Tuesday, March 15, 2011

Why a nuclear reactor can never turn into a nuclear bomb

Edit: Karl Withakay has pointed out a couple of omissions and a small factual error in this post. I have indicated this in the footnotes. Please do read his illuminating comments, especially if you are interested in the more technical aspects of nuclear technology.

The news from the Fukushima Daiichi nuclear plant continues to be horribly depressing. For me, nothing mitigates this kind of discouragement more than being overly technical about it, so let's do that.

As bad as the situation is, there's been lots of really just stupidly over-the-top fear-mongering, and other websites have done a good job addressing that. One thing I've noticed has been omitted, though, is that while there are many resources out there reassuring people that a nuclear explosion is not possible in this case, because a nuclear reactor and a nuclear bomb are two completely different things, I haven't seen anyone offering a lay explanation of why that is the case. So I will attempt to do so here.

Say you've got a lump of Uranium-235, the fuel used in most nuclear reactors. Really, you'd have a lump of rock that contained some amount of U-235, which you would have attempted to purify as much as possible, but this is all beside the point1. This lump of U-235 can be subcritical, critical, or supercritical.

To understand what these terms mean we need to briefly revisit the basics of a nuclear fission reaction. I'll leave the detailed explanation to the Wikipedia article, but for our purposes, the important point is that when a U-235 atom splits into two pieces, along with the energy that is released, it also ejects three stray neutrons. It turns out that what induces the U-235 atom to split in the first place is being struck by a stray neutron. So you fission one atom, which causes three more atoms to split, which in turn trigger nine more fission reactions, then 27, then 81, and so on exponentially until you get a whole heapin' load of energy.

Except not quite. The stray neutrons don't always hit a U-235 atom. Sometimes they miss, and just go shooting off into the distance. Actually, since all the matter around us is mostly empty space, they usually miss.

And here's the key point: The more dense your lump of U-235 is -- the closer together the atoms are -- the more likely it is for an ejected neutron to bump into one of the atoms. I'm sure this makes good intuitive sense, since obviously it's easier to hit one of a whole bunch of targets clustered close together than it is to hit a one of a few targets scattered far apart.

If the odds of a neutron hitting a U-235 atom are less than 1 in 3, i.e. on average, each time an atom splits and ejects three neutrons the average number that go on to trigger another fission reaction is less than one, then we say the mass is subcritical. You will get some energy released, but the reaction will rapidly peter out as you run out of neutrons.

If the odds of a neutron hitting a U-235 atom are exactly 1 in 3, i.e. on average, each time an atom splits and ejects three neutrons an average of one of them connects, then we say the mass is critical. You will get a fairly constant release of energy until all of the fuel is used up. This is how you want to run a nuclear reactor.

If the odds of a neutron hitting a U-235 atom are more than 1 in 3, i.e. on average each split atom causes more than one other atom to split, then your mass is supercritical. All other things being equal (which they aren't; more on this in a second) fission will continue in a chain reaction style, releasing energy faster and faster, until you get a mind-numbingly large explosion.

This is how you want to build your nuclear bomb. But the thing is, just being supercritical isn't enough. Your mass has to be really supercritical to get a bomb of any serious yield. This is because supercriticality tends to be self-limiting. To see why this is, let's examine what (probably) happened with North Korea's unsuccessful atomic bomb test: Fizzle.

The North Korean bomb test managed to create a pretty supercritical mass of U-2352. The chain reaction starts, and, as intended, a really impressive amount of energy is delivered in a really short period of time, causing an explosion. But in that unsuccessful bomb test, the resulting (relatively small) explosion blew the mass of uranium apart before most of the U-235 had a chance to fission. So there was an explosion, but not nearly as big as they were looking for.

It turns out it's really hard to get around this problem. The North Korean engineers had to work pretty damn hard even just to get the result they did. If you tried to just slowly squeeze some U-235 together to make a supercritical mass, you'd never get there, because as soon as you got very slightly supercritical, you'd either burn up enough uranium that you weren't supercritical anymore, or you'd heat it up and the density would go down enough to take you out of the supercritical zone.

And that's what makes it so damn hard to build a nuclear bomb. Your bomb contains some amount of subcritical material, and you need to smash or squeeze it together so that it becomes not just a little supercritical, but hugely amazingly supercritical, and it does it so fast that the bomb doesn't blow itself apart when you are only halfway there. Even successful nuclear bombs of the type described so far have a fairly low percentage yield, which is why engineers have designed all sorts of clever ways to mitigate this problem.

(For a moment, I must digress because I noticed my enthusiasm is showing through here. I find the technology behind nuclear weapons to be absolutely awe-inspiring; it is just such a remarkable feat of pure engineering. But from a human perspective they are also terrible terrible things, and I am confident the world would be better off without them. Just to be clear. My fascination with the technology does not in any way diminish my opposition to the horror that these devices can wreak upon humankind.)

So what does all this have to do with nuclear power plants? Well, as mentioned before, you want the fuel for your nuclear power plant to be right around (actually just under) the critical mass. That means it's not even close to exploding. (The explosions at Fukushima Daiichi were hydrogen combustion explosions, not nuclear explosions, and other blogs have already explained how that happened far better than I ever could)

Even if somehow the fuel got compressed so that it became supercritical, it would rapidly self-correct down to the critical level, by heating, melting, or (if somehow it got really supercritical, which it wouldn't) blowing apart. It's just so damn hard to get uranium to the level where you'd have a legitimate atomic bomb explosion, there's just no way it could possibly happen by accident.

We might have intuitively expected this, since the first artificial nuclear reactor was built by Enrico Fermi and a handful of grad students on a freakin' abandoned tennis court (and in fact it even occurs naturally in at least one place in the world), whereas the first successful nuclear bomb test required scores of the world's top physicists, a massive industrial support operation, and god knows how much money and resources. But it's worth understanding the reasons anyhow.

This doesn't mean that the situation at Fukushima Daiichi couldn't get really bad. The worst case scenario for a nuclear power plant is more akin to a dirty bomb, which is not exactly super happy fun time either. And, I struggle how to say this tactfully, but the real tragedy may be that this torpedoes our last best shot at a politically tenable solution to (at least temporarily) dodge the problem of global warming. It is only a little bit hyperbolic to say that this tsunami may in the end kill billions. More about that in a future post, perhaps.

1Turns out this is not so much "beside the point" as I thought. Karl Withakay tells us that while the fuel used in commercial nuclear reactors is enriched to contain about 3-5% U-235, the minimum purity requirement for a bomb is around 20%, and in practical devices it is much much higher. So not only is it impossible for even highly-enriched uranium to "accidentally" become supercritical enough to create a significant explosion, you couldn't even do it on purpose with the fuel used in nuclear reactors.

2Karl also points out that the North Korean test used plutonium rather than uranium. For the purposes of this explanation, the concept is similar enough to suffice. But please do read Karl's comments, which provide some additional technical background and clarify a few minor errors I made in trying to whip off this post using top-of-my-head knowledge rather than actually doing the research.


  1. FYI: North Korea's nuclear bombs were Plutonium-239 devices.

    I suspect the fissile was a result of "reactor grade" Plutonium with too high a concentration of Pu-240 which has a high rate of spontaneous fission, which causes the fissile by starting the chain reaction before full assembly can be achieved. The assembly speeds or tamping requirements for a weapon made from reactor grade Plutonium with a high concentration of Pu-240 are fairly impractical and so a lower concentration of Pu-240 is needed.

    My theory is supported a little by North Korea's subsequent decision to pursue Uranium enrichment. U Enrichment is time and energy intensive, but is an easier road to weapons grade material Plutonium breeding if you don't have a purpose built breeder reactor that is optimized for weapons grade production in both design and operation.

  2. Ah, thanks for the clarification. On a similar note, I'm not 100% sure I was right when I said, "...Uranium-235, the fuel used in most nuclear reactors." I think that is true, but I really have no idea how prevalent plutonium reactors are. Nor do I have a good feel for the differences.

    Really, I probably should have done some research for this post, but instead this was mostly all just off-the-top-of-my-head knowledge -- and I'm no expert, so there may be some other serious mistakes. I think I got it mostly right, but no guarantees!

  3. A really really well made nuclear bomb is MUCH cleaner than the worst possible disaster with a nuclear reactor! (Radioactively that is). Just sayin'

  4. Bah! Stupid blogger software erased my much longer comment.

    Shorter version: You are exactly right, and that is one reason why the Fukushima Daiichi disaster is really troubling despite the title of this post.

  5. Yes U-235 is the fuel used in most power reactors, especially commercial reactors.

    Natural Uranium is about 1% U-235, the rest being U-238.

    In commercial reactors, the fuel is usually enriched to around 3-5% U-235.

    The minimum enrichment required to make a nuclear weapon is ~20%, but that would very difficult to make into a bomb. The assembly speeds and tamping requirements, etc would be difficult to achieve. Nuclear weapons that use Uranium typically use uranium enriched to at lest 85% U-235. (Little Boy was ~80% u-235)

    Even with highly enriched Uranium or weapons grade Plutonium, it's not easy making a bomb. The shapes of the pit and explosive lens, the tamping, and numerous other design elements are all very critical to a successful bomb design.

    There's really not much practical difference between U and Pu for power generation (there are some significances for weapons design: U -235 has a higher critical mass, and Pu-239 cannot be practically be made into a little boy style bomb), but PU has greater weapons proliferation concerns (because it can be chemically separated and doesn't require difficult enrichment), so it's not really (directly) used commercially. Pu is bred to some degree in any U-235 reactor, which is why fuel reprocessing has weapons proliferation implications as well.

    Your post got most of the basics correct in that the reactor can't turn into a bomb. The main point I would add in support is that the reactor's Uranium is not nearly enriched enough to even be manufactured into a bomb, let alone become one by accident.

  6. "A really really well made nuclear bomb is MUCH cleaner than the worst possible disaster with a nuclear reactor! (Radioactively that is). Just sayin' "

    How so?

    Actually a really, really well made nuclear bomb is very, very dirty. The more fuel that is fissioned, the more highly radioactive daughter products are produced. Weapons are designed to maximize the amount of fuel fissioned. Reactors cannot sustain a supercritical chain reaction long enough to fission a significant portion of their fuel during the accident, and most of the highly radioactive daughter products produced by the reactor before the accident will have already decayed to less active nucleotides.

    Additionally, most thermonuclear bombs (nearly all nuclear weapons deployed today) produce the majority of their yield through the fast fissioning of the secondary tamper made of U-238. This produces a large amount of fission daughter products in the fallout.

    The real problem for a Chernobyl type accident is that nuclear reactors are not nearly as remote as nuclear test sites.

  7. I guess it depends what you mean be "well-made" and what you mean by "dirty".

    As to the first point, my understanding is that smaller nuclear bombs can be constructed so as to have very little fallout. Some people might not call a smaller bomb "well-made", but it depends what your goal is. Is my understanding mistaken here?

    As to the second point, my understanding is that while thermonuclear bombs are very dirty in an absolute sense, the ratio of kaboom-to-fallout is higher than with other bomb designs. More fallout total, but the yield is increased proportionally more than the fallout. Do I have that about right?

    In any case, many thanks for the fact-checking. I'm fascinated by this stuff, but am far from an expert.

  8. "In commercial reactors, the fuel is usually enriched to around 3-5% U-235."

    Oh wow. Yeah, I knew weapons-grade stuff was more highly enriched, but I guess I didn't realize just how much more. I would have definitely mentioned this in my original post if I had known. You can see where I off-handedly referred to purification in the 3rd paragraph, but I figured it was somewhat tangential to the point I was trying to make. Turns out not!

  9. Well, yes, lower yield will mean less fissioned material and fewer daughter products.

    You'd probably be talking about a low yield, pure fission device but nobody really makes such low yield bombs/warheads. Pretty much all current designs are thermonuclear and use Tritium boosting in the primary to increase fission in the primary. The way lower yields are achieved in current designs is by dial a yield by (I believe) varying the amount of Tritium in injected into the hollow primary pit. Less Tritium = fewer nuetrons to help fission the pit in the primary. Low enough yield in the primary can mean no detonation of the secondary and even an intentional fissile. The dirtiest thing you can do on the dial a yield bombs is to dial them up to full yield.

    Another way to reduce fallout is to use an air blast instead of ground blast or lay down detonation.

    Actually, most thermonuclear bombs are very dirty relative to their yield; the higher the yield, the dirtier. Contrary to what most people think, typical thermonuclear bombs/warheads do not produce most of their yield directly from fusion. Although it is possible to produce "cleaner" thermonuclear bombs with non-fissionable lead or tungsten tampers for the secondary, the yields are very low relative to the weight of the weapon, and very few of these types of weapons are made. In most thermonuclear designs, fusion is more of a source of fast neutrons to induce fast fission in the U-238 tamper on the secondary than it is a direct energy source for the yield. (U-238 is not fissile- it cannot fission from thermal neutrons produced by fission events- but it is fissionable and can fission from fast neutrons produced by D-T fusion)

    In regards to a reactor accident vs a typical thermonuclear weapon detonation, what matters is the relative dirtiness of the incident. I could be wrong, but I think I'd rather deal with the radiation from a Chernobyl type accident than a thermonuclear ground detonation at the same distance. Consider also that much of the immediate exclusion zone in Chernobyl would have been destroyed in a thermonuclear blast rather than just irradiated, so there's a bit of an apples and oranges thing for the blast zone comparison. (You can, of course, compare apples and oranges; you just can't equate them.)

  10. Hello from Yokohama-- *excellent* post! If I have to write anything else about the Fukushima reactors, I'm sending everybody straight to you! For a layman you certainly do explain well:-)) (You do in biology, too, over at WEIT--always enjoy your posts there). As soon as I get everybody to bed, I'm re-reading this (a natural reactor?? Had *no* idea! Now I'm hooked...)

  11. Thank you for the kind words! Yeah, when I read about the natural fission reactor in Gabon, it kinda freaked me out :D It makes sense if you think about it, but still... woah...

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