asahi.com（朝日新聞社）：No. 4 reactor poses more danger than other reactors
Asahi Shimbun with an update on the situation at reactor four with the spent fuel cooling pool. What’s bothering me is that spent fuel rods generally are not usually in need of this much cooling - as is being stated here. I am wondering if they had partially defueled the reactor in advance of their maintenance, and moved relatively unused fuel rods to the spent fuel pool.
An explosion and fire at the pool containing spent fuel rods at the No. 4 reactor of the quake-stricken Fukushima No. 1 nuclear power plant Tuesday could prove more serious than explosions at two other reactors.
The fire at the pool could potentially pave the way for another explosion that could destroy the building that houses the reactor and release a large amount of highly radioactive materials into the atmosphere.
At the No. 4 reactor, the pool that contains spent fuel rods is located outside the nuclear pressure vessel and the containment vessel.
The only thing that separates the pool from the outside world is the concrete building that houses the reactor.
The spent fuel rods used at the reactor were stored in the pool, which sits next to the reactor.
The spent fuel rods require constant cooling with water because the residual heat is so high it is capable of producing the equivalent of tons of steam per hour.
The supply of water stopped after batteries used to run the water pumps ran out, apparently leaving the remaining water in the pool to evaporate due to heat.
As a result, the spent fuels were exposed and generated hydrogen from the alloy covering the spent fuel rods.
The hydrogen reacted with oxygen, causing the explosion.
To reduce the danger, water needs to be quickly pumped into the pool to cool the fuel rods.
The No. 4 reactor had been shut down before the quake for safety checks.
That would make it easier for workers to fix the cooling problem than the No. 1 and No.3 reactors.
But a highly radioactive substance is believed to have already been discharged into the atmosphere after the fire damaged the building.
More here - With a pool temp of 85 degrees celsius or 185 degrees Fahrenheit you can see where the water went over the past four days, and the pool turns out to be on the fifth floor…
UPDATE: I get a needed correction in a comment from Merkin (see below,) basically spent fuel rods generate more heat than new ones do when they are out of the reactor. Thanks for helping.