I'm a layman, so be patient with me.
Your statement above is that an either/or conditions or you need BOTH quantities and enrichment levels?
If it's an either or, I know they've mentioned one of the reactors has a stockpile of used rods.
Would it be possible for that reactor to go super-critical?
Think of it as needing density - the original atom bombs worked by increasing material density - either by imploding the material and achieving density under pressure, or by mechanically assembling to sufficient density by bringing two parts (designed for the purpose) together.
The worst situation there in Fukushima would seem for the material to melt, coalesce and pool. Once that happens, it's a big problem - heat will be intense, poison emissions into the ground and air will go up and that blob will be hard to stop from flowing.
That's often likened to a lava flow, but that's not correct - lava doesn't generate its own heat, so that blob will stay hot.
Once you're there, you go for what Kaku described as the Chernobyl Option - encase it in a sarcophagus, monitor it for years and sign up for sarcophagus maintenance. By the way - the Chernobyl sarcophagus isn't so great. The idea behind a sarcophagus is to encase it until such time that technology finds a way to deal with it.
Whether measures that drastic will be called for remain to be seen.
Homan13PSU - please freely cross-check and criticize anything I've gotten wrong.
