Like most I've been watching US television for news on the reactor disasters at Fukushima.
The one source, outstanding above all, has been Frank von Hippel, director of the Program for Science and Global Security at Princeton.

Von Hippel was on Rachel Maddow last night and for the show he gave the clearest-for-the-layman explanation of what one of the primary threats is at Daichi.

Any mistakes in this interpretation are mine.

It centers on the spent fuel rods pool at Reactor 4. Like the distressed and failing reactor cores, it needed constant circulative cooling. When it no longer had it because of power failure, the decay heat of the stored fuel rods, no longer dissipated, boiled off or split what water covering remained or that which was added in insufficient volume in the emergency.

As it happened the temperature of the rods rose precipitously, destroying their cladding, blistering and blowing it off. Explosions occured, or remain likely, because of the generation of quantities of hydrogen gas, leftover from the oxidation of the hot zirconium metal fuel cladding.

According to von Hippel there was no easy way to determine the state of the infrastructure and the rods because of intense radiation -- which means some large quantity of radioactive metal was exposed -- and the destruction of sensors and cameras at the site.

When the fuel rods are uncovered by the water mediator/shield and the cladding perforated, burned or destroyed, the heat also drives off the spent fuel's volatile radioisotopes. And that process is the spraying of radioactive waste into the prevailing winds. Unless it's contained by intact walls.

Each Daichi reactor contains between 60 and 80 tons of fuel rod assemblies.

Spent fuel rod pools concentrate exhausted fuel rod assemblies.

Von Hippel said he had heard estimates of anywhere between 2 and 8 reactor cores being present in the spent fuel rod pool in question.

And so the catastrophe, in terms of raw numbers, is rendered quite clearly.

One reader amplifies:

Thirty feet of (borated) water normally absorbs [the radiation]. Boron was necessary ?! to keep neutron reactivity down ... As the water uncovers the rods you get ... [no longer absorbed] spontaneous fission neutrons getting amplified by all that fissile material [which generates more heat.

Anyway, the problem is once the zirconium is burning you now have the worst case ... scenario you can imagine. Magnesium-class pyrotechnics surrounding a stick of nasty. The phrase "whichever way the wind blows" comes to mind...

Yep, 10 reactor's worth of fuel, some quite reactive, all of it full of long lasting isotopes. Cesium volitilizes quite easily being the most extreme in that column...

Additional material on combustible metals and their pyrophoric qualities -- including zirconium and uranium -- is here.

An early edition of this post was published at Dick Destiny blog.

This post has also been updated.