George Washington’s Blog 
Monday, April 25, 2011
NHK reports :
The operator of the troubled Fukushima Daiichi nuclear power plant is carefully monitoring the situation at the Number 4 spent fuel pool, where the water temperature is rising despite increased injections of cooling water.
Tokyo Electric Power Company, or TEPCO, says it will inject 210 tons of water into the pool on Monday, after finding on Sunday evening that the temperature in the pool had risen to 81 degrees Celsius.
On Friday, TEPCO found that the pool’s temperature had reached 91 degrees, so it began injecting 2 to 3 times the amount of water.
The Number 4 spent fuel pool stores 1,535 fuel rods, the most at the nuclear complex.
(Bear in mind that the amount of radioactive fuel at Fukushima dwarfs Chernobyl .)
As I noted  on April 2nd:
Nuclear engineer Arnie Gundersen notes that the spent fuel rods in reactor number 4 have no water, and the rods are exposed:
These are very high levels of radiation. As I noted  on April 10:
Radiation levels were apparently about 300 sieverts per hour  … right after Chernobyl exploded.
To be clear, the Chernobyl figure is radiation released into the environment, while the reactor 4 figure is radiation within the containment vessel. I have seen no evidence to date that reactor 4 is leaking.
This is especially odd given that reactor 4 was supposedly shut down prior to the earthquake  for maintenance. In other words, reactor 4 was – according to official reports – shut down, and shouldn’t have very much radiation at all. Something doesn’t add up.
In contrast, the radiation inside the cores of the other reactors are much lower:
- Reactor 1: 0 
- Reactor 2: 26.3 
- Reactor 3: 11.1 
- Reactor 5: 0 
- Reactor 6: 0 
Indeed, the Japanese nuclear agency prominently displays the radiation data for all of the reactors except number 4 on it’s main page. Number 4 is conspicuously absent , and you have to type in the url for the correct web page to find it .
The building housing reactor 4 doesn’t seem to be quite as badly damaged as those housing other reactors:
However, a Fukushima engineer says he helped cover up a cracked containment vessel  at reactor number 4 for decades.
On the other hand, the fact that no radiation is being reported in the drywell of reactor 4 (noted by “D/W” in the nuclear agency’s tables) – while there it is for several of the other reactors – might imply that the containment vessel has maintained its stability.
At this point, I don’t have enough information to determine why the radiation levels inside reactor 4 are so high compared to the other reactors, let alone what it means. It might mean that reactor 4 is in trouble. On the other hand, it could mean that reactor number 4 is the only reactor which still has core integrity. In other words, maybe the other reactor cores have much lower radioactive levels because most of the radiation has already leaked out.