Get off the beach! Tsunami coming!

[Karlos]

No, as that reactor is of a different type.

When Chernobyl went out of control, the reaction accelarated making it into something like a little nuke.

Actually, that's not the complete story. Although there was a criticality incident at Chernobyl, it was not directly responsible for the main explosion and resulted only in the reactor core being broken up whilst still in containment.

As you may recall, it was an engineering test that resulted in the disaster. The plant operators wanted to see if they could use the residual inertia in the the steam turbines to provide power to the coolant pumps after an emergency shutdown of the reactor core, since without this, they'd be left without active cooling for up to a minute, whilst alternative power was brought on line. The theory was, the coolant pumps could be powered for about 45 seconds from the slowing turbines.

In the experiment, the reactor was not fully shut down but left in a low-power operational state. However, this state was (in hindsight) not stable and a power surge occurred during the experiment. The first explosion was caused by the sudden heating of the core and ruptured some of the control rod channels, which prevented the control rods from being inserted further, resulting in a loss of control over the reactor. The second explosion, seconds later, is believed to have been caused by the criticality incident you refer to. This secondary explosion wrecked the core and separated the materials sufficiently to extinguish the critical state, but everything was still contained at this moment.

However, the big explosion, which ripped the 2000 ton roof off the reactor and vented it to the atmosphere was purely chemical in nature. Hot steam was able to reach the graphite, wherein it was converted into carbon monoxide and hydrogen. The resulting reaction produces 2 moles of gaseous product for every one mole of gaseous reactant (ie the steam) and this sudden jump in internal pressure was enough to lift the containment plate clean off.

The moment this mixture of hot CO and H2 hit the atmosphere, it ignited, the plume sucking more of the reactor content out into the open.

If hot water/steam were able to come into contact with the reactor core at the Japanese installation, a similar explosion would be entirely possible.

[Karlos]

BTW - If any of you were watching CNN a few minutes ago you would have heard Bill Nye mention that cesium was detected in the material ejected during the explosion at the one nuke plant in Japan.
For those of you who don't know what that means, cesium is the material used in the control rods of a nuclear plant (it absorbs radioactive particles and slows the reaction rate).
For cesium to be found in the vapor expelled in the hydrogen explosion, some part of the control rods has not just melted, but vaporized.
Further, if the cesium is melting, then the reaction rate will increase.

Isn't it cadmium that's used in the control rods? IIRC, caesium is one of the longer-lived fission product from uranium. Caesium wouldn't be used in any part of the reactor (unless as a coolant fluid) since it melts at ~28 C and boils at ~671C, so even if it were used, it wouldn't be indicative of a core meltdown.

Radioisiotopes of caesium tend to come from the decay of more immediate fission products such as xenon and iodine. Detecting it now would indicate that it was something rather more serious than merely radioactive steam (containing eg N-16) that was vented and that radioactive iodine/xenon from the core had been vented into the atmosphere. That would explain why the Japanese government is issuing iodine tablets too.