Why do we use Uranium and Plutonium for nukes? Is it possible to use other elements?
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Nuclear weapons work through nuclear fission. While a majority of elements can undergo nuclear fission, some are able to do so more easily than others, requiring less work. Scientists decided that Uranium-235 and Plutonium-239 were the best options as they are able to achieve nuclear fission very easily. It is possible to make nuclear weapons with the isotopes of other elements, but none are as easy, and possibly as cheap, as using uranium and plutonium.
In order for a nuclear bomb to work you need a material that this specific property:
When it is struck by a fast moving neutron, it splits.
When it splits, it creates multiple new fast moving neutrons.
The way it work is that its a chain effect, one atom is split making multiple neutrons, these neutrons go into a new atom and splits that, and more and more neutrons are released and shit goes boom.
U-235 and Pu-239 are the only elements that does that as far as I know, so those are used.
So to make it truly ELI5
You need a material that doesn’t explode too easily but isn’t nearly impossible to explode.
It needs to be easily obtainable to mass produce bombs or missiles.
The amount needed to produce boom can’t be too heavy so as to make transport or flight impossible.
Adding to other answers here, some other nuclides could conceivably be used in a bomb (e.g. Uranium-233, Neptunium-237), but producing them in a concentration that can be made into a critical mass would not be cost-effective.
Some other fissile nuclides (capable of sustaining a chain reaction) would be unsuitable because their half lives are too short. Heat generated by radioactive decay could be problematic, and helium from alpha decay could build up within the material.
We use them because certain isotopes are just the right amount of unstable. This means they radioactively decay at a certain rate and release neutrons. There is a certain quantity and density of these elements that will cause the neutrons they release to start a chain reaction which accelerates the decay and split, releasing more neutrons. With uranium-238 and plutonium-239, that “critical mass” is achievable and the elements are stable enough to stick around long enough to use. Other elements are either too stable or not stable enough to make a good fission bomb. Some other elements can be used to generate power through fission, but make bad bombs, like Thorium. Then there are always fusion bombs, which use a fission bomb to make hydrogen fuse into helium, which is an even bigger boom.
They are the only ones that are economically fissile. Other elements have been examined in the past and are fissile. Neptunium-235, -236, and -237 are all believed to be fissile, but aren’t found in nature and they’d have to be extracted from spent uranium or plutonium fuel – so there’s no point in using them.
For a long time it was thought that the vanishingly rare isotope protactinium-231 was also fissile. During the 1950s, there was a great deal of interest at using it in bombs in the UK who looked at extracting it from uranium mining tailings. They spent about $500,000 on a pilot scheme and created the world’s largest stockpile of the isotope – all 125g of it. More recent calculations show that the isotope is not fissile, so goodness only knows what we’ve done with it.
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in a very eli5 way:
when things are broken (nuclear fission) they release energy. it takes alot of effort to make very stable elements explode, for example gold. uranium is verrrrrrrry unstable and can release ALOT of energy when “broken”. lots of other elements can do this but you run into other problems like money, obtainability, weight, so reactive it reacts with the bomb casing, explodes randomly without warning(very bad).
this release of energy looks like a big mushroom floating in the air which i am sure you are familiar with.
if u want to know why things release energy when they are broken i can try to explain it but i will have to go into it a bit haha
In addition to other comments:
U235 is relatively easy to make a bomb from, but it’s very expensive and noticeable to other nations to make enough quantity of it so that a bomb will work.
Plutonium 239 is relatively easy to make, but very difficult to make a working bomb from. Because it’s relatively easy to make, it’s possession is extremely tightly regulated.
And with any radioactive elements in more than extremely tiny quantities, it’s very difficult to hide that you have them from the authorities. Radioactive decay chains are near impossible to hide.
For example, in Chernobyl, when the Swedes detected decay nearly immediately, it was near obvious to them that something was wrong with a Soviet reactor, nearly immediately.
They were used because the idea of creating “hydrogen fusion” was unthinkable at the time. Uranium and plutonium nukes are called “fission” bombs which rely on using very fast neutrons to collide with and split the atoms of uranium/plutonium which then releases more neutrons.
Fast forward ~20 years and the Russians would detonate a Hydrogen Fusion bomb which causes two separate nuclei to merge and release a vastly higher amount of energy. The energy required for this process is supplied by a much “weaker” fission bomb detonation. Hydrogen nukes or “thermonuclear bombs” use a “traditional” fission bomb to supply the energy needed to fuse the nuclei in the second half of the hydrogen bomb.
However, fusion bombs are so much more ridiculously powerful that the entire planet basically agreed to stop testing them. The US detonated the hydrogen bomb “Bravo” which was a complete disaster and blew a massive hole in the ocean floor. The next one was the Tsar Bomba which was 50MT of force and was so devastating that no other Hydrogen bomb that size or larger has been made in the 60 years since to put it in perspective, the uranium/plutonium nukes we dropped would kill a few hundred thousand people in New York City. The hydrogen bomb the Russians tested would have instantly killed every living creature in New York City.
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