Is it possible to create a hydrogen fusion bomb without a fission stage?

Maybe just using conventional explosives. Would such a bomb have much radioactive fallout?

I’m confused. Did you mean withOUT a fission stage?

Yes, sorry. Title edited.

IANANuclear scientist but ISTM you need very high temperature and pressure to initiate a fusion reaction (unless someone figures out cold fusion which was a hoax).

Without that fission stage it is difficult to imagine how you get the temperature and pressure needed in a bomb to get fusion to happen. I seriously doubt conventional chemical explosives can manage it. At least not in a bomb that can be carried around dropped on enemies.

Conventional explosives, as I understand it, do not get remotely close to the temperature needed for the fusion stage, which can reach 400 million Fahrenheit, IIRC.

Temperature as high (150 million °C) as needed to initiate Fusion are only attainable by nuclear Fission (Uranium or Plutonium).
Another possibility could be lasers but the power will need enormous size.

Need answer fast?

There is a discussion on this wikipedia page, including current progress (no one has managed to make a practical weapon yet) as well as some possible theories about how one might be made in the future.

Well, gather enough and you will get fusion due to gravity…


Aside: This is why fusion bombs are called thermonuclear. In a fission bomb, the chain reaction is perpetuated by neutrons, while in a fusion bomb, it’s perpetuated by just plain heat.

My understanding of “hydrogen bombs” is that the fusion reaction is not used in particular for the heat that it generates, but the fact that it creates a lot of very high energy neutrons that are capable of causing fission in U238, which is otherwise not fissionable. This apparently allows for much higher yields than using only U235 in a fission bomb. I’m not a nuclear scientist so I can’t say exactly how this works, and it’s probably highly classified anyway. They might even have intentionally misled people with regard to how it works in order to keep the real method secret.

Basically a small (tiny, even) fission bomb is used as the mechanism to compress a “secondary” stage that’s typically made of lithium deuteride (apparently the lithium gets in on the party and the yield is higher than straight deuterium or tritium). That’s the “fusion” bomb part, and the fusion secondary is nearly always considerably higher in yield than the fission primary.

Due to the fact that fusion reactions generate massive quantities of fast neutrons, if you surround the fusion secondary with uranium or plutonium, those fast neutrons have enough energy to fission that uranium or plutonium and generate even more yield- more than the fusion secondary. So in a sense, they’re fission-fusion-fission bombs and much of the yield can be due to that final fast neutron fission.

But those are extremely “dirty” relative to fusion bombs. Of course, you can do other things with this- you can use that fusion secondary to drive an even bigger fusion tertiary stage, or you could even go so far as to use the fission primary to compress a larger fission secondary.

The US “Dial a Yield” bombs have ways to enable or shut off these various parts of the warhead - they can be the primary only, the primary boosted with tritium, the secondary, the secondary and the uranium tamper, etc… giving the pilot multiple options as far as the bomb yield is concerned.

So, I gather then that a pure fusion weapon would create a lot of neutrons, in a large explosion, but little to no long lived radioactive fallout.
If someone had such a weapon, would it’s usage be restricted by current treaties?

The most energetic bombs are the ones that use fusion to trigger even more fusion. Once you have the initial fission trigger, it’s not particularly hard to make a fusion bomb with an arbitrarily-high yield: It basically just requires adding more hydrogen (of whatever isotope), and even tritium is cheap compared to uranium or plutonium.

The reason this mostly isn’t done any more is because there just isn’t any practical use for a bomb that big. At some point, the city is already destroyed, and you’re just digging a deeper crater.

Not Hydrogen fusion but Boron-Hydrogen fusion has shown some promise for fusion based energy generation. The high temperatures (Boron fusion requires higher temps than hydrogen) are generated using lasers.

I am not a physicist and only get involved with the engineering aspects.

Beyond that, it’s generally more effective in terms of destruction to blanket an area in a lot of smaller yield warheads, than to wallop the area with one really big bomb.

I can’t find the image, but it was in some book from 20-ish years ago, and showed the effect of one large sized warhead (IIRC it was the 9 megaton W53 on the Titan II ICBM) on St. Petersburg, versus the effect of the warheads of a single Trident missile deployed in a pattern.

Basically the W53 obliterated a large area, but the 12 W88 warheads (475 kilotons each) actually did more destruction over a wider area, as they maximized the area with (IIRC) 5 psi overpressure, which is the level of blast at which most buildings are completely destroyed, versus the larger W53 which basically had insane overkill at ground zero, and a smaller total area of absolute destruction.

Indeed. One of the “cleanest” weapons ever detonated was also the largest ever detonated. The Soviet “Tsar Bomba” got something like 97% of its 50 megaton yield from fission alone.

Presumably, you want to be able to take out hard targets, not just knock down a bunch of buildings. This may require some ground-penetrating bunker-busting rather than just an absurdly large yield.

I assume you mean fusion. That is good, because who needed the fallout from testing a dirty bomb.

Isn’t that the Fission-fusion-fission bomb, another mad scientist creation to increase yield?

No, fission is correct. The massive neutron flux initiates a second burst of fission in the tamper. The Tsar Bomba could actually have been bigger with an additional tamper of fissile material. Ironically, just like a set of Russian dolls.

Dirty bombs come about from various means. What makes a bomb dirty can be as simple as the amount of dirt it picks up and irradiates, which then falls back to the ground. A deliberate dirty bomb can be 'salted" with material that will become horridly radioactive in response to the neutron flux, Cobalt being an example. A high airburst of a primarily fissile bomb is not particularly dirty, simply because there isn’t much raw material to work with to create nasties. The fissile products themselves, whilst reasonably nasty, are not nearly as bad as can be created by either deliberate salting, or a near ground burst. Lots of neutrons wrecking stuff is your path to a properly dirty bomb.