Core Meltdown Nuclear Experts Explain Worst-Case Scenario

 at Fukushima Power Plant

The type of accident occurring now in Japan derives from a loss of offsite 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 |http://www.scientificamerican.com/article.cfm?id=fukushima-core

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/

First came the earthquake, centered just off the east coast of Japan, near Honshu. The 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 1:30pm EST on March 12, Americannuclear experts gathered for a call-in media briefing. While 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 Laboratory in New Mexico. "And the type of accident that is occurring in Japan is known as a station blackout. It means loss of offsite 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 offsite power and of onsite 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], 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.

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