Nuclear Experts Explain Worst-Case Scenario at Fukushima Power Plant

[jpak]

http://www.scientificamerican.com/article.cfm?id=fukushima-core

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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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[Lefta Dissenter]

1. excellent article

thanks for posting.