As of 9 am Tuesday morning, March 15, it appears that the spent fuel pool
in Unit #4 of the Fukushima Dai-ichi reactor complex has probably caught fire due to a dryout condition caused by a failure of cooling to the spent fuel bay.
This can lead the zirconium cladding to catch fire in the moist environment, reacting with the steam to produce zirconium oxide and hydrogen gas. (In fact this is the source of most of the hyrdogen gas that exploded in units 1 and 3. It is quite possible that the even more damaging explosion in unit 2 was a steam explosion caused by the sudden contact between some molten fuel and the water that was being used to try to cool the core.)
Back to the spent fuel bays. Not only does the burning of the zirconium clad allow the release of the fission products that are in the “gap” between the fuel pellets and the cladding (gases and vapours) but the intense heat of this exothermic reaction (think of burning magnesium) raises the temperature of the irradiated fuel so that 10 to 100 percent of other volatile fission products such cesium can be released.
To make matters worse, the spent fuel pool is not within the primary containment envelope and is located at an elevated position higher than the core of the reactor. Thus when the secondary containment is damaged by hydrogen gas explosions the spent fuel bays are then open to the atmosphere. Thus the potential releases are much greater and much quicker than from the core of the reactor itself.
The spent fuel bays of the other reactors are equally vulnerable.