Tuesday, March 15, 2011

Japan Nuclear Crisis: What Is a Nuclear Meltdown?

by Josh Dzieza

What’s the difference between a “partial” and “full” meltdown?
Brenner: Both phrases are not technical phrases. What they're to do with is the radioactive core of the nuclear reactor, which needs to be covered with water to keep it cool. What one means by meltdown is that at some point the core isn’t covered by water. It could mean that a few inches are uncovered for a few seconds, or that the entire core is uncovered. The phrase covers a multitude of sins.
Ballinger: In that context they're talking about fuel that's been damaged and partially melted. Some of the fuel has probably been oxidized and breached and melted at the top of the core where the heat rises. The core height is about 4 meters, so the top meter of the core has probably been damaged.

And a full meltdown?
Ballinger: If they don't cool the plant, if they're not successful... then eventually the entire core would melt. Then it would melt into the bottom of the vessel. Then you get to this theoretical point where if they can’t cool it, then eventually the vessel itself, the steel, would melt, and you’d end up with a bunch of melted fuel and steel on the bottom of the concrete faceplate of the plant, in the containment vessel. And then it would have to get out of there. That’s what I would call a full meltdown....
Are meltdowns necessarily dangerous?
Brenner: They’re certainly not good. You can contrast the two major nuclear incidents of the past: Both Chernobyl and Three Mile Island were meltdowns, but the difference in scale is enormous. Chernobyl was the equivalent of 1 million Three Mile Islands. A “meltdown” certainly is not a good thing, but the ultimate consequence is how much radioactivity is released into the environment. You can have a situation like Three Mile Island, where it’s extremely small amount, or a situation like Chernobyl.
Which does Fukushima look like?
Certainly looks much more like a Three Mile Island. There are a lot of similarities between this and Three Mile Island. In both cases they were able to shut the reactor down almost immediately. That was not the case in Chernobyl. The whole point was that they couldn’t shut the reactor down. In Three Mile Island and in the Japanese reactors, they shut it down.
Once you shut it down, there’s still a low-level reaction going, so you have to keep water covering the fuel. What happened in Three Mile Island and Japan is that they couldn’t do that. The secondary cooling system that pumps water over the core failed.
All of the Daiichi reactors shut down automatically when the earthquake struck. The problem is that it fission reactions don’t just stop; they fade slowly, continuing to produce energy and tremendous heat for days. Normally a cooling system would run water over the core after it shut down, but that system lost power, first when the power station was cut off from the grid, then again when the tsunami swamped the backup diesel generators.
Now that the cooling system has failed, what happens?
The core is going to get hotter and hotter. The nuclear material is enclosed in a metal cylinder, zirconium, which can react with water at high temperatures and produce hydrogen, which is explosive in the right situation. So when you start to get buildups of hydrogen, you have to vent it. But when you vent it, you also vent the radioactive material in the air inside the container. That’s probably where the radioactivity detected comes from.
Ballinger: There are two vectors going on. There’s the decay heat generated by the fission products in the fuel, and that heat has to be removed. If they can’t remove the heat, then the thing heats up. But the decay heat rate is decreasing with time, because the radioactive fission products are decaying away, at the same time you’re having to remove the heat. So the amount of heat you have to remove is decreasing with time, so the amount of cooling they need is going to decrease with time.
The other source of vector is the reaction between the zirconium and water. The zirconium alloy will react with water to produce hydrogen and oxide, but it also produces heat that has to be removed. So one source of heat—the decay of the fission products—is decreasing with time, and the other is a function of temperature, so you decrease the temperature, you decrease the oxygenation rate. It’s like baking a cake. If you set the oven at 300 degrees it’ll cook in a hour. If you set it for 350 degrees it will cook in 20 minutes. So as they cool the plant down, the rate of oxygenation will also go down. And it’s not a linear function. For every 50 degrees Centigrade, you change the chemical reaction rate by a factor of two.
Are there any signs that indicate how successful they’re being in cooling the reactor?
You can get an idea of how successful that is by looking at how often they have to vent the gas—the non-condensable gasses, the hydrogen and stuff. That’s going down and down and down. So they’re having success at cooling. It doesn’t mean there’s not a lot of fuel damage, it just means the oxygenation rates are going down, so they’re having success at cooling it.
There were explosions at the No. 2 and 3 reactors when they vented them. Why do they keep exploding? And what can they do to prevent an explosion?
The trick when you’re venting is to make sure you have a lot of dilution, to make sure you don’t have a hydrogen concentration above 5 or 6 percent. So I’m sure what they’re doing is they’re venting it slower and using a lot of blowers to make sure the concentration doesn’t get that high. Hydrogen is a funny gas. It tends to pool. It’s lighter than air, so it rises, and in a building—think of where the fans are, they’re in the ceiling, well that’s where the sparks from the motors are. Hydrogen will tend to rise and pool in the ceiling area, so the hydrogen concentration could be less than flammable on average, but in certain areas if you’re not careful it can get above the flammable point. They either vented too fast or didn’t realize it was concentrating.

I’ve read that there’s spent fuel stored near the reactor. Is it common practice to store fuel on site?
Yes, there are two places where they put spent fuel. When they take it out of the reactor it’s still generating heat. The decay heat is still there. So they put it in pools full of water. After a long enough period of time they can take the fuel and put it in these monstrous cement casks that you could fire a missile at and nothing happens, and they put them out on a pad and it’s cooled by natural convection.
Officials have expanded the evacuation radius around the stricken reactors. First it was 3 kilometers, then when they vented the reactor it was expanded to 10 kilometers. When the reactor exploded, it went up to 20. Then the Nuclear Regulatory Commission in the United States said radiation was unlikely to reach the West Coast in harmful amounts.
Is the amount of radiation emitted when they vent the reactor dangerous?
Brenner: Depends on how much comes out. From the point of view of the surrounding population, probably not. But the situation is still ongoing, so we don’t know. There’s one good thing in this terrible situation: Winds are offshore at the moment, blowing what radioactivity is in the air out into the ocean.
Could the wind blow the radiation to North America?
Brenner: Yes, but the question is how much. The Chernobyl accident was far larger than we can imagine this one to be. You could detect the radioactivity worldwide. But it’s a matter of how much radioactivity would arrive at the West Coast. Right now it’s absolutely negligible. And even in a worst-case scenario, it’s hard to imagine it would be significant. It’s hard to imagine a significant exposure to anyone on the West Coast simply because of the distance involved: As the wind blows the plume further, it gets more and more dispersed. The worst case still wouldn’t be Chernobyl.
Would it be dangerous to people nearer the reactor? They’ve evacuated people within a 20-kilometer radius.
Brenner: It’s not an unreasonable precaution. In any scenario, the dose will be less and less as you get further from the source. But it will certainly be closer to Three Mile Island than Chernobyl.
How long will the radiation last?
Brenner: It depends on the isotope. Iodine has a half-life of a week. Cesium will be around for years. But the consequences depend on how much is released. Even if cesium is around for a long time, if there’s not much of it, it won’t be an issue. And it depends on which direction the wind is blowing, and again, that’s favorable right now.
Josh Dzieza is an editorial assistant at The Daily Beast.

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