighty minutes after the tsunami strike , the temperatures inside Unit 1 of the Fukushima Daiichi Nuclear Power Plant reached over 2,300 ˚C , causing the fuel assembly structures , control rods and nuclear fuel to melt and form corium . ( The physical nature of the damaged fuel has not been fully determined but it is assumed to have become molten . ) The reactor core isolation cooling system ( RCIC ) was successfully activated for Unit 3 ; however , the Unit 3 RCIC subsequently failed , and at about 09:00 on March 13 , the nuclear fuel had melted into corium . Unit 2 retained RCIC functions slightly longer and corium is not believed to have started to pool on the reactor floor until around 18:00 on March 14 . TEPCO believes the fuel assembly fell out of the pressure vessel to the floor of the primary containment vessel , and that it has found fuel debris on the floor of the primary containment vessel
tsunami strike
2011 Tōhoku earthquake and tsunami
Corium (nuclear reactor)