At the time of the quake, Reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance. The remaining reactors shut down automatically after the earthquake, with emergency generators starting up to run the control electronics and water pumps needed to cool reactors. The plant was protected by a seawall designed to withstand a 5.7 m (19 ft) tsunami but not the 15 m (49 ft) maximum wave which arrived about 47 minutes after the earthquake.TEPCO announced on 8 July 2011, based on the computer analysis, tsunami height was 13.1 m (42.9 ft) at time of impact, 51 minutes after earthquake. This height is the result of pile up by the number of small tsunami at 30 km (18.6 mi) off the coast caused by six faultdestruction.The entire plant was flooded, including low-lying generators and electrical switchgear in reactor basements and external pumps for supplying cooling seawater. The connection to the electrical grid was broken as the Tsunami destroyed the power lines. All power for cooling was lost and reactors started to overheat, owing to natural decay of the fission products created before shutdown. The flooding and earthquake damage hindered external assistance.
In the hours and days that followed, reactors 1, 2 and 3 experienced full meltdown. Hydrogen explosions destroyed the upper cladding of the buildings housing Reactors 1, 3, and 4; an explosion damaged the containment of reactor 2; multiple fires broke out at Reactor 4. With the remnants of its reactor core fallen to the bottom of its damaged reactor vessel, Unit 1 continues to leak cooling water approaching three months after the initial events; similar conditions are hypothesized to exist at the other two melted-down reactors in the complex.