Arney: "Maybe someone noted this earlier, but I just realized that the spent fuel cooling ponds in reactors 4,5,6 may not actually be holding "spent fuel" as in depeleted fuel rods. Reactors 4,5,6 were already shut down and emptied for inspections when the quake hit, not for refueling, so to me, that that raises the possibility that those fuel rods were intended to be reloaded into the core and are still hot enough to restart fission (outside the protection of a containment vessel!) in the worst case scenario. Or maybe they are being inspected at a natural refueling point, I don't know. "

Arney ... I've got a co-worker who used to be a nuclear engineer, and actually he did some of the analysis of the reactor core that came out of Three Mile Island. I had a fairly long talk with him this morning. A few things came out of that discussion:

1. The general consensus amongst nuclear engineers who reviewed the Three Mile Island incident is that you don't want fuel rods to get uncovered by water (doesn't matter what kind, reactor rods or spent fuel rods). And IF it does happen, you want to get water back on them as soon as possible. So this leads to the conclusion that the Japanese need to get a lot of seawater back onto those exposed rods as fast as possible. This is not to say that the process will be straightforward, or without a lot of risks and complications, just that it's probably the best option going forwards.

2. When fuel rods get hot (as in several hundred degrees), they can start developing exothermic chemical reactions if water or oxygen is present. These reactions are "oxidation" reactions. But they create extra heat. So it's possible that some of the events from Japan that have been called "on fire" in the news reports are really oxidation reactions taking place in the fuel rods. Both the zirconium and the uranium in the rods can oxidize and liberate heat.

cheers,
other Pete