spent fuel pools

Add this to your reading list. A new potential issue in Japan has arisen base on the spent fuel pools. To be brief, after fuel rods are spent they are transferred to the spent fuel pool to further cool. To do this the rods are arranged in a specific manner and cooled to avoid going critical. Critical in the fission sense, not the atomic bomb sense.

The water does a number of things:

1. It shields the radiation from the spent fuel from harming anyone.
2. The water is recirculated to keep the fuel rods in a cooling mode

There are some reports that the pool in #2 and/or #4 are damaged (leaking water) or that they were neglected and the water boiled off (partially or otherwise).  There's no verification of this however.  Ultimately this could be a much bigger issue than the core reactor because recooling them could be difficult (too hot).  These pools contain a lot of spent fuel rods and if they heat up they can melt.  If they melt they'll sink to the bottom of the pool and be in such a dense, close packed configuration that they could go critical and start generating heat again.  This would be bad.  But as of yet I don't see any confirmation other than some reports that #4 had a fire that some are suggesting was started by the spent fuel rods igniting and/or producing hydrogen and detonating.