Tsar Bomba and the Limits of Thermonuclear Warhead Power

[fenderzilla]

So I’m not particularly learned on the topic of nuclear physics, but I do know basically how thermonuclear warheads work. My understanding is that at the core of the warhead is a nuclear fission device, surrounded by a shell of deuterium. When the fission device is ignited, the adjacent deuterium has so much energy dumped into it that it actually undergoes fusion, which generates a huge amount of energy as well. As far as I know, a great deal of the bomb’s destructive power comes from the fission at the core, and the energy of fusion is just a way of boosting it.

So here’s the question. I was reading a little about the Tsar Bomba and how the fission device was so huge, yadda yadda yadda, and there was a lot of concern about the environmental impact the fallout would cause. But from what I know, radioactive fallout is just spent fuel from the fission reaction that has been spread out by the blast. So instead of scaling up a standard nuclear warhead and dropping that, would it work if you just used a normal fission device and encased it in tons more hydrogen than usual, since the heat of the fusion would spread throughout the entire shell and ignite it all? Would that work, and if so, would it be a more environmentally safe option? And if it does work, is there actually any limit to the strength of a thermonuclear warhead, since you could just keep packing on hydrogen?

[Brotoro]

No, you do not understand how a thermonuclear explosive works.

The output from the fission primary is focused to compress and heat the fusion secondary (the high energy neutron output from this secondary can be used to produce additional fission, if desired). The output of the secondary can be focused to set off a third fusion stage...so it is possible to make a very large design using multi-staging.

[Bill Phil]

There's also some tritium in some warheads. You can control the output within a certain range depending on the amount of tritium.

[sgt_flyer]

The tsar bomba was among the cleanest thermonuclear bombs relatively to it's yeld (the russians did not want to create huge fallout on their own country on purpose ) - on 'classic' smaller bombs, a uranium tamper is used between the stages - which create additionnal fast fission for triggering the second / third stage fusion. (And those tampers, becomes of course radioactive & create additionnal fallout). on the tsar bomba, those tampers were made of lead

There's also some tritium in some warheads. You can control the output within a certain range depending on the amount of tritium.

Well, technically, the 'Hydrogen' fusion is a Deuterium Tritium reaction (so you'll get 1 helium atom + 1 neutron from the fusion)

However, using deuterium and tritium as they are would need cryogenic storage for a complete bomb (that, and tritium as a relatively short half-life of 12 years, so it can't remain in storage for long.)

so instead of that, they use a Lithium/Deuterium molecule as fusion fuel. The lithium is bred by the fast neutrons to create tritium, which then undergo fusion with the deuterium,

Edited July 4, 2015 by sgt_flyer

[YNM]

Multi staging is possible - though it's better off you use fissionable pusher (equivalent to piston) for the fusion, fissionable "spark plug" and fissionable shell. That way you can even produce additional fusion and fission fuel within the bomb (via neutron irradiation) while being ignited. More than two stage is AFAIK never really tested, probably confronted with limited rich-fuel productions, and the high efficiency of two-stage bomb.

[Brotoro]

Well, the Tsar Bomba was supposedly a three-stage design...which is why I mentioned it. As was the American B-41...supposedly.

[sgt_flyer]

Well, wikipedia states that the tsar bomba might have had multiple thrid stages (all ignited by the second stage at the same time) instead of a single large one.

[jwenting]

your understanding of what fallout is is also flawed. Fallout is radioactive material from the bomb indeed, but there's more to it. Dust blown up by the blast and heat wave gets coated by that residual bomb material, increasing the total amount of radioactive material that needs cleaning up in many bombs (in volume and mass, not in becquerels or radioactivity obviously).

Other materials may undergo state changes and even fission when bombarded with high energy particles released during the nuclear explosion, adding both mass and volume AND becquerels to the amount of fallout resulting.

The main methods of controlling fallout are bomb design (both materials chosen and fission efficiency (iow, making sure there's as little as possible nuclear material that doesn't undergo fission, which also increases yield, thus more bang for your buck), and deployment profile (higher atmospheric bursts cause less material to be sucked into the fireball and become coated in radioactive debris, thus reducing the amount of fallout produced).

In strategic military applications of course limiting fallout is not the main concern when designing weapons and deployment profiles, maximising destructive power for the amount of money and bomb size/weight is what matters. But for many strategic purposes those concerns tend to overlap to a large degree.

Atmospheric explosions cause the shock and thermal pulse to hit a larger area with overpressure, thus increasing the efficiency of the weapon (unless you're hitting a seriously hardened point target), and making the weapon more efficient reduces its size and weight for the same yield, thus allowing you to use a smaller delivery vehicle and/or increase range or number of weapons per delivery vehicle.

The Czar bomba was deliberately scaled down from the design yield by about 50% through the replacement of a Uranium tamper with a Lead tamper. This was not done because they were scared of potential fallout (that would be roughly the same) but because the full 100MT design yield would make it impossible for the dropping aircraft to escape the blast and because the scientists weren't sure they could guarantee the safety of the observation vessels and aircraft (which carried some rather high ranking military and party officials, not guys you wanted to blow up in the 1950s USSR and expect to get away with it by saying "oops").

[DerekL1963]

So here’s the question. I was reading a little about the Tsar Bomba and how the fission device was so huge, yadda yadda yadda, and there was a lot of concern about the environmental impact the fallout would cause. But from what I know, radioactive fallout is just spent fuel from the fission reaction that has been spread out by the blast. So instead of scaling up a standard nuclear warhead and dropping that, would it work if you just used a normal fission device and encased it in tons more hydrogen than usual, since the heat of the fusion would spread throughout the entire shell and ignite it all? Would that work, and if so, would it be a more environmentally safe option? And if it does work, is there actually any limit to the strength of a thermonuclear warhead, since you could just keep packing on hydrogen?

What you describe won't actually work - in order to fuse, the fusion fuel has to be compressed. Even a thermonuclear weapon doesn't generally generate enough heat to force fusion in an uncompressed fuel mass. The solution to that problem was the staged Teller-Ulam configuration. (There does exist a middle configuration between pure fission and Teller-Ulam thermonuclears, the boosted fission configuration, but explaining that... would require more background material than I care to type.)

As to how to create a "clean" (minimal fallout) weapon, see sections 4.5.2 ("Dirty" and "Clean" Weapons) and 4.5.4 (Minimum Residual Radiation (MRR or "Clean") Designs) of the Nuclear Weapons FAQ. (Warning: serious science/physics content.)

[fenderzilla]

Yeah, that's always the part that I forget. Fusion isn't just about heat, it's also about density.

[ZetaX]

Density is surely relevant for speed and efficiency. But it is not strictly necessary for fusion to occur. The density in a tokamak is very low, for example.

[kiwi1960]

There are restrictions on the size of warheads.

If you imagine a device that can be destructive with a 1000 foot radius, then for every extra foot, you need about double the power of the original... this is why the "doomsday" device is impossible, unless its tied into the Earth's core of course (or some other implausible idea like that)

The Neutron bomb is different, there is a radioactive blast with a very small destructive potential. The crazy logic is that it would kill people but leave the buildings intact.

Tsar Bomba was not that clean, it blasted a huge hole in the Ozone layer which is where most of the fallout went, but even so, like the Ozone hole over the Antarctic which extends itself partially over New Zealand if the conditions are right, can lead to skin cancer.

The awful truth is... there is no such thing as a clean, green nuclear bomb.

[Brotoro]

There are restrictions on the size of warheads.

If you imagine a device that can be destructive with a 1000 foot radius, then for every extra foot, you need about double the power of the original... this is why the "doomsday" device is impossible, unless its tied into the Earth's core of course (or some other implausible idea like that)

I don't know where you get that calculation from. The destructive radius of a nuclear explosion scales as the yield to the 2/3 power.

A "Doomsday Bomb" would not be a deliverable weapon. It would be a multistage nuclear device of enormous yield that could, supposedly, spread enough radioactive fallout to kill all people on the surface of the planet. Because it isn't designed to be deliverable, it could be built any size needed. Just remember, Dimitiri: the whole point of the doomsday machine...is lost if you keep it a secret!

The Neutron bomb is different, there is a radioactive blast with a very small destructive potential. The crazy logic is that it would kill people but leave the buildings intact.

No...The logic of the neutron bomb was to kill an attacking Soviet tank column while minimizing the collateral damage to the infrastructure of the friendly nation that's being invaded. Or, in the case of neutron bombs used as missile interceptors, the logic is to irradiate the fissile material of the incoming warhead, causing fissions that release energy to ruin the incoming warhead.

Edited July 4, 2015 by Brotoro

[Nothalogh]

The Teller-Ullam device is one of the single most amazing pieces of engineering ever concieved

[K^2]

No...The logic of the neutron bomb was to kill an attacking Soviet tank column while minimizing the collateral damage to the infrastructure of the friendly nation that's being invaded.

Ayup. A neutron bomb is just a low yield nuke designed to make sure that all of its products, both from fuel and irradiation of the casing, are very short-lived isotopes. The idea is that these isotopes will burn through in a few weeks, making area safe to enter, rather that leaving an irradiated wasteland for decades to come. Low yield also ensures that it doesn't lift quite as much of a dust cloud, hopefully, reducing any chance of irradiating anything downwind.

Density is surely relevant for speed and efficiency. But it is not strictly necessary for fusion to occur. The density in a tokamak is very low, for example.

It's the combination of density and confinement time that are important. If your confinement is very short, such as during a nuclear explosion, density needs to be very high. Tokamak achieves fusion by having very long confinement times instead.

[KerikBalm]

There's also some tritium in some warheads. You can control the output within a certain range depending on the amount of tritium.

There is no tritium in the vast majority of hydrogen bombs... basically no free hydrogen in a hydrogren bomb at all...

So why do they call it a hydrogen bomb?

The tritium/hydrogen isotopes are produced by fission of lithium-7 and lithium-6, due to the neutron flux from the primary.

Those may then undergo fusion.

https://en.wikipedia.org/wiki/Castle_Bravo

Castle Bravo was the code name given to the first United States test of a dry fuel hydrogen bomb

...

The device detonated for the test was named "Shrimp" and was the same basic configuration as the experimental Ivy Mike device, except with a different type of fusion fuel. The Shrimp used lithium deuteride, which is solid at room temperature; Ivy Mike used cryogenic liquid deuterium, which required elaborate cooling equipment. Castle Bravo was the first test by the United States of a practical deliverable fusion bomb (hydrogen bomb).

...

The yield of 15 megatons was three times the yield of 5 Mt predicted by its designers.[1][8] The cause of the higher yield was a theoretical error made by designers of the device at Los Alamos National Laboratory. They considered only the lithium-6 isotope in the lithium deuteride secondary to be reactive; the lithium-7 isotope, accounting for 60% of the lithium content, was assumed to be inert.[8] It was expected that lithium-6 isotope would absorb a neutron from the fissioning plutonium and emit an alpha particle and tritium in the process, of which the latter would then fuse with the deuterium and increase the yield in a predicted manner. Lithium-6 indeed reacted in this manner.

It was assumed that the lithium-7 would absorb one neutron, producing lithium-8 which decays (via beryllium-8) to a pair of alpha particles on a timescale of secondsâ€â€vastly longer than the timescale of nuclear detonation. However, when lithium-7 is bombarded with energetic neutrons, rather than simply absorbing a neutron, it captures the neutron and decays almost instantly into an alpha particle, a tritium nucleus, and another neutron. As a result, much more tritium was produced than expected, the extra tritium fusing with deuterium and producing an extra neutron. The extra neutron produced by fusion and the extra neutron released directly by lithium-7 decay produced a much larger neutron flux. The result was greatly increased fissioning of the uranium tamper and increased yield.

[sgt_flyer]

Yup, only a few bombs (notably neutron bombs) since castle bravo were enriched with tritium - but with the tritium short half-life, in order to keep those bombs ready, they needed to replace the decayed tritium each year (roughly 1 gram of tritium per year per bomb, and remove the byproduct from the decay too. So, very intensive maintenance required.)

(As a nice thing to look at, the anti ballistic missiles with neutron bombs warheads (for taking out incoming warheads from ballistic missiles, either by frying the incoming missiles circuitry or fissioning enough of the target's fissile material to prevent the target warhead from detonating correctly) those have some serious TWR

http://m.liveleak.com/view?i=33b_1372005455&comments=1

[SomeGuy12]

Can you just keep on staging? Can you build a 10 stage device, where every stage drives several stages after it, resulting in an oil-tanker sized bomb?

[Brotoro]

I would think that ALL modern thermonuclear bombs have some tritium in them because they use boosted primaries. The secondaries also contain hydrogen in the form of deuterium in the lithium deuteride.

Yes, in theory a multi-stage device the size of an oil tanker should be possible.

[kiwi1960]

I don't know where you get that calculation from. The destructive radius of a nuclear explosion scales as the yield to the 2/3 power.

I didn't want to get al sci-ency, so used an example of sorts. Its like most things, the bigger the thing, the more energy it needs.

In the case of a nuke, the bigger the blast doesn't always mean a bigger blast radius. (using non sciency talk again) because the blast circle is bigger, say, 1 one foot blast compared to a 2 foot blast, most of the enery would still be wasted in the first one foot making the hole bigger. So, obviously, you'd need more energy the further out you wanted to go.

[K^2]

I would think that ALL modern thermonuclear bombs have some tritium in them because they use boosted primaries. The secondaries also contain hydrogen in the form of deuterium in the lithium deuteride.

Modern boosted fission devices also use lithium deuteride for all the same reasons explained above. So a modern thermonuclear warhead contains no tritium.

[Brotoro]

Modern boosted fission devices also use lithium deuteride for all the same reasons explained above. So a modern thermonuclear warhead contains no tritium.

I think you are incorrect (although I suppose it would depend on how you define "modern"). The Savannah River Nuclear Laboratory website claims that they make the high pressure deuterium-tritium gas systems for the W87 warhead

[K^2]

Hm. That's strange. Maybe TD gas produces more neutrons for the fission. I'll look into it.

[cicatrix]

As a matter of fact, the Tsar bomba was a scientific experiment to prove exactly that - that there is no limit to its yield. Technically speaking, our Sun is also a thermonuclear blast but it has so much fuel (hydrogen) that it's been 'exploding' for already ~4 billion years. Actually, they worried not about environmental impact (nobody cared all that much back then) they worried about ocean water being involved in fusion reaction as well. I'm not in a position to say if it is really possible to 'ignite' our oceans and what it takes to do that, but at that time this theoretic possibility was considered seriously. That was the reason the bomb yield was reduced by 50%.

[DerekL1963]

Modern boosted fission devices also use lithium deuteride for all the same reasons explained above. So a modern thermonuclear warhead contains no tritium.

The DOE is producing tritium for something. Three guesses what it is, and two of them don't count.

According to this (PDF link), tritium is associated with the W76 and W88 (Trident II) weapons, the W78 and W87 (Minuteman III) weapons, as well as the W80 (ALCM and Tomahawk) and the B61 (gravity bomb).

Hm. That's strange. Maybe TD gas produces more neutrons for the fission. I'll look into it.

Tritium allow you to easily build dial-a-yield weapons. It also (and unlike lithium deuteride) enhances nuclear safety because it remains isolated from the pit unless specifically injected.