A very good article by Bela Liptak about the back up systems at Fukushima mentions the following:
"The earthquake destroyed the electric power supply of the plant (the connection to the grid) which by itself should not have been a serious problem, because backup diesel generators (18) were provided. It seems they failed because they were not elevated and the 18-ft waves of the tsunami reached and damaged them. The reason for their being installed at low elevation was probably both convenience and concern for their stability. The destruction of these generators could have occurred because water entered the diesel fuel tanks and sank to the bottom because water is heavier than the diesel fuel. As the engine takes its fuel supply from the bottom of the tanks, water instead of oil reached it. It is also possible that the air intakes of the engines were not elevated and ended up under water. If either or both of these conditions existed, the engine could not operate.
The secondary battery backup (19) was of no use either because it was drastically undersized. It provided only about eight hours worth of electricity, while about ten times that would have been needed to supply the electricity needed for a safe shutdown. (It should be noted here that of the 104 American reactors, 93 are provided with only four-hour battery backups). Another problem in the Fukushima plant was the lack of automatic battery recharging. This could have been provided because the plant was still generating steam at a rate of about 5% of full capacity and, therefore, some of the turbine-generators could have been kept in operation.
No other backup was provided at the Fukushima plant. This is unfortunate, because electricity itself is not essential to cool the reactors. For example, if emergency cooling water tanks were provided on the roof, would have made it possible to charge water just by gravity, and if those tanks were properly sized, the accident could have been prevented."
Read the full article in this link
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