Headline版 - 日本核反應堆部分融化 語焉不詳 What's a full meltdown? （轉載） |
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发帖数: 92 | 1 Japan Nuclear Crisis: What Is a Full Meltdown?
Tuesday March 15, 2011 2:50 PM - Leave a Comment
Japan is on the brink of a nuclear disaster in the wake of its
devastating earthquake and tsunami, with a third explosion at the
Fukushima Daiichi nuclear power station damaging the steel containment
structure of one reactor, and a fire at another spewing radioactive
material into the air. Before the latest explosion and fire, as workers
raced to stay ahead of a full meltdown, The Daily Beast spoke with Ron
Ballinger, professor of nuclear science and engineering at MIT, and
David Brenner, director of the Columbia University Center for
Radiological Research, about the difference between partial and full
meltdowns, hydrogen blasts, and windblown radiation.
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.
Cooling
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.
Radiation
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.
{The Daily Beast/Matzav.com Newscenter} |
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