As we all know (at least, those of us who aren’t living under a rock) North Korea recently set off their largest nuclear explosion to date. They claimed that it was a full-blown thermonuclear device; some have accepted this statement at face value while others have been dubious. Rather than weighing in on North Korea’s truthfulness, let’s instead briefly discuss the different types of nuclear weapons (fission, fusion, and boosted fission) and then see how we might be able to tell the difference.
When a uranium atoms splits a tremendous amount of energy is released. Breaking a chemical bond (which is what happens to conventional explosives) releases a few electron volts worth of energy; breaking an atomic nucleus releases about a hundred million times more energy. So splitting a single atom releases as much energy as about ten to a hundred million molecules reacting in a block of, say, C4. The trick is getting the atoms to start fissioning, and then keeping them fissioning until you have enough to make an explosion.
As far as initiating the fission process – this is where weapons-grade uranium and plutonium come in. One isotope of uranium (U-235) fissions fairly reading, as does one isotope of plutonium (Pu-239). Although U-235 is fairly rare, we can process uranium to enrich its concentration to the point of sustaining a chain reaction; the plutonium has to be produced in a nuclear reactor. With either of these, we must have enough material to sustain the reaction – this is called the critical mass. Not only that, but it has to be in the correct configuration – called the critical geometry – in order for the reaction to progress.
So – we have a critical mass of fissionable material, but we don’t want to keep it in a critical geometry all the time because we don’t want it to detonate prematurely. Instead, we can keep two sub-critical masses separated, or we can have a critical mass that is in a subcritical configuration. In both cases, at the appropriate time, conventional explosives are set off to achieve the critical geometry. The can be done by slamming two masses into each other, or by squeezing a single mass into the correct size and shape. Either way, once there’s a critical mass AND a critical geometry (oh – and with a small neutron source to get things started) the atoms will start to fission.
Once the fission starts it gets out of control quickly. One neutron causes one fission; that fission releases a few more neutrons, which cause fissions of their own. Each of those fissions releases multiple neutrons, which cause still more fissions…and it grows exponentially, with each new “generation” of fissions occurring in a nanosecond or so, and with each fission releasing a few hundred million electron volts of energy. In less than a microsecond there have already been over 1024 fissions, and the energy released is equivalent to over 10,000 tons of TNT – this is about the size of the device that destroyed Hiroshima. I should also note that the fission stops fairly quickly because all of this energy produces enough heat to vaporize the fissionable material, as well as the explosive force to blow it all apart. As soon as the density of the fissionable material falls too much it’s no longer in a critical configuration and the fission stops.
Pure fission devices can produce yields ranging from less than a kT to a few hundred kT – the largest fission-only device put out about 500 kT (give or take a little). The limiting factor is trying to put together as much fissionable material as possible without having it go off prematurely, as well as keeping it together long enough to get a high fission yield.
As bad as the simple fission bomb sounds, it’s horribly inefficient in its use of fissionable material, and the great majority of uranium or plutonium atoms are wasted – they go unfissioned. This is where a weapons designer can boost the yield without going to a full-blown thermonuclear design.
If you slam two hydrogen atoms together hard enough they’ll fuse together – that’s what power the Sun and it’s what powers the H-bomb. But hydrogen comes in a few different “flavors” – normal hydrogen has only a single proton, deuterium has a proton and a neutron, and tritium (which is also mildly radioactive) has a proton and two neutrons. If the atoms that are being fused are atoms or tritium or deuterium, there are extra neutrons that can be released. A good weapons designer can put these neutrons to use – even if the energy produced by fissioning a little bit of tritium is trivial compared to the nuclear explosion, the neutrons can go on to cause additional fissions in the uranium or plutonium that’s flying away – these extra fissions will significantly boost the yield of the device.
Boosted fission weapons are NOT full-blown thermonuclear weapons – they’re the nuclear equivalent of mixing aluminum powder with an explosive. They give you a bigger bang, but they’re just a minor tweak to the technology rather than a whole new level of science and engineering. Boosting can increase the yield of a fission device by a factor of 10, give or take a little, and can increase the efficiency of the devices by even more.
Fusion (the H-bomb)
Finally, we get to the real mother of all bombs – the thermonuclear device. These are much more difficult to build than a simple fission or boosted fission device, but they are also far more powerful. The basic principle here is that, when hydrogen fuses, it releases about 10% as much energy as a single uranium atom splitting – but hydrogen atoms are a LOT lighter than uranium atoms – only about 1% the mass. So on a kilogram-by-kilogram basis, hydrogen fusion releases about ten times more energy than fission. The catch is that protons are all positively charged – they repel each other – unless you can get them to VERY close distances, and the only way to get them close enough to stick together is by heating them to extraordinarily high temperatures and subjecting them to extraordinarily high pressures. On the surface of the Earth, the only place (so far) that we can create these temperatures and pressures on a large scale is with a fission explosion.
A lot of work went into trying to figure out how to make an effective thermonuclear device, even using a fission bomb to trigger it; the design that finally evolved uses a few different stages to initiate the hydrogen fusion. The first stage is a pure fission explosion; producing a tremendous wave of energy and radiation. The second stage is a combination of plastic and lithium that’s saturated with deuterium and tritium, all surrounding a plug of plutonium. And this is where it gets complicated. Neutrons from the fission bomb will initiate fission in the plutonium “spark plug” and it begins exploding outwards, into the lithium. At the same time, heat and radiation from the fission bomb bombard the plastic, vaporizing it almost instantly – the plastic explodes both inwards and outwards and the inward part of the explosion squeezes the lithium from the outside. So now the lithium and deuterium and tritium is not only being bombarded with energy and radiation, but it’s also being squeezed between the exploding plastic on the outside and the exploding plutonium spark plug on the inside. All of this energy and all of these forces are sufficient to ignite the deuterium and tritium, giving a tremendous explosive yield. And a lot of neutrons.
From here, the weapons designer has some choices – what to do with all those neutrons. If yield is the primary concern, you can surround the bomb with something like depleted uranium – this not only helps to contain the explosion for another few nanoseconds, but it also fissions when it’s hit with a lot of fast neutrons. So adding a third stage (fission-fusion-fission) can give you a very big – and very “dirty” – explosion. Alternately, letting these neutrons fly unencumbered into space reduces the blast, but gives very high radiation levels that can be fatal to the people in the area.
Thermonuclear devices have no theoretical upper limit for their yield, although for practical purposes it just doesn’t make sense to make them too large. In practice, most thermonuclear devices vary from several hundred kT to several MT, and the largest ever set off (the Soviet’s “Tsar Bomba”) was about 50 MT.