Is a criticality event possible in the Japan nuclear crisis?

[sadasiva]

I keep hearing how it is not as serious as Chernobyl and many other conclusions.. but I see a siituation that keeps getting worse.. 4 reactors in danger of melting down, exploding fission materials.

What is it going to take to get it under control?

With most workers sent home due to exposure fears and others with firehoses trying to keep the reactors cool and rods cool.. how will this crisis resolve.. ?

How will they get a handle on it?

I mean to say, will the material eventually be burned off? DO they need to get power restored so they can shut something off?.. or something?

 

[promecheng]

http://nei.cachefly.net/newsandevents/information-on-the-japanese-earthquake-and-reactors-in-that-region/

Latest update from NEI link above:
"Seventy percent of the fuel rods Unit 1 and one-third in Unit 2 have been damaged, TEPCO said. The cooling water level in both units is being maintained."

Maintaining cooling water, assuming it's in the reactor, is good news. I don't see an update on Unit 3 however.

 

[sadasiva]

Is there a scenario where the power comes back on and some "normal" operations can resume and get this under control or is it going to be men with fire hoses until the fuel is spent (or something).

 

[Azael]

It is very hard to speculate since all information is filtered through media and both TEPCO and the Japanese authorities are quite silent.

Restoring power to the site would most probably be very beneficial, if the plants own pumps are still in operational condition after the explosions at reactor 1 and 3(they should be since they are below the concrete structure) then restoring power would give the operators much higher capacity to cool the cores. If they can get higher coolant flow into the core they are more likely to prevent the core from melting its way through the pressure vessel.

It won't be anywhere close to normal operation, but it would be much preferable to the current situation where they are using fire pumps to inject seawater into the core. I have no knowledge on the power etc of those pumps compared to for instance the low pressure injection system.

 

[PietKuip]

http://nei.cachefly.net/newsandevents/information-on-the-japanese-earthquake-and-reactors-in-that-region/

Latest update from NEI link above:
"Seventy percent of the fuel rods Unit 1 and one-third in Unit 2 have been damaged, TEPCO said. The cooling water level in both units is being maintained."

Maintaining cooling water, assuming it's in the reactor, is good news. I don't see an update on Unit 3 however.

Now http://www.bbc.co.uk/news/science-environment-12762608" that the stored fuel in reactor 4 is completely dry. Radiation levels are very high.
According to TEPCO, there is some risk of recriticality.

 

[minerva]

According to TEPCO, there is some risk of recriticality.

Used fuel pool water is usually loaded with boron to keep it subcritical. Furthermore, if there is some kind of fuel-pool LOCA, you will have even less reactivity in the system because you've taken away the moderator, just like in a light water reactor.

So, to me, criticality seems quite unlikely, especially if the water is lost.

I think to get a criticality you would need to lose the pool water, then re-fill the water without adding more boron (plausible in an emergency LOCA situation?) and then disrupt the geometry, putting all the fuel too close together.

It doesn't seem very plausible. Would anyone else out there with a more solid knowledge of nuclear engineering than me like to comment?