The most important lesson of Fukushima No.1 plant, therefore, is that we should have a multiplicity of means to provide a continuous electric supply and heat sinks. This is not the same as “You should not put all the eggs in one basket.” We should have eggs and apples in a few different baskets.
If a country or company wants to operate a nuclear reactor, it should not assume anything about potential disasters – be they earthquakes, tsunamis, terrorist attacks, or a plane crash. No matter what happens, the reactor must be brought to cold shutdown, which requires electricity and heat sinks. It is a pretty simple principle.
There is a more general lesson for all operating nuclear facilities: If you have made assumptions, then you are not prepared.
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All the nuclear reactors in the world have been designed by probability assumptions, originally proposed by Prof. Norman Rasmussen of MIT. It is a scientific way of expressing what the public will accept.
For example, what is the probability of a plane crushing into Yankee Stadium with a full audience during the World Series? This can be calculated with certain assumptions, and, the theory goes, that “level of probability of accidents” is something people tacitly accept because it is very unlikely to happen. The same principle was followed at Fukushima: Assumptions were made about possible causes of nuclear plant accidents, and engineering precautions were made accordingly so that “the reactor is safe.”