Recriticality

Reuters has a pretty good recap of events at Fukushima (thanks
Zero Hedge) but I'd like to take exception to one point they make.

The risk that the spent fuel pools could reach recriticality seems remote, as long as there are workers and firefighters willing to douse the reactors with water each time temperatures start to rise.

Criticality has nothing to do with temperature. It has to do with neutron efficiency and moderation. Water is a good moderator and a neutron reflector. Pouring water on a potentially critical mass of uranium/plutonium is the wrong thing to do if you want to do everything possible to prevent recriticality. OTOH it may be a balancing act. i.e. conflicting requirements. They may have to keep things cool enough to prevent further melt down while hoping to avoid a recriticality accident.

I think that they have already had a recriticality accident. From the lack of recent reports I estimate that the mass of junk that went critical has gone subcritical. But there is no certainty that it will remain in that condition.

If the Japanese start reporting neutrons on site (well above background) at any time or measurements of Iodine 134 ten days from now I'd be worried.