Nuclear Experts Explain Worst-Case Scenario at Fukushima Power Plant

[BOILING-WATER REACTOR SYSTEM: The system's inverted lightbulb primary containment vents below through pipes to a pressure-suppression torus. Once that torus breaches due to overpressure, the secondary containment is all that separates the released radioactive steam from the outside environment.
Image: http://www.nucleartourist.com/]

The type of accident occurring now in Japan derives from a loss of off-site AC power and then a subsequent failure of emergency power on-site. Engineers there are racing to restore AC power to prevent a core meltdown

By Steve Mirsky | March 12, 2011 | Scientific American Magazine

First came the earthquake, centered just off Japan's east coast, near Honshu. The added horror of the tsunami quickly followed. Now the world waits as emergency crews attempt to stop a core meltdown from occurring at the Fukushima Daichi nuclear reactor, already the site of an explosion of the reactor's housing structure.

At 1:30 P.M. Eastern Standard Time on March 12, American nuclear experts gathered for a call-in media briefing. Whereas various participants discussed the policy ramifications of the crisis, physicist Ken Bergeron provided most of the information regarding the actual damage to the reactor.

"Reactor analysts like to categorize potential reactor accidents into groups," said Bergeron, who did research on nuclear reactor accident simulation at Sandia National Laboratories in New Mexico. "And the type of accident that is occurring in Japan is known as a station blackout. It means loss of off-site AC power—power lines are down—and then a subsequent failure of emergency power on-site—the diesel generators. It is considered to be extremely unlikely, but the station blackout has been one of the great concerns for decades.

"The probability of this occurring is hard to calculate, primarily because of the possibility of what are called common-cause accidents, where the loss of off-site power and of on-site power are caused by the same thing. In this case it was the earthquake and tsunami. So we're in uncharted territory, we're in a land where probability says we shouldn't be. And we're hoping that all of the barriers to release of radioactivity will not fail."

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 in Pennsylvania], 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."

Bergeron discussed the specific power plant in question, the General Electric design BWR Mark 1. "This is a boiling-water reactor. It's one of the first designs ever developed for commercial reactors in this country, and it's widely used in Japan as well. Compared to other reactors, if you look at NRC studies, according to calculations, it has a relatively low core-damage frequency. (That means the likelihood that portions of the fuel will melt.) And in part, that's because it has a larger variety of ways to get water into the core. So they have a lot of options, and they're using them now—using these steam-driven turbines, for example. There's no electricity required to run these steam-driven turbines. But they still need battery electricity to operate the valves and the controls.

"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 [nine to 12 meters] 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 [do]. 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."

Finally, Bergeron summed up the events so far: "Based on what we understand, the reactor has been shut down, in the sense that all of the control rods have been inserted—which means there's no longer a nuclear reaction. But what you have to worry about is the decay heat that's still in the core—that will last for many days.

"And to keep that decay heat of the uranium from melting the core, you have to keep water on it. And the conventional sources of water, 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. And I'm sure they're working on it."