Why didn't the control rods in Fukushima shut down the reactor? 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?
 A: 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.

A: Using the Unit 1 electric power rating of 460MW and the 3% figure above give us ~14MW at scram, that is still a lot of power!  So every second 14 million Joules is released(1 Watt = 1 J/S). Now it takes ~4200 joules to heat 1 kilogram of water by 1 degree C. So 14 million joules would heat 1 kg of water to ~3300 degrees C in 1s. You can see why we have a problem! 
Can someone check my math, it has been a few years since physics class
A: In a nuclear power plant, it is designed so that, if the power fails, the rods will slip into place (and in fact, the problem in Chernobyl was that they didn't). The control rods DID slip into place - but the amount of damage caused to the plant was such that other cooling systems began to fail. The problem was not an ongoing chain reaction, but rather the inability to keep cool the reactors. As a result, water began to evaporate off, which meant even more radioactivity was given off.
In short, the control rods weren't the issue. Nuclear power plants have such complex safety structures, and quite right. Just turns out that Fukushima Daiichi wasn't prepared for this disaster. But then, what can you do? It's impossible to prevent all outcomes.
