Tepco has warned: "The possibility of re-criticality is not zero".

[flamingdem]

Nice nerd explains criticality and re-criticality events
http://anti-proton.com/

article:
http://www.bbc.co.uk/news/mobile/science-environment-12762608

Tokyo Electric warned on this:
Over the last few days there have been reports suggesting water levels were low and the water "boiling"; and now the US Nuclear Regulatory Commission (NRC), which has a team of 11 experts advising in Japan, says the pool is completely dry.

This means the fuel rods are exposed to the air. Without water, they will get much hotter, allowing radioactive material to escape.

More remarkably, the Tokyo Electric Power Company (Tepco), which owns the power station, has warned: "The possibility of re-criticality is not zero".

If you are in any doubt as to what this means, it is that in the company's view, it is possible that enough fissile uranium is present in the cooling pond in enough density to form a critical mass - meaning that a nuclear fission chain reaction could start.

[AndyTiedye]

1. All Bets are Off Once That Stuff Melts

:nuke::nuke::nuke::nuke::nuke::nuke: :hide:

[Wilms]

2. I'd say "admitted".

Nothing announced prior has led me to think we're out of the woods. If we dodge a really major bullet, it may be a while before we get there.

Not good. Not good at all.

[nadinbrzezinski]

3. Good luck. 10-17 over and out

[jimlup]

4. My observation agrees with this post

We are in for a major shit storm before things get better. The media seems to have dropped this story because of the events in Libya but honestly, I think events here are more significant and with reports of smoke coming from 2 and 3 we are still in major trouble. The media is busy dropping the ball yet again.

[flamingdem]

5. No one wants to be on assignment there

Even in Tokyo I bet.. and the experts usually have an agenda and no idea of what is really happening.

I imagine there will be a big cover up of the amount of radiation and lot of apologies down the line.

[jimlup]

6. Agreed! And leaking evidence of contamination seems to back us up....

[TNLib]

8. That and there is no real information coming out

The media sees a bunch of plains and water cannons dumping water on the reactors and think oh well problem solved. The Japanese government and Tepco don't seem to want to tell anyone what is really going on and I'm willing to bet the NRC is keeping a lid on things as well for diplomatic reasons.

[Statistical]

7. Critical mass is not based merely on density.

You mis-characterize what criticality is.

Raw (no enrichment) uranium is critical in a Russian RBMK reactor for example. The same uranium in a light water reactor of fuel pond would do nothing. Likewise a nuclear bomb has fissile uranium at much higher densities than occur in nature or are present in fuel but it will not go critical until a specific set of conditions exists. Ironically one of the requirements for criticality is a moderator. Neutrons emitted by fissioned uranium are too fast. They need to be slowed down. Water is a moderator, and the one used in light water reactors. Water is also present in fuel pond at it is the theoretical (as in not 0%) chance of recriticality. There is nothing else in a fuel pond that could act as a moderator.


Your combining of two fears is unfounded. Without water there is a risk of fuel rods melting and that brings new dangers but it actually reduces the likelihood of criticality.

"it is that in the company's view, it is possible that enough fissile uranium is present in the cooling pond in enough density to form a critical mass"

No that is your mistaken assumption about what criticality means.