Actually, I think the IAEA concluded that a criticality incident occurred at Chernobyl, and that's what ignited the subsequent fires.

Since the hydrogen at Fukushima came from the breakdown of water in the reactor, and the Chernobyl reactor was moderated by graphite, it's not clear where any hydrogen at Chernobyl would have come from. Chernobyl used water to produce steam, but it wouldn't have been in contact with the core until *after* the initial explosion had already occurred, bursting the pipes.

As a side note, this whole conversation has been tossing terms like "explosion" around with wild abandon, and I think at least some of the confusion may result because different people are using them in different ways.

Technically speaking, an explosion is simply the rapid release of energy, from any source. Increase the pressure in a steam boiler beyond its design limits, and it will explode. Controlled explosions are what make internal combustion engines work. So I don't view an "an explosion at Fukushima" as necessarily any more hazardous to human health than "an explosion on the Deepwater Horizon." Suppose, for example, that one of the fuel tanks for the backup generators had blown up: well clear of the reactor vessel, and therefore not in itself a terribly big deal. And yes, I suppose that a fuel tank explosion would technically meet David's claim that a "nuclear plant can blow up," but I think we all know that wasn't what he had in mind.

The scary stuff starts to happen if the rapid release of energy takes place *inside* the reactor vessel. That's what happened at Chernobyl, and that's what shot highly radioactive material all over the place. But there is no evidence that such a situation has occurred -- or could occur -- at Fukushima.

Katherine