2:46PM, 11th March, the great earthquake occurred. Nuclear reactors 1 to 3 made emergency shut-downs successfully, while the other reactors 4 to 6 were not operating because of regular inspection. As the electric feeder lines to Fukushima NPP No.1 were damaged by the earthquake, emergency generators started to power the emergency cooling system. However, the first wave of the tsunami arrived at 3:27PM, and the second wave at 3:35PM, inundating the buildings housing the nuclear reactors and generators with water 4 to 5 m deep. As a consequence, the emergency generators stopped, and reactors 1 to 5 lost electricity for cooling systems.
After 12th March, cooling water levels were lowered in reactors 1 to 3, resulting in damage and partial melting of nuclear fuel rods. To protect the nuclear reactors, pressure was lowered by releasing water vapor out of the reactors. This diffused radioactive matter into the atmosphere. At the same time, as used nuclear fuel rods were stored in pools in the buildings, there was also a need to cool the pools continuously. As of the end of April, 2011, though the situation is becoming stable, unprecedented long-term efforts are necessary to stabilize the rectors.
Obviously, the tsunami triggered the accident. TEPCO (2011) recently reported that the tsunami run-up elevation was 14 to 15 m, whereas the height of the design tsunami was 5.7 m. The ground elevation of the Fukushima NPP No.1 is about 10 m, and the emergency cooling pumps were set in the basements of the buildings. Therefore, the reactors lost the cooling system due to tsunami flooding.
There are other nuclear power plants located on the Pacific coast, such as Onagawa (Miyagi), Fukushima No.2, and Tokai No.2 (Ibaraki). These were also affected by the tsunami, but no critical accident occurred. This is mainly because in each case the elevation of the ground were high enough compared with the tsunami’s run-up. For example, in Fukushima No.2, the run-up height was 6.5 to 7 m, higher than the design height of 5.2 m. However, the major buildings were situated on ground which is 12 m high. The damage from the tsunami was not large (TEPCO, 2011). Significantly, the Onagawa NPP of the Tohoku Electric Power Co. was hit by a 13 m tsunami, but, as the ground level was 13.8 m, it only suffered from a small inundation (Tohoku Power Co. 2011).
As already mentioned, the Pacific coast of the northeast Japan has a history of damage caused by big tsunamis such as Meiji Sanriku Tsunami in 1896, Showa Sanriku Tsunami in 1933 and Chilean Earthquake Tsunami in 1960. Moreover, recent research has found evidence that a giant tsunami called the Jyogan Earthquake Tsunami occurred in 869 (Minoura and Nakaya, 1991). It is estimated that the earthquake’s magnitude was about M8, and that the tsunami penetrated 2 to 4 km inland (Sugawara et al. 2001; Satake et al. 2008). Similar wide inundation areas might have happened 1200 years ago. Considering that accumulation of stresses between the Pacific and the North American plates generates big earthquakes periodically, such big earthquakes and tsunami can occur every several hundreds to a thousand years. Based on such findings, the Headquarters for Earthquake Research Promotion plans to investigate the big earthquakes and tsunamis off the Pacific coasts. Unfortunately, such a major event happened before such scientific findings could be incorporated into practical policies, plans and actions.