Dear Dr. Zoomie – I just heard that North Korea claims to have developed a hydrogen bomb, but our experts say it’s probably just a regular fission bomb – or maybe a boosted device. This is all Greek to me – what’s the difference between these?
Good question! And I know the terminology can be a bit confusing, so let me see if I can help shed some light. But first, some basics.
First, where the energy comes from. Conventional explosives get their energy from breaking chemical bonds – breaking or rearranging chemical bonds releases a few electron volts each (the electron volt is a unit of energy that makes sense on an atomic or molecular level). By comparison, nuclear reactions involve rearranging the nuclear structure of an atom and nuclear reactions release millions of electron volts. So a single nuclear reaction releases as much energy as at least a million chemical reactions.
Next – where the energy comes from in a nuclear reaction. Some atoms are so big that they barely hold themselves together; hit them with a neutron and they’ll split apart, releasing all that energy. They also release additional neutrons, and if those neutrons are absorbed (and cause fissions in) additional nuclei then the reaction will grow, as will the energy release. And since all of this happens in the merest fraction of a second (timescales are on the order of nanoseconds), the power output grows…well…explosively. Fission weapons (using uranium-235 or plutonium-239) make use of this process exclusively. But fission weapons can be horribly inefficient – it’s not uncommon for over 90% of the fissionable material to be blown apart before it can be fissioned. We’ll get back to that in a moment.
Another way of producing nuclear energy is from slamming light atoms together hard enough that they stick, forming a larger atom. This is how the sun makes energy – hydrogen atoms stick together to form helium; three helium atoms can fuse to form carbon, and so forth. But fusion can only happen under extraordinary conditions – specifically the conditions that we see in the center of the sun. In a weapon, these conditions are generated using a fission explosion – a fission bomb goes off, igniting the fusion reaction. Now the question becomes how much fusion takes place and how much energy does it produces.
Most importantly, this fusion also generates neutrons, and these neutrons are vitally important in a boosted weapon. In a boosted weapon, enough fusion fuel is put in the center of the bomb to produce copious numbers of neutrons, but not enough to produce a lot of energy. But these neutrons are crucial because they can be captured by some of that 90% of the fuel that normally is untouched – if you can simply double the number of U-235 or Pu-239 atoms that fission you’ll double the weapon’s yield. So this is a boosted weapon – a “typical” fission bomb with a smidgeon of fusion fuel in the center – but the fusion fuel is there to produce neutrons. You can think of this as the nuclear equivalent of blowing on a fire – you’re not directly adding significant energy to the fire, but you’re helping the existing fuel to burn more efficiently.
Of course, if you put more fusion fuel (this can be a mixture of hydrogen isotopes deuterium and tritium, often combined with lithium to form lithium deuteride or lithium tritide) then you get more energy from fusion – at some point the fusion is not only producing a ton of neutrons, but a significant amount of energy as well. This is where we transition from a boosted fission weapon to an out-and-out thermonuclear (or hydrogen) bomb. And this, too, is where the weapons designers have to decide what to do with all of the fusion neutrons – they can use them to cause still more fission, to produce a great deal of radioactivity (for example, adding stable cobalt to the weapon can produce radioactive cobalt-60), or they can let them escape to make a “neutron bomb” that produces high levels of radiation while sparing the infrastructure. Since fusion doesn’t result in radioactive fission products it’s considered to be fairly clean – especially compared to fission weapons.
Finally, there’s one more point I’d like to address – North Korea’s claims that their weapons have been miniaturized. First, this is potentially important because unless a device can be delivered, it can’t really be considered to be a weapon. The smaller (physically) a weapon is, the more easily it can be used. And to be used on a missile, where every cubic inch and every ounce matters, the smaller a weapon can be made, the better. The problem is that it’s not easy to make a compact nuclear weapon – physics itself places some constraints (you have to have a critical mass of fuel, in addition to the explosives to set it off plus the electronics plus the casing and so forth. You can trim a lot of this somewhat – for example, a boosted weapon will require less uranium to achieve the same yield – but there are limits. It takes a lot of science and engineering to press up against these limits, as well as a lot of testing to make sure that, when you’re pushing the limits of the science, that your weapon will actually work the way you intend. Think of electronics – it’s easy to make something that’s large, but making something tiny can be hard. So North Korea’s claims to have developed miniaturized nuclear weapons is potentially alarming – but also somewhat dubious.
There’s a LOT more discussion we could have here, but to go beyond this level would take up the better part of a book (rather than a blog posting). But if you’re interested in how nuclear weapons work (at an unclassified level) you can go online to the Nuclear Weapons Archive; if you’re interested in the effects, try reading The Effects of Nuclear Weapons – these are all unclassified documents. In addition, Richard Rhodes’ books The Making of the Atomic Bomb and Dark Sun: The Making of the Hydrogen Bomb are outstanding histories of these two projects. In addition, the magazine The Progressive published a piece titled The H-Bomb Secret in their November, 1979 issue; this can still be downloaded at no charge.