What you may not realize is that nobody uses these designs anymore. Atomic bombs are SO fifties. Even as the rubble in Japan was still cooling, physicists were looking for a more powerful, energetic weapon. If fission is good, then adding a little fusion to the mix must be even better. Virtually every nuclear warhead deployed today is a thermonuclear device patterned after the original "Teller - Ulam" device. And good lord, it's the scientific equivalent of a madman's ramblings on full yellow legal tablets. Except, to the best of our knowledge, the damn thing seems to work.
The details? Where to start? Teller Ulam is a multistage weapon. It
starts with a conventional explosive that initiates a fission explosion.
And that's where things get dicey. The fission explosion, even if it is
a 'boosted' fission explosion, isn't enough. So the goal is to use that
initial fission bomb to create a fusion reaction. The challenge is that
from the moment that fission is initiated, you have only milliseconds to
create your fusion reaction before the whole thing is vaporized. You
are essentially creating a complex chain reaction inside an atomic bomb in
the process of detonating.
The idea is that you use a typical first generation atomic bomb (as crudely described above) to focus a blast of X-Rays on a 'secondary' - a target that, under the right conditions will initiate nuclear fusion. You focus the x-rays, you reflect them with a 'tamper' and you enhance them by surrounding the secondary with a fissionable shell. If all goes according to design, for a very brief moment you get nothing short of a miniature star - a hydrogen fusion reaction. Of course, to understand the engineering challenge this represents, you have to go back to the very first step. The whole process was kicked off by detonating an atomic bomb. And everything that happens afterward, the whole complex set of steps and processes, all happens inside the casing of the warhead in the microseconds before that initial atomic blast vaporizes the whole kit, kat and kaboodle.
The part that nobody outside of the nuclear weapons community is clear on is precisely the mechanism by which the secondary is compressed enough to begin a fission reaction. We know that the X Rays from the primary are somehow 'focused' across the 'interstage' section of the weapon, and by some process (there are several competing theories, any or all of which may be in use), those X Rays initiate a fission reaction in the secondary. The most likely method is one where a very secret formula 'foam' surrounds the secondary, and the X Rays from the primary convert the foam into a highly energetic plasma that compresses the secondary, which fissions and compresses the Lithium Deuteride fuel to thermonuclear temperatures, initiating the fusion reaction.
The term 'hydrogen bomb' is a bit of misnomer. It's true that the entire device is designed to produce hydrogen fusion, but most of the energy released is from good old fission. The fusion reaction's primary purpose is to add enough energy to greatly enhance the efficiency and output of the fission reactions. The original bomb designs were less than ten percent efficient - that is, they needed to contain a large amount of fissile material so that getting 10% of it to fission before the whole thing was vaporized would result in a large enough yield. Now, despite the unlikely complexity of the modern thermonuclear device, you get much more energy out of a given amount of fuel than you did before. Which is ironic, because the modern warheads such as the W-88 are designed to have a maximum yield of just a few hundred kilotons, while the earlier weapons often yielded 10 megatons or more.
There's a few lessons to take away from all this. First, even though the first Teller - Ulam device was tested in 1952, and despite it's rather unlikely design, nobody has figured out how to improve on it or replace it. There's also a lesson here about letting the physics and math dictate the engineering. In order to make something work in the real world, you sometimes might have to compromise on the most elegant applied science to get the desired result. But most of all, it's kind of bizarre to consider that human culture could be ended for all time by these goofy, Rube Goldbergian bombs.