Why are some nuclear weapons so much more powerful than others?

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Why is the science behind it so differing, between one country who has nuclear capabilities, and another (U.S, Russia, U.K) etc who have vastly more powerful devices? Why can’t any country who have ‘cracked it’ just make the most powerful one?

In: Physics

Just because you can make a nuclear weapon doesn’t mean you can automatically make a more powerful one. A good analogy is cooking. Just because you can make an apple pie does not mean you can make one good enough to win a pie making competition.

Learning to make a more powerful nuclear weapon is expensive. Even the first ever nuclear bombs ever made were powerful enough to wipe out entire cities. There’s essentially no point to making a bigger explosion except as a symbol of military strength. Most countries have more important things to spend their money on.

There are different ways to make a nuclear weapon, and several different basic designs for modern weapons.

You don’t always want the biggest possible bomb though. You can, if you really want to, build an arbitrarily large nuclear weapon. It’s just that a bomb the size of a cargo ship isn’t all that useful. A bomb that is small enough to put on a missile, and accurate enough to land where you want it to is a lot more useful than a massive doomsday device.

It requires testing, requires rare resources, costs a lot of money, and ultimately doesn’t really give you much more deterrent than a “small” nuke. First off, the current nuclear powers try pretty damn hard to make sure no one else joins the club. So efforts to import and refine fissile material will be directly hindered. The whole process from development to production is also horribly expensive, so thats a crappy investment when one of the nuclear powers is going to just send a cruise missile into your refinement facility. And if you somehow reached the point of having a workable bomb, thats a pretty effective deterrent on its own. You dont need a hydrogen bomb when you have a regular nuke and a way to strike an enemy city, its somewhat overkill.

At the very basic level, you can say there are two types of nuclear reactions. Fission and fusion. Fission releases less energy relative fusion but costs less to research and build. Fusion is more powerful but also more expensive. Like others have mentioned, these things even at the basic level from ww2 are way more powerful than we really need from a conventional warfare perspective. The only point to have the biggest nuke is for national pride and added deterrence. To answer your question though in one word: money.

There is a big variety of nuclear weapons, and they depend both on the materials and the designs. As for power, that’s mostly either theoretical or from test ranges, thankfully nobody has used more than two in actual warfare.

Uranium was used in Little Boy, the bomb dropped on Hiroshima. Plutonium is a bigger reaction and was used in Fat Man, dropped on Nagasaki. Hydrogen bombs are more powerful still, and they basically use a small uranium bomb to start the H-bomb’s more powerful detonation. Cobalt bombs and neutron bombs have also been designed, with different purposes.

Design factors can change how the detonation works. Implosion versus explosion make a difference. Fusion reactions release far more energy than fission reactions. The amount of material and the quality of the material also matter. Just because a nation knows the pattern to create a nuclear bomb doesn’t mean they can refine the metal and create enough pure isotopes to craft the bomb.

The height they’re detonated also makes a difference. A large explosion (nuclear or not) close to the ground will have most of the energy absorbed by the ground. Too high in the air will be ineffectual. Depending on the design there is a maximum potency above the ground to do the most damage. Most of the above-ground nuclear bomb tests in the 50’s and 60’s was designed around testing these factors.

I don’t know specifically about atomic bombs, but I know in general for chemical explosions more material doesn’t automatically make a bigger bomb. It takes time for the reaction to occur, so just adding more stuff won’t create a larger blast, instead there will be a smaller blast which has parts reacting for a longer time. Basically the stuff on the outside prevents the stuff on the inside from expanding. There is an optimal level for any given mix where what’s on the inside can reach a maximum potential the instant the outside breaks open. If there is more than that, the stuff on the outside will block it from maximum potential resulting in a smaller explosion.

The big, most powerful bombs that were designed are thermonuclear, not simply nuclear. They start with a fission reaction (the type used in Japan at the end of WW2) and use that to ignite a hydrogen fusion reaction (like what happens in stars), which is hundreds of times more powerful. Beyond that are cobalt bombs, neutron bombs, and even antimatter-triggered bombs (it isn’t just Star Trek). Few nations have the means to manufacture the higher power bombs, both due to isolating all the materials and the know-how to build successful designs.

Nuclear weapons are an engineering & supply chain problem, not a science problem. “Everyone” has known the basic science/physics for even the most powerful weapons since the 1980s (ish).

However…the materials to build it are incredibly hard to get and work with, and the machinery to make it work are incredibly hard to obtain or build. And it’s all a giant industrial undertaking that’s nearly impossible to keep secret, so as soon as you start trying to build one everyone else knows and makes as hard as possible for you to continue.

Start with the fuel. You can use highly enriched uranium or plutonium. Uranium is naturally occuring (yay) but not in that many places. And what is naturally occuring is 99.3% U-238 (not helpful) and only 0.7% U-235 (the good stuff) and it’s stupidly difficult to separate the two, particularly at industrial quantities. And the equipment to do it is large, expensive, specialized, and not good for anything else.

So just to make even the most basic simple warhead, you need to get ahold of tons of uranium (which nobody wants to sell you) then build a multi-billion dollar complex industrial facility to refine it (which very few people have the expertise to do and nobody wants to help you).

Or you can use a commercial reactor to turn the low-enriched uranium (assuming you can get it) into plutonium, then use *that* to build a bomb. But nobody will help you design or build the kind of reactor that makes lots of plutonium…commercial power reactors are (mostly) designed specifically to *not* do this, for this reason. So you either need to design & build your own nuclear reactors (hard & expensive and nobody will share their expertise or equipment with you) or buy it or steal it from somebody who has one…which is generally large nation states who don’t like people trying to buy or steal their plutonium.

But let’s assume you get through that and actually obtain a sufficient quantity of enriched uranium or plutonium to build one or several bombs. You now need to craft that into actual warheads…and uranium and plutonium are both TERRIBLE materials to work with. They’re toxic, radioactive, wildly dangerous, and difficult to cast/machine/etc. So you need yet another giant specialized industrial facility (different than the giant specialized industrial facility you already built to refine the fuel) to build the warheads.

And then you need to set them off…which requires insanely precise conventional explosives and detonators and timers. Which, surprise surprise, are hard to build and nobody will sell them to you because they only have one use and everyone knows what that is, so you have to bootstrap another entire specialized R&D program from scratch.

But let’s say you do get through all that, and manage to not piss your neighbors off enough for them to slap additional sanctions on you, crippling your economy so you can’t do any of this stuff.

Now you “just” have to build an intercontinental range rocket that’s sufficiently reliable to loft a wildly dangerous and toxic warhead over your own population without blowing up and scattering uranium/plutonium bits over you and the neighbors. So now you need to basically bootstrap your own space program, or hand your precious (you’re in for several billion $ now) warhead over to someone else who has a rocket…and do you really want to give your warhead to someone else?

There are different types of weapons that require vastly different levels of technological advancement and theoretical understanding to make. It’s not one thing that you can “crack,” it’s like learning to work on a car – just because you can change the oil or fix a flat tire doesn’t mean you know how to completely rebuild an engine.

The simplest type is a pure fission weapon, which is what all countries that have nuclear weapons develop first. These max out at around 500 kilotons. The next step is a boosted fission weapon. This involves injecting a small amount of fusion fuel into the core of a fission bomb to boost its yield. This tops out at probably less than a megaton, and requires more advanced technology and physics knowledge. The final type is a fusion bomb. This is an incredibly complex device that uses a fission stage to start fusion in a second stage. These are the types of bombs that go into the multi-megaton range. Just because a country figured out how to make a simple fission bomb doesn’t mean they know how to make the much more advanced type of bomb that can make bigger explosions.

Basically money and difficulty. Big bombs use more fuel, and the biggest bombs use a different, more expensive type of fuel and are also much harder to make.

For fission-type nuclear bombs that use uranium or plutonium, you need to mine/refine/create enough fissile material to make the size bomb you want. Now sure, you could pack all of this material into one big bomb and say you have a “2 megaton nuke”. But strategically, it’s probably better to break it up and have a few dozen 100kT nukes. In terms of delivery vehicles (missiles, planes, etc.) smaller nukes are also easier to deliver to their target. There’s no point in making a massive nuke if you can’t actually fire it at anyone.

For the largest nukes though, you need thermonuclear fusion. This is much more technical than creating something like a “gun-type” nuclear weapon (which, if I had a few kg of U-235, I could probably make myself) and requires precision and expertise. Additionally, the fusion fuel (things like lithium and deuterium) are expensive to make and refine, much moreso than Uranium or Plutonium. Hence, you have to have a very well established and well funded nuclear industry to create these types of bombs.

Finally, since nuclear tests are banned through a variety of treaties, there’s no way for countries wanting to build nukes to test them. Sure, they could make them and simulate them pretty accurately with computers, but at the end of the day you still need to test the things you build. If you break the treaties (even if you’re not a signatory to them), you’ll recieve international condemnation and probably sanctions. This is why North Korea got slammed starting in 2006 for their nuclear tests, and why every time they test a nuke they get a new round of sanctions.

So if you ever read about the WWII nukes you know that only a little fraction of the uranium in the first A-bomb actually fissioned. The more fissions happen the more energy gets released. So what you want is an A-bomb that uses its uranium efficiently. But as it turns out H-bombs are more powerful, well more efficient. They combine fission with hydrogen fusion and fusion releases a lot more energy, but its more expensive cause tritium (a hydrogen isotope) is super rare. So the most powerful nukes are H-bombs. But the way they are made are secret for reasons. So with basic knowledge and enough money you can easily make a bomb a few kilotonnes but for megatonns you need a shit load of money and science, so these stuf are top secret.