What Does “Nuclear Meltdown” Really Mean?

March 18th, 2011, by

Boiling water reactor design
Boiling water reactor design at Fukushima Daiichi | Photo courtesy: Nuclear Energy Institute

Turn on any television news program and you’ll notice the phrase “nuclear meltdown” being used to help describe the reactor crisis in Japan.

And while it is a dramatic phrase, and one that calls to mind a lot of awful images, after speaking to Jeff Merrifield, a former commissioner for the Nuclear Regulatory Commission, it became clear that “nuclear meltdown” is not a scientific term.

“It’s not a technical term,” Merrifield said by phone. “It’s more of a cultural term.”

Merrifield served as NRC Commissioner from 1998 through 2007, has visited 50 percent of the world’s nuclear plants and has led emergency response efforts dealing with radiation protection. So, I asked him to explain what event this unscientific, non-technical term “nuclear meltdown” is actually referring to.

Here’s what he had to say:

You have the fuel from the reactor. And you need to keep that fuel wet. In the absence of water, the fuel begins to heat up.

The fuel assemblies are made of two components. One is the zirconium cladding, which basically is housing the uranium fuel pellets inside of tubes, and then you have the uranium fuel pellets. The melting temperature of the zirconium cladding is a couple of thousand degrees. The melting point of the uranium inside is about five thousand degrees. So, the spent fuel pools that you’re seeing on TV, if they can’t get more water into those spent fuel pools…then there’s the possibility of the zirconium cladding catching on fire.

This is not going to cause some huge conflagration. What it’s going to do is cause more of a sparking…then the uranium inside, the temperature…would not be sufficient to melt the uranium.

So, you would have the uranium that would slump to the bottom. When that process is happening, you have the release of certain radioactive gases with a very short half-life. Krypton-85 and Xenon-133…You could have the release of Iodine-131, Cesium-137 and Strontium-90.

When the zirconium is melted and the uranium settles at the bottom, there is a potential for “re-criticality,” where basically that material becomes critical, and you could have gamma radiation coming off of that, and that is an issue of concern. If you have that level of gamma radiation, it would make it very difficult if not impossible to get near the reactor.
But is that actually happening in Japan? Merrifield said that we can’t really know for sure.

You don’t want to speculate, but I think it’s fair to say that when you focus on the four reactors that have the most issues, they have a containment system, which is holding the fuel from the reactor itself. There have been some hydrogen leaks that have come from that, but really not anything significant coming from the reactors themselves.

In the spent fuel pools, what we’re likely seeing in some of the boiling off of the water in the spent fuel pools, there may have been some hydrogen gas that was released as well, as a result of that activity…Subject to, perhaps, clarification, it’s not clear that there’s been destruction of the fuel yet.

Last modified: May 25, 2011 at 6:32 pm