[little jeremiah]

http://english.kyodonews.jp/news/2011/03/78318.html

At 6:14 a.m., a blast occurred at the No. 4 reactor and created two square-shaped holes about 8 by 8 meters in the walls of the building that houses the reactor. At 9:38 a.m., a fire broke out there and smoke billowed from the holes.

The utility said it could not deny the possibility that the early morning explosion was caused by hydrogen generated by a chemical reaction involving the exposed spent nuclear fuel and vapor

[brityank]

I noted this from the link at Kyodo:

The agency said among the three, the situation is the severest at the No. 4 reactor because all the fuel rods are stored in the pool due to the change of the reactor's shroud. At the No. 5 and No. 6 reactors, up to one-third of the rods are being kept in the pools. The more fuel rods that are kept in a pool, the more radioactive substances could be emitted.

I'm not certain of how the "packages" are constructed that hold the tubes of fissile pellets, but groupings of 9, 27, and 81 tubes per package are vaguely important. I am also given to understand that the spent fuel pool has to be kept running in cool-down mode continually, as the packages will heat back up if left uncovered too long. Plus it looks as if 2/3rds of the packages are very 'hot', having been relocated from an active reactor. Once cooled, they can be handled outside of the pool, but if left outside of continual water cooling will slowly start to heat back up from their own radioactivity and groupings.

If so, then the loss of water in the pool could be disastrous and put extreme heat on the outside of the reactor, which was not built for it in that manner. That should be a serious concern for all of the plants we have here in light of the refusal of the government to honour its commitment for a Yucca Mountain type repository.