As of 9 am Tuesday morning, March 15, it appears that the spent fuel
pool
in Unit #4 of the Fukushima Dai-ichi reactor complexhas 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 mnuch greater and much quicker than from the
core of the
reactor itself.

The spent fuel bays of the other reactors are equally vulnerable.

Gordon Edwards ccnr@web.ca