How does a hydrogen bomb work?

[TechnicalK3rbal]

Just what the title says. It seems like it would be nuclear fusion, but I've heard that that hasn't been done yet.

[TheGatesofLogic]

a hydrogen bomb IS fusion, however it is not pure fusion (we have not achieved true breakeven with pure fusion yet). The fusion of hydrogen atoms in a 'hydrogen-bomb' is triggered by the heat and pressure waves of a fission explosion usually using the "gadget" model of nuclear exllosion devices. The gadget model uses plutonium or uranium (typically higher yield with plutonium) in a sphere with a dimpled polonium activator in the cenfer surrounded by specially shaped and bilayered explosive charges which compress the plutonium to supercritical levels. In a hydrogen bomb this gadget is shrrounded by a thin separating layer, then a highly pressurized layer of hydrogen, typically enriched with deuterium, then a strong containing wall and bombshell. When the plutonium explodes the incredible energy waves are used as fuel to trigger the much more energetic fusion process.

[Aghanim]

No way. As far as I know H-bomb uses Teller-Ulam design: http://en.wikipedia.org/wiki/Thermonuclear_weapon

The fission primary uses implosion design, as gun type is too inefficient

And no one have invented pure fusion nuke, meaning H-bomb without its fission primary, and its (un)fortunate depending on your point of view

[TechnicalK3rbal]

So the heat & pressure of the fission explosion causes fusion? So it's basically just a fission bomb made more powerful?

[brooklyn666]

So the heat & pressure of the fission explosion causes fusion? So it's basically just a fission bomb made more powerful?

The fission acts as the primary, which sets of the secondary fusion reaction. And we're talking orders of magnitude more powerful. The most powerful pure fission device was 500 kilotons, which is about the upper limit for pure fission devices. The most powerful Fusion device was ever tested was 50 megatons, but there is no theoretical upper limit for a fusion bomb.

[Brotoro]

Take a fission bomb capable of knocking over a small city.

Use the x-ray output of that "primary" (redirected in clever ways) to compress and heat deuterium/tritium/lithium (the lithium makes more tritium when bombarded with neutrons) so you get fusion reactions.

Extra oomph can be had if you use the high-speed neutrons from the fusion reactions to fission U238. A lot of extra oomph in many H-bomb designs.

The result knocks over a large city.

Do not do this at home, kiddies.

[travis575757]

A hydrogen bomb works by bombarding lithium 6 isotope to create tritium and detritum then under incredible force/energy it causes fusion to take place. The bomb is made to allow enough time for the reaction to take place. Though fusion happens it is not really a "fusion" bomb because of the fission that also takes place.

[DonLorenzo]

A fun thing to note is that the final breakthrough in the creation of the H bomb had nothing to do with (nuclear) physics. It was the development of the computer.

People here have noted that the fission primary 'ignites' the fusion through 'clever shapes'. The principle of that had been well theorized, but physicists couldn't quite work out how to make them exactly. Remember we're talking about a structure that directs kilotonnes worth of energy in a very specific way on milli- if not nanosecond timescales WHILE IT'S VAPORIZING. It took the computational power of (I think) the ENIAC to work that out.

[Sillychris]

A fission bomb's efficiency is dependent on lots of neutrons zipping around.

Using deuterium (heavy hydrogen), you can boost the neutron flux by inducing fusion.

That fusion costs energy, but you gain more energy from the boosted fission reaction.

That is how a "hydrogen" bomb works.

[KvickFlygarn87]

A fun thing to note is that the final breakthrough in the creation of the H bomb had nothing to do with (nuclear) physics. It was the development of the computer.

^{[citation needed]}

[DonLorenzo]

Ok, alright. Bit of a snarky way to ask for more reading material, but go ahead

read this: http://en.wikipedia.org/wiki/ENIAC

and/or this: http://scholar.lib.vt.edu/ejournals/SPT/v3n3/FITZPATR.html

Take it from there, they've got more references

[SargeRho]

Sillychris, a hydrogen bomb works by inducing fusion by compressing fusionable material (deuterium and tritium) to fusion-conditions, the energy from the fission-part of the bomb is actually very very small compared to the fusion part of the total energy released. Not by using fusion to boost the fission reaction.

And only I think something like 35mg of deuterium and tritium actually fusioned in the Tsar Bomba.

[Winter Man]

Ok, alright. Bit of a snarky way to ask for more reading material, but go ahead

read this: http://en.wikipedia.org/wiki/ENIAC

and/or this: http://scholar.lib.vt.edu/ejournals/SPT/v3n3/FITZPATR.html

Take it from there, they've got more references

It comes from http://xkcd.com/285/ , now people do it to look clever because xkcd is clever so logic.

But yeah, that was a damn cool computer and pretty much the entire reason we look into fusion at all.

Just what the title says. It seems like it would be nuclear fusion, but I've heard that that hasn't been done yet.

Pure fusion has been done in the lab, the issue is it currently takes more energy to create than we get out of it. If you're not worried about containing it and using its energy, it's relatively easy.

Edited February 27, 2014 by Winter Man

[wasmic]

Sillychris, a hydrogen bomb works by inducing fusion by compressing fusionable material (deuterium and tritium) to fusion-conditions, the energy from the fission-part of the bomb is actually very very small compared to the fusion part of the total energy released. Not by using fusion to boost the fission reaction.

And only I think something like 35mg of deuterium and tritium actually fusioned in the Tsar Bomba.

Actually, the fusion process /does/ initiate a second fission reaction, at least in some bombs. Plutonium fission -> hydrogen fusion -> uranium fission.

[Aghanim]

Actually, the fusion process /does/ initiate a second fission reaction, at least in some bombs. Plutonium fission -> hydrogen fusion -> uranium fission.

More specifically, the uranium-238 at the tamper that is used to push the fusion fuel inside

Uranium-238 usually doesn't do fission, but because fusion reactions create a huge amount of energetic neutrons (fast neutrons) so they split and create huge amount of energy

However, it also creates huge amount of fallout, so Tsar Bomba replaces it with lead, so that 50 MT bomb explosion energy comes from 97% fusion power

[travis575757]

H bombs also do not conduct fusion the same way stars do. Stars fuse with just hydrogen 1 atoms. In h bombs tritium and detritum are used. Fusion this way is done in nature though in brown dwarfs

[Aghanim]

H bombs also do not conduct fusion the same way stars do. Stars fuse with just hydrogen 1 atoms. In h bombs tritium and detritum are used. Fusion this way is done in nature though in brown dwarfs

Because the first time man successfully do breakeven DT fusion is in nuke, not in a peaceful fusion reactor. Why? Even DT fusion is really hard. You need to accelerate the atoms and make them hit together, and hope both of them reach close enough that their electrostatic repulsion is overcome by nuclear force. And we are only talking about 2 atoms, deuterium atoms and tritium atoms.

In hydrogen fusion, we need to fuse FOUR atoms (actually its protons) together, which, I quote this from Project Rho:

Proton-proton fusion is what the Sun uses, and what Bussard Ramjets would like to use. Four protons fuse to create an atom of helium-4 and 26.73 MeV of energy. Trouble is that the Lawson criterion is off the top of the chart. Trying to get four protons to simultaneously fuse is almost impossible, short of using an actual star.

Even our sun cheat this by using quantum tunneling:

[K^2]

H bombs also do not conduct fusion the same way stars do. Stars fuse with just hydrogen 1 atoms. In h bombs tritium and detritum are used. Fusion this way is done in nature though in brown dwarfs

It is, indeed, DT fusion in fusion bombs, but modern devices don't store them as deuterium and tritium, because tritium is unstable. Instead, the fusion payload (as well as the fusion booster for the primary) are made out of lithium-6 deuteride which can be stored for a very long time without decaying into something inert. During the explosion, the fast neutrons strike lithium-6, taking it to lithium-7*, which has high odds of decaying into helium-4 + tritium. That tritium, consequently, fuses with deuterium to produce another helium and a fast neutron. So there is actually one more fission stage in a modern fusion bomb.

The fact that lithium deuteride absorbs neutrons overall is the main reason why a fissile core is inserted into the secondary of a modern TU device.

By the way, most implosion type fission bombs are also fusion-boosted. The yield of a pure fission bomb is only about 20kT, which isn't enough to light up a fusion bomb. A fusion-boosted bomb has lithium deuteride core sandwiched between the plutonium hemispheres. That takes the primary to about 200kT. This can be used either in its own warhead, such a smaller tactical nuke, or used to ignite the secondary of the TU device. In the later case, you get your .5 - 2MT yield of a typical ICBM warhead. Or more, if you needed, but warheads larger than 1-2MT are not cost-efficient.

Proton-proton fusion is what the Sun uses, and what Bussard Ramjets would like to use. Four protons fuse to create an atom of helium-4 and 26.73 MeV of energy. Trouble is that the Lawson criterion is off the top of the chart. Trying to get four protons to simultaneously fuse is almost impossible, short of using an actual star.

Fusing four protons at the same time is impossible even in an actual star. That's not how proton-proton fusion works. p-p fusion follows a p-p chain.

And even in some stars, that's not the dominant cycle. See also the CNO cycle.

Edited February 27, 2014 by K^2

[Idobox]

One thing about the fusion boosted bombs, to clear the common confusion.

As K2 said, many bombs have some DT fuel inside the primary, and that's a different thing from thermonuclear devices (H-bombs).

A basic fission device has a rather low yield because it explodes too fast and doesn't have time to fission all the fuel. If you put DT or DLi6 fuel in the core of an implosion bomb, the explosion will cause some fusion, which will release lots of neutrons that will in turn cause more fission in the plutonium. The energy released by that fusion is not really significant, but it allows to use a much larger part of the plutonium, and hence gives a bigger yield.

Another advantage is that you can put more or less fusion fuel to change the yield of your weapon, to adapt to your target (sometimes, you don't want maximal power).

About the p-p chain. It's terribly difficult to get any energy out it because of the first step: p + p -> He2. He2 is very unstable and will very often decay back to p + p. Once in a while, it will decay to H2 (deuterium), allowing the rest of the chain to happen.

To give an idea, the core of the Sun has a power output less than 300W/m³. That's comparable to a pile a fermenting fodder or reptile metabolism.

[K^2]

Precisely. That's why it's fusion-boosted, rather than a fusion bomb. Almost all of the energy still comes from fission, but way more of your fuel burns through. Yeah, I probably should have made it clear in my post.

As for p-p chain, yes, the branching fraction of He-2 -> D is very low. But it's still dramatically higher than probability of more than two protons fusing together at the same time. And so as slow as it is, it's the dominant process in a lot of stars. As I've mentioned, the other process that may be dominant is the CNO cycle. p-p-p is nowhere even close, and p-p-p-p is just silly.

[DonLorenzo]

Oh what, I did not know about the fusion-boosting of fission bombs. I knew that it's important to shoot a lot of neutrons into the fissioning core, and that the materials involved had to be picked for that, but not exactly this awesome.

[rpayne88]

Why?...I can understand reactors, but bombs? Lets just say one is a controlled reaction, the other isn't (and that is my whole understanding of nuclear physics.)

[TheGatesofLogic]

no bomb ever "shoots neutrons" that would be an easy way to make your bomb fizzle out. initiators for pure fission bombs and also fusion boosted fission primaries are technically still classified but it's HIGHLY likely that they use a foil coated polonium neutron source that is commonly suspected to have a shape similar to a golf ball in order to provide neutrons at exactly the right moment.

the difference between a bomb and a reactor btw is that a reactor is maintained to be exactly critical, so that for every split atom of fuel only one more atom is split by the neutrons released. in a bomb the goal is to have as many neutrons as possible from any one atom split other atoms in the shortest amount of time so that the maximum amount of energy possible is released before heat causes the fuel to expand beyond criticality.

[rdfox]

...actually, the initiators aren't really classified any more, at least in generalities. The early weapons used roughly spherical polonium neutron sources coated in a beryllium reflector that were located at the center of the "physics package" and compressed with the fissile material to start the reaction. Later designs (as in, 1951 or so) switched to using external initiation with a miniaturized betatron neutron source contained within the bomb case that would provide extremely precise initiation timing.

Reactors are not designed solely to maintain exact criticality (if they were, they couldn't be started up or shut down), but instead to have highly controllable levels of criticality. During the startup process, a certain degree of supercriticality is maintained to bring the core reaction rate up to full power levels; during a shutdown, subcriticality is created to stop the reaction. The big thing about a weapon is that it goes prompt critical, generating enough prompt neutrons to acheive runaway supercriticality and, rather than have the reaction reach a level of stable criticality, instead just keep generating more and more power until the released energy blows the core apart.

The major aspects of nuclear weapon design that are still "I could tell you, but then I'd have to kill you" level secrets are things like the precise geometry of the physics package and the mechanisms by which the radiation implosion process for fusion ignition works. That said, I could *probably* construct a highly-inefficient gun-assembly weapon that would yield a fizzle in the 250 ton to 1 kiloton range in my stepfather's garage, solely using what I know from open-source materials... but now that I've triggered a dozen NSA red flags, I'll specify that I'd first need to get my hands on enough fissile material to build three or four proper Little Boy weapons, and that this is entirely theoretical.

As for published descriptions of the operation of nuclear weapons in fiction, all of the ones that go into detail deliberately leave out one or two critical steps to make sure that they can't be used as a how-to manual by terrorists or rogue states... and according to actual weapon designers, there's one or two items that they've kept hidden from all the open-source literature all these years, too, just in case. As for exactly what those items are, I'd rather not publicly speculate, even regarding the open-source stuff left out on purpose.

[TheGatesofLogic]

eh, that's what i meant be specifying the "somewhat"ness of the initiator design. Also when i mentioned exact criticality i assumed people would recognize that the precision would necessarily have To be done via external control. I was unaware of the neutron beam initiator designs however.

I too could build such a design, they really aren't very complex. Enriching the uranium would take me several centuries though.