With all the hubbub over the reactors in Japan and nuclear safety, I was wondering why the insertion of the control rods in Fukushima did not shut down the reactor? Shouldn't they slow the fission reaction so far that cooling is not needed? Why do they need to continue cooling the core when I would expect no more exothermic reaction to take place?
Answer
Per this article on the subject: http://theenergycollective.com/nathantemple/53384/how-shutdown-and-core-cooling-japanese-reactors-likely-functions
Even with rods inserted, the reactor continues to produce heat equivalent to about 3% of its full power level. This is not the same as taking a pot off the stove and letting it cool. There are still some atoms splitting and fission products decaying that produce heat. This drops off slowly and is why there needs to be layers of redundant cooling with backup power. During such an earthquake, power from outside the plant would not be expected to be available.
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