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Posted February 25, 2001

I had a call last week from someone who urgently needed to know the difference between fission and fusion. This can be a touchy subject, so I'll warn you right now that I am not going to tell you how to make a nuclear bomb during the show.


The definition of the word "atom", outside of physics, is something that can't be split. Something that can't be split into parts, because it has no parts. In physics and chemistry, the word "atom" refers to a lump of protons and neutrons and perhaps some nearby electrons. The lump of protons and neutrons is called the nucleus, and it is usually indivisible, and you usually can't make bigger ones out of smaller ones.

Of course, we're here to hear about the unusual cases, which are called nuclear reactions.

There are two kinds of nuclear reactions. Fission, and fusion. Fission is splitting an atom into smaller parts; and fusion is forcing several atoms together into one big atom.

Both kinds of nuclear reactions work by exchanging mass for energy. One unit of mass is worth c2 units of energy. c2 is 10 thousand billion, and what that means is that you can produce a lot of energy if you can sacrifice a small amount of matter.

Fission and fusion are two different ways of coming up with that small amount of mass.

Fusion is the kind of nuclear reaction that happens in the Sun. If you subject a bunch of hydrogen atoms to enough heat and pressure, they will start to collide with each other with such force that their electrical repulsion is not strong enough to keep them apart. When several hydrogens get sufficiently close to each other, they get pulled together by the strong nuclear force. The strong nuclear force is the one that holds the neutrons and protons together in every atomic nucleus. And that's exactly what it does to these hydrogens; it holds them together in a single nucleus, which now looks like a helium nucleus.

It takes a huge amount of energy to make this reaction happen in the first place. You might have to heat up your hydrogen to 40 million degrees before you fuse any hydrogen. But there's a big payoff, because when you do fuse it, you also convert a little bit of mass into energy. So you get a lot of energy. Enough energy to cause a serious problem, if you happen to have more hydrogen sitting around. Because if you've got just enough energy to start a single fusion reaction, and you create a new atom, then the resulting energy will cause a few more fusion reactions. And in a fraction of a second you'll have a hydrogen bomb.

So don't try this at home.

Even if you do have a 40 million degree oven.

Fusion is not for kids.

Fission, now that's a different story. Fission is done with atoms which are unstable to begin with. Certain isotopes of some elements will split readily if you hit them with a neutron. Uranium-235, uranium-233, and plutonium-239 are popular examples. When you hit a uranium-235 atom with a neutron, it might split into a krypton-91, a barium-142, and 3 stray neutrons. You can check the math and find that 235+1 = 91+142+3, so the total number of nucleons is preserved, but more interestingly, you can do the math and find that you have some disappearing mass. The mass, of course, is converted into a large amount of energy. Meanwhile, the three neutrons can go on to collide with other nearby uranium atoms, and start more fission reactions.

There are various other ways that uranium can break down, besides into krypton and barium, and some give off different numbers of neutrons as a side effect, but the average number of neutrons coming out of a uranium-235 fission reaction is 2.4. Which is also the average number of babies per North American family, and that is probably not a coincidence.

Depending on how pure your uranium-235 is, getting 3 neutrons out of each fission reaction can result in a bomb or a power plant. If you're using a big piece of metal, with little bits of uranium-235 here and there, most of your extra neutrons will not end up colliding with one of them. You can fire more neutrons at it if you want more energy to come out, and stop when you have enough.

If, on the other hand, you've got a piece of solid uranium-235, then your extra neutrons will be causing new fission reactions left and right. They'll have the whole thing fissioned into smaller elements in no time, the energy will come out all at once, and you'd better be far far away at that point, if you don't want to be vaporized.

If you're planning to make an atomic bomb, then you're probably wondering a couple of things. The first thing is how to make a hydrogen bomb without the mythical 40 million degree oven, and the second thing is how to make pure uranium-235, since if you've done any digging at all you know that it's only found inside large pieces of unfissionable uranium.

A hydrogen bomb is called a "thermonuclear device" because you have to put in lots of energy before you can start getting any energy out. The only reasonable way to generate enough energy to start a fusion reaction, is by starting an uncontrolled fission reaction right next to it. So if you have a fission bomb, and you want a hundred times as much vaporizing power without having to attract attention by buying a hundred times as much pure uranium, you can use readily available hydrogen to make a fusion bomb, and set it off with a small fission bomb. Simple.

And that highlights the second nagging question, because in order to make an A-bomb or an H-bomb, you need pure uranium. The way you do this is to make something called UF6, and I'm not allowed to tell you how, but it's a gas form of uranium, so you can use something like a centrifuge to separate it based on the difference in atomic weight between radioactive uranium-235 and regular uranium-238.

One last thing; don't throw away that left-over uranium-238, because when your hydrogen bomb enters its fusion stage, it will generate enough high-energy neutrons to cause even boring old uranium-238 to undergo fission. That extra fission can add a lot to your energy output, and it has a special added benefit. The uranium breaks down into a diverse array of smaller elements, and many of these elements are themselves radioactive. These elements are referred to as fallout in your nuclear holocaust survival guide.