FYI, Reuters energy correspondent is answering questions on their blog:
Japan earthquake | Page 83 | Liveblog live blogging | Reuters.com
@REUTERS, thanks for being willing to answer questions Scott DiSavino. My question is what everyone I know is asking: Is the largest threat an uncontrolled fire on the spent fuel rods? And what would the results be if such a situation happens?
Hi, Thanks for your question - energy experts say the two biggest threats are the further melt down of Unit 2 at Daiichi and the spent fuel pool at Unit 4
if the reactor core at Unit 2 melts through the vessel, it would fall into the primary containment - which has a crack in it BUT most of the radiation should still be contained in the primary containment even with that crack
as for the spent fuel pool - TEPCO needs to keep the water in the pool over the rods or they will set on fire and release radiation, including cesium, strontium and plutonium into the environment