UPDATE ON FUKUSHIMA-1 NUCLEAR MELTDOWN

I just re-checked the Union of Concerned Scientists website and they have an excellent explanation which seems to explain why the hydrogen explosions occurred in the upper portions of the reactor buildings' secondary containment structure: click here for report!

There's also an improved graphic that I find useful for understanding the design of the reactor building and its major components.

In summary, when the back-up pumps failed the temperature began to increase within the reactor vessel and reached a point where hydrogen gas was being produced. At a certain point (at an elevated pressure) the engineers attempted to vent the hydrogen gas to the outside. Evidently before they did that the seals above the reactor vessel began to leak hydrogen gas into the secondary containment structure, which subsequently ignited and exploded (think leaky head gasket).

This type of problem was first identified for this kind of reactor design from tests taken back in the 1970's at a nuclear reactor in Brunswick, North Carolina. It's unclear what if anything the Nuclear Regulatory Commission did with these test results. Damn!

Charley Noble