As far as I know, access to uranium is tightly controlled for obvious reasons, but hydrogen is everywhere, and even getting access to deuterium shouldn’t be too hard.
There is also the fact that most modern thermonuclear bombs “only” use the fission bomb to trigger the hydrogen bomb.
People demonstrate achieving fusion all the time. The problem is getting useful energy out of it. When building a bomb, we don’t really care about useful energy; we just want to release a lot of it.
So why aren’t people building purely fusion based thermonuclear weapons left and right?
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Because fusion is much harder to ignite than fission, really. I’m sure it’s possible, theoretically.
We’re only able to achieve fusion at all with the use of hugely powerful lasers, requiring massive amounts of energy. Currently, there’s no way to pack that kind of gear into a warhead. Fission is relatively compact.
> People demonstrate achieving fusion all the time.
I mean, not *all* the time. It takes a lot of equipment and power to create fusion, much less get a net energy gain out of it. If you aren’t in a massive test reactor you aren’t going to be demonstrating fusion.
> When building a bomb, we don’t really care about useful energy; we just want to release a lot of it.
We actually do care about useful energy since the aim is to make the fusion reaction release more energy than it took to start. Otherwise we would just use whatever we used to start the reaction alone as the explosive.
And therein lies the problem: Starting fusion is hard. It requires immense temperatures and pressures which you aren’t going to get out of conventional explosives. You can’t just have a fuse leading to deuterium and while hydrogen will burn violently when exposed to oxygen it isn’t nearly as powerful as conventional chemical explosives. Even carefully shaped charges using our most powerful conventional chemical explosives aren’t going to fuse the material of a warhead. Instead what we need is the power of a fission bomb to provide the heat and pressure to make the fusion fuel actually fuse and release even more energy.
Hydrogen bombs are achieved through fusion, which requires insane amounts of energy to happen.
So much energy that they need to use nuclear fission to create the energy, which is where the uranium comes into it.
You need fission as a stage 1 to ignite your stage 2 fusion reaction. I didn’t read far but it seems like the goal is to have your fission and fusion reactions both happening at the same time, each one making the other more intense. Wiki https://en.wikipedia.org/wiki/Thermonuclear_weapon
Perhaps another way to answer the question is that getting a fusion reaction to output more energy than you put in is very difficult and IIRC has only bearly been achieved in lab settings.
No one knows how to make a practical pure fusion weapon yet (or if they do they’re not publicizing the fact). The problem is that it takes a ton of energy to trigger fusion. If you don’t particular care about how big your setup is you can use things like lasers to do this, which is what the guys “achieving fusion all the time” tend to do. Nuclear weapons however are designed to fit onto places where here space and weight are at a real premium, like missiles. As a result if you want to make a worthwhile fusion bomb you need to come up with something that is very small but capable of creating the intense conditions needed to trigger fusion. We don’t really have options besides nuclear fission at the moment.
As an aside, if you give up practicality, supposedly there are some designs out there for “pure fusion” systems, but when you factor in the weight of everything involved they don’t have an advantage over a similar mass of high explosives (unless you consider bathing everything within a few hundred meters of your multi-ton ‘bomb’ in a sea of radiation to be an advantage).
A uranium or plutonium fission bomb is the only way to get the energy required to start fusion in a size that is transportable.
[The NIF](https://en.wikipedia.org/wiki/National_Ignition_Facility?wprov=sfla1), where they just achieved breakeven, uses lasers for fusion. The length the laser light has to travel is over 1500 meters long. There is no way to make this even remotely transportable as a complete package.
>People demonstrate achieving fusion all the time. The problem is getting useful energy out of it. When building a bomb, we don’t really care about useful energy; we just want to release a lot of it.
It is not useful energy that you get more out of than you put in with the recent laser-based fusion announcement from the National Ignition Facility. It is more energy released from fusion than laser energy. Lasers are not 100% efficient and you can convert 100% of the released energy to something useful like electricity. The electricity used to power the laser was more then the electricity that could be extracted from the heat the reaction created.
But even with a nuclear bomb, you do care about energy. Warheads need to be delivered to the target and be stored for a long time. If you need a hure amount of energy to initiate the fusion reaction directly where will it come from? And if it is not a lot less the fusion release why do fusion to begin with and not just release that energy directly?
The energy wet talks about in the National Ignition Facility experiment is 2.05 megajoules of laser energy and 3.15 megajoules of energy released. The lasers required vell over 400 megajoules of energy. So even if you could extract all the energy from the fusion that is still less the a hundred of what the lasers consumed.
The facility has a footprint of three football files, how to you deliver it to a target from a nuclear weapon? The weapon will need to be a ship that you use like a nuclear torpedo.
The is question how muse energy is 3.15 megajoules? 3.15 megajoules = 3,140,000 joules. A kilocalorie that is called just a calorie for food is 4,184 joules. so 1000 food calories is a bit over 4 megajoules.
The sugar we eat contains 17 MJ/kg usable for us. So 3.15 megajoules is the energy in a bit less than 200 grams of sugar.
TNT which is an explosive that is often used for comparison contains 4.184 megajoules per kg, the fusion reaction released less energy than 1 kg of explosives. Explosive contains less energy the sugar, and one reason it the need to release it quickly. Another is sugar cheats, the energy is from sugar and oxygen that is taken from the atmosphere.
A 105mm artillery shell, which is a smaller caliber used for howitzers today has around 2 Kg of explosives in it, the total projective weight is around 15 kg. A hand grande have around 0.2 kg of explosive so
So the ship-sized laser-initiated nuclear fusion weapons release less energy than quite a small artillery shell or five-hand grades.
So we do care about releasing energy for bombs and using nonfission triggered fusion reaction make conventional explosives look extremely efficient
There’s roughly 3 things human can make that can trigger nuclear fusion.
1. A lot of extraordinarily powerful magnets.
2. A lot of extraordinarily powerful lasers.
3. A fission bomb
1 and 2 are with current technology are either the size of a large building and thus unsuitable to be launched by missile.
Or only capable of fusing tiny amounts of hydrogen at once.
A fission bomb, or even multiple fission bombs are small enough to launch in a missile, and capable of fusing absurd amounts of deuterium/tritium.
According to [wikipedia](https://en.m.wikipedia.org/wiki/Pure_fusion_weapon#:~:text=A%20pure%20fusion%20weapon%20is,in%20fission%2Dfusion%20thermonuclear%20weapons.) it might be possible:
>It has been claimed that it is possible to conceive of a crude, deliverable, pure fusion weapon, using only present-day, unclassified technology. The weapon design[[2]](https://en.m.wikipedia.org/wiki/Pure_fusion_weapon#cite_note-2) weighs approximately 3 tonnes, and might have a total yield of approximately 3 tonnes of TNT. The proposed design uses a large [explosively pumped flux compression generator](https://en.m.wikipedia.org/wiki/Explosively_pumped_flux_compression_generator) to produce the high power density required to ignite the fusion fuel. From the point of view of explosive damage, such a weapon would have no clear advantages over a conventional explosive, but the massive neutron flux could deliver a lethal dose of radiation to humans within a 500-meter radius (most of those fatalities would occur over a period of months, rather than immediately).
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