The explosions @ the Japanese reactors have been subject to an increasing amount of analysis. Here’s Arnie Gunderson at Fairewinds Associates:

Gunderson suggests that a hydrogen explosion in the service area of unit 3 distorted fuel stored in the spent fuel pool to such a degree that a prompt criticality took place.

What is a prompt criticality, you ask?

I am glad you asked because here is an answer:

The SL-1, or Stationary Low-Power Reactor Number One, was a United States Army experimental nuclear power reactor which underwent a steam explosion and meltdown on January 3, 1961, killing its three operators. The direct cause was the improper withdrawal of the central control rod, responsible for absorbing neutrons in the poorly-designed reactor core. The event is the only known fatal reactor accident in the United States. The accident released about 80 curies (3.0 TBq) of I-131, which was not considered significant due to its location in a remote desert of Idaho. About 1,100 curies (41 TBq) of fission products were released into the atmosphere.

Fissioning Uranium 235 releases neutrons instantly when atoms divide, these ‘prompt’ neutrons cause knock-on reactions that are amplified by the accompanying increase in heat generated by the reactions. The positive feedback loop can cause an explosion.

When the central control rod in SL-1 was removed, fission reactions took place much faster than the heat generated by the fission could be removed by water boiling within the reactor core. The reactor ran away and the core blew up, scattering bits of itself everywhere and shutting down the reactions. All of this took place within a small fraction of a second.

The three operators were killed by the blast.

A similar criticality took place @ Chernobyl reactor unit 4. Both criticality incidents involved large steam explosions and fuel melting.

Did a criticality incident take place in the fuel pool (or in the core or below the core?) Nobody knows for sure but the release of more information regarding isotopes found on the reactor site will be revealing.

Meanwhile, TEPCO is trying to decide whether unit one containment is leaking:

TEPCO checks to see if water is leaking from No. 1 reactor container

The operator of the crippled Fukushima Daiichi nuclear complex on Tuesday started checking to see if water is leaking from the No. 1 reactor container ahead of work to flood the vessel with water as a step to stably cool the troubled reactor.

Tokyo Electric Power Co. said that remote-controlled robots did not find ”notable water leakage” during Tuesday’s observation at the No. 1 reactor building, and it plans to continue checking by increasing the amount of water currently being injected into the reactor and measuring the change of reactor pressure and other factors.

Of course it’s leaking, all the reactors @ Fukushima Dai- ichi are leaking. The entire site is a mess that gets worse with every passing day. The more the reactors leak the more water must be pumped into them, the more radiation carried by the leaks, the increased difficulty in doing ‘repairs’ in areas flooded by water.

The water becomes more radioactive with more water added daily. The water storage areas become too full to hold any more so ‘mildly contaminated’ water is then pumped into the ocean so that even more contaminated water can take its place. ‘Round and ’round the nonsense goes, a point is reached where the water is too radioactive to approach with more and more being pumped into the leaking reactors.

Then what, sports fans?

No more water and the cores start melting and more ‘prompt criticality’ incidents.

Good grief!