Bergeron explained the basics of overheating at a nuclear fission plant. "The fuel rods are long uranium rods clad in a [zirconium alloy casing]. They're held in a cylindrical-shaped array. And the water covers all of that. If the water descends below the level of the fuel, then the temperature starts going up and the cladding bursts, releasing a lot of fission products. And eventually the core just starts slumping and melting. Quite a bit of this happened in TMI [Three Mile Island], but the pressure vessel did not fail."
Former U.S. Nuclear Regulatory Commission (NRC) member Peter Bradford added, "The other thing that happens is that the cladding, which is just the outside of the tube, at a high enough temperature interacts with the water. It's essentially a high-speed rusting, where the zirconium becomes zirconium oxide and the hydrogen is set free. And hydrogen at the right concentration in an atmosphere is either flammable or explosive."
"Hydrogen combustion would not occur necessarily in the containment building," Bergeron pointed out, "which is inert—it doesn't have any oxygen—but they have had to vent the containment, because this pressure is building up from all this steam. And so the hydrogen is being vented with the steam and it's entering some area, some building, where there is oxygen, and that's where the explosion took place."
..."So there's some advantages to the BWR in terms of severe accidents. But one of the disadvantages is that the containment structure is a lightbulb-shaped steel shell that's only about 30 or 40 feet across—thick steel, but relatively small compared to large, dry containments like TMI. And it doesn't provide as much of an extra layer of defense from reactor accidents as containments like TMI. So there is a great deal of concern that, if the core does melt, the containment will not be able to survive. And if the containment doesn't survive, we have a worst-case situation."
And just what is that worst-case scenario? "They're venting in order to keep the containment vessel from failing. But if a core melts, it will slump to the bottom of the reactor vessel, probably melt through the reactor vessel onto the containment floor. It's likely to spread as a molten pool—like lava—to the edge of the steel shell, and melt through. That would result in a containment failure in a matter of less than a day. It's good that it's got a better containment system than Chernobyl, but it's not as strong as most of the reactors in this country."
...the electricity that provides the power for pumps, have failed. So they are using some very unusual methods of getting water into the core, they're using steam-driven turbines—they're operating off of the steam generated by the reactor itself.
"But even that system requires electricity in the form of batteries. And the batteries aren't designed to last this long, so they have failed by now. So we don't know exactly how they're getting water to the core, or if they're getting enough water to the core. We believe, because of the release of cesium, that the core has been exposed above the water level, at least for a portion of time, and has overheated. What we really need to know is how long can they keep that water flowing. And it needs to be days to keep the core from melting.
"The containment, I believe, is still intact. But if the core does melt, that insult will probably not be sustained, and the containment vessel will fail. All this, if it were to occur, would take a matter of days. What's crucial is restoring AC power. They've got to get AC power back to the plant to be able to control it.