Q: What is going on in a nuclear reactor, and what happens during a meltdown?

Physicist: Nuclear reactors are very 19th century in a way.
The nuclear fuel is basically a bunch of very hot metal, and the more of it you get together in one place, the hotter it gets.  That heat is used to turn water into steam, which turns turbines.

Uranium and plutonium (nuclear fuel) undergo radioactive decay on their own, at random.  This is the base radioactive rate.  But some isotopes can also be made to decay by hitting them with an extra neutron (it’s called being “fissile“).  As luck would have it, when uranium (and plutonium) decays it generates a spray of neutrons, and can induce nearby material to decay as a result.

So, you can “throttle” how much heat the fuel rods (bars of nuclear fuel) produce by shielding or not shielding them from each other: the more they’re exposed to each other, the more they make each other react, and the more heat they make.  So you can never “turn them off”.  Each fuel rod will continue to produce heat, even on it’s own (just less).  We’re used to thinking of things cooling off on their own, but that’s exactly what nuclear fuel doesn’t do.

In a reactor, the average number of other atoms set off by each decaying atom is less than one.  This means that the rate of radioactive decay levels out at a relatively low level.  I mean, you wouldn’t want to be in the room, but it’s not going to blow up either.  The base radioactive rate (the random decays) keeps the rate from dropping all the way to zero.

However, if the average is more than one, then the rate of decay will keep increasing exponentially.  This is called a “run away nuclear chain reaction” or, in the more common vernacular; “a bomb”.  The amount of material you need to bring together to produce this effect is called “critical mass”.

In an emergency, the rods are generally dropped into slots so that they can’t interact (this didn’t happen at Chernobyl, and resulted in a runaway chain  reaction).  However even without critical mass, if there’s a failure in the cooling system then the rods, and all the supporting material, will eventually melt. Depending on the types of materials used you can also get nasty chemical effects, like the coolant water being broken down into hydrogen and oxygen (this is one of the things that happened at 3 Mile Island).
There isn’t really a maximum temperature that the nuclear fuel can reach.  Or at least, it’s really, really high.  It’ll just keeps heating up until it can spread out, somehow.

And that’s a meltdown.

Some of the worst case scenarios are: the fuel melting through the floor of the power plant and getting into the water table, melting and pooling together at critical mass, or getting so hot that it vaporizes.  There are a lot of safety precautions in place to keep this stuff from happening.

It would be bad if all of the fuel in a nuclear plant vaporized (mostly to nearby people).  But, to put it in perspective, it’s much, much worse to leave coal plants running normally.  It has been estimated that in 1982 alone more than 10,000 tons of Uranium and Thorium (both nasty) was released into the air by coal powered generators world wide.  There are a lot more coal plants open these days, but I can’t find the exact data.

Not to go off on a tangent; but the world would be much better off with a Chernobyl sized disaster every month, than it is with the amount coal pollution we produce today.

For those of you (not presently living in Japan) worried about radiation exposure, just spend an extra minute or two not in direct sunlight.  That should more than make up for it.

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10 Responses to Q: What is going on in a nuclear reactor, and what happens during a meltdown?

  1. John says:

    Why exactly is a meltdown bad?
    Why is water splitting up in hydrogen and oxygen bad?
    Lately, the news seems to focus on the scaryness of these big bad plants, but they don’t bother to explain why exactly we should be scared of the effects.

  2. The Physicist The Physicist says:

    It’s bad when the radioactive material gets out.
    The hydrogen build up is a problem because hydrogen has a way of exploding suddenly (as oppose to just getting hot).
    The danger of a meltdown is that it releases radioactive stuff into the surrounding area, but it’s a very local danger.

  3. Lewis Mead says:

    I’m glad you stated how much more dangerous coal plants are, the media seems to have a vendetta against all things nuclear.

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  5. Scott says:

    Radiation is scary because it’s generally undetectable, and related to bombs and cancer. Nuclear plants and meltdowns are scary because there is a very small, but very real danger of major catastrophe. Probably similar to the way that our brains are programmed to hold on to losing stocks much longer than we hold on to winning ones, people seem to be much more comfortable with a low level of statistical mayhem than we are with a remote chance of catastrophe, even if that catastrophe is less harmful in total than the mayhem.

  6. tonyf says:

    Actually the absolutely dominat heat production after the nuclear reactor is switched off by the inserting the control rods fully is from the decay of the fission products (if the reactor has before that been running for a while so a lot of fission products has been produced). That is e.g. what causes the problems in the Fukushima reactors. The heat from the residual radioactivity of the uranium fuel is negligible in comparison.

  7. lemony says:

    You could look at Japan as proof that, so far, the statistics don’t show nuclear to be that deadly, considering that the earthquake and tsunami were so catastrophic as to kill upwards of 12,000 people and yet no one yet died from the nuclear reactors. Even Chernobyl only killed a couple hundred people, and that was without containment.

    For coal statistics you can see the Energy Information Administration website, http://www.eia.gov/coal/ . Somewhere on the site there are also data tables on coal use internationally, coal imports and exports, estimates of emissions, and a million other things.

  8. JRS says:

    Two quibbles:

    “Critical mass” is not the mass needed to produce “a bomb”, it’s the (smallest) mass needed to produce a self-sustaining reaction. Criticality very specifically means for every one atom that fissions, one additional atom (no more!) is caused to fission – a sustained, controlled burn; not an explosion. What you’re describing as a “runaway chain reaction” is a *super*criticality, not criticality.

    Quibble the second: in all the plant designs I’m aware of, you don’t “insert fuel rods into slots to keep them from interacting”, you insert *control* rods into slots in the core; the control rods (which are not fissile) absorb free neutrons, thus greatly lowering reactivity in the core. Fuel rods are an immobile part of the core itself; they’re bundled together into fuel “elements” which are never moved for anything but complete replacement once spent.

  9. Ron says:

    Sure, nuclear is relatively safe yet sometimes causes problems. Sure, coal is fairly safe yet still causes problems…

    Solar, however, is ALWAYS safe! It is even economical too, if we are willing to let economies of scales work in our favor by abandoning the idea that we can escape from the energy grid. Photo-voltaics are expensive, however solar troughs are not.

  10. Daniel Moreno says:

    Thank you for putting fossil fuels into perspective. I work in Japan and many of my coworkers are always surprised, somehow repeatedly(I blame the Japanese hive mentality), that burning fossil fuels releases immense amounts of “natural” radiation. What are your thoughts on the Experimental Molten Salt Reactor at Oak Ridge National Laboratory?

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