DOE source - Multiple tests have been run with spent fuel cooling turned off. Temperatures do not reach boiling, not even close. Evaporation rates go up, and that requires daily makeup of gallons of water, not thousands of gallons.
UPDATE: At Unit 4 on March 14 at approximately 8:38 p.m. EDT, a fire was reported in the reactor building. It is believed to have been from a lube oil leak in a system that drives recirculation water pumps. Fire fighting efforts extinguished the fire. The roof of the reactor building was damaged. [The fire does not seem to have been initiated by the spent fuel - at least not directly.]
World nuclear news - There was a fire at the unit 4's cooling pond and it was put out
Fire at unit 4, concern for fuel ponds
Prime minister Naoto Kan confirmed a fire burning at unit 4, which, according to all official sources, had never been a safety concern since the earthquake. This reactor was closed for periodic inspections when the earthquake and tsunami hit, therefore did not undergo a rapid and sudden shutdown. It was of course violently shaken and subject to the tsunami.
Kan's spokesman Noriyuki Shikata said that there had been "a sign of leakage" while firefighters were at work, "but we have found out the fuel is not causing the fire." The fire is now reported extinguished.
The International Atomic Energy Agency did confirm that the fire had taken place in the used fuel storage pool. The Japan Atomic Industry Forum's status report said the water was being supplied to make up for low levels.
Similar to the need to cool fuel in the reactor core, used fuel assemblies in cooling ponds require a covering of water to remove decay heat. The main differences being the amount of decay heat to be removed decreases exponentially with time and that fuel ponds are much less of an enclosed space than a reactor vessel. At the same time, ponds may contain several years of fuel.
JAIF reported that temperatures in the cooling ponds at units 5 and 6 are increasing, but the reason for this is not yet available.
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