ceebs:

Lochbaum, who formerly taught reactor operation for the Nuclear Regulatory Commission, said the pools measured about 40 feet long, 40 feet wide and 45 feet deep. The spent fuel, he added, rested at the pool's bottom and rose no higher than 15 feet from the bottom. That means that in normal operations, the spent fuel is covered by about 30 feet of cooling water.

Assuming the bozos at TEPCO haven't stacked two rows of fuel on top of each other for a total height of 30 feet, one would need 40x40x15 feet of water to fill the space the fuel assemblies occupy (yeah, I know the assemblies take up some volume so less water will be needed to cover them, but let's be conservative and keep the math easy).

That's 24000 cubic feet of water times 27 litres per cubic foot gives 648,000 litres of water, or 648 tonnes of water.

tuasfait:

Whether or not my outburst last night worked, now Army choppers dropped over 21 tons of water on No. 3. The riot squad is preparing to spray water now, at 11 am Tokyo time.

21 tonnes of water is 21000/27 < 800 cubic feet. Since the pool has a 40x40 feet footprint, 800 cubic feet of water adds a half-foot or 15cm to the pool. Fuel assemblies are over 4m high (15 feet)

In addition, according to Gaianne:

At high temperature the oxide of zirconium flakes away and the zirconium keeps burning.  Also, the affinity of zirconium for oxygen is so fierce at high temperatures that zirconium will pull the oxygen out of steam (vaporized water) leaving hydrogen which will burn as soon as it meets more oxygen--which is presumably what fueled the several large explosions of reactor buildings.  

So throwing water onto a burning fuel assembly only makes matters worse.

So, in what may be my last act of "advising", I'll advise you to cut the jargon. -- My old PhD advisor, to me, 26/2/11