"The tsunami of 11 March was the 'direct cause' of the accident at the Fukushima Daiichi nuclear power plant, concluded an official investigation report. It dismissed the idea that earthquake damage was a major factor in the accident.
A safe emergency shutdown was achieved within seconds of the magnitude 9.0 earthquake, said the Fukushima Nuclear Accident Investigation Commission composed of experts independent of plant owner Tokyo Electric Power Company. Control rods were fully inserted within seconds and all 13 diesel generators started as per design when tremors disconnected the grid connection. Instrumentation was working correctly, as were cooling systems.
Shaking recorded at the site was around the maximum that the plant was designed to cope with and still maintain nuclear safety but walk-down checks by plant staff showed no indication of significant damage to coolant systems.
Within an hour of the earthquake, however, almost the entire site was submerged to a depth of up to nine metres by a series of tsunami waves. Over about ten minutes these flooded six of the diesels and ruined the supporting equipment of another six. Only one diesel unit survived and this was used alternately to maintain essential systems at units 5 and 6 - using one of only three power distribution panels that had not been submerged. Some 36 other distribution panels throughout the emergency diesel generator system were made useless by water.
This situation was noted as the 'direct cause' of the accident. The lack of emergency power and the inability to restore it eventually led to the loss of four reactor units and a significant release of radioactivity."
02 December 2011
by World Nuclear News
Japan always been hit by the earthquake and tsunami, thus the engineers there already consider the probability that they been hit by the tsunami. Not they did not prepares, by the disaster that them to them is far beyond their imagination. I did hear some of the Malaysian to said that the Japanese is stupid enough to built the NPP near to the ocean where it is easy to been hit by the tsunami. What that they didn't see is that the Japanese is prepare that to take the risk and took it in their account for the building of their nation.
Not only that their withstand the shook of the earthquake and been hit by the tsunami. That old NPP also did not receive any damage due to its design. Rather the overheat of the reactor happen because of the malfunction of the backup generator and the power grid that supply the energy to circulate its coolant. after the backup generator is offline, the backup system is dependent in 8-hour batteries(which act as reserve for the basic system to been restore to its basic operation).
This mean the meltdown of the Fukushima Daichi is happen because of the unexpected accident which is outside of its design. The incident may become the guide for the Japanese engineers to take a into account if they want to design a future NPP of their country.
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