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.