How does simply splitting one atom expel so much energy? There’s no way that much energy could fit into a hydrogen atom. There’s only one electron, which has the energy. How does one atom make a big explosion?
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As other posts already said, you don’t split hydrogen; you can’t even split basic hydrogen (“protium”) as its nucleus is just a proton, nothing else.
Instead, you want to fuse atoms, and to get the basic conditions for this to happen, you detonate a atom bomb based on splitting (“fission”). It _could_ be, theoretically, done differently, it is simply not worthwhile, prohibitively expensive and has never been done. Instead you wan to merge (“fuse”) atoms, _hydrogen_ in particular. But it is not that simple!
**Proper ELI5 explanation:**
Hydrogen comes in 3 variants: **P**, **D** and **T**. Almost all of it in nature is **P**, and the rest is **D**. **T** is effectively only created as side-product in nuclear fission reactors by us.
We want to smash some of those to together to create _helium_ and energy. We don’t actually care about the helium (**He**), but all our options for lots of energy end with it anyway.
Making He from **P**’s takes four and is so hard to get going, even the sun that lives off that produces less energy than a human per volume (the sun is just THAT big). Definitely not bomb material, also not enough for potential reactors, both would at best fizzle.
So we instead use **D** and/or **T**. Smashing two of those together immediately gives **He** and the energy we want. Hence a bomb and maybe a reactor some day. This alternative process is so much better and easier, it makes it totally worth to take the extra steps to extract the little bit of **D** from the oceans, and make **T** artificially.
**Now for ELI[gifted 5 year old]:**
**To be on a common base point:**
Lets quickly recap that atoms have a nucleus (“core”) and only that part matters for splitting and fusing. This nucleus potentially consists of _protons_ and _neutrons_. The number of protons determines the _element_ and (mostly) the chemical properties; the number of neutrons determines what we call the _isotope_. Both numbers matter a lot for nuclear reactions, you cannot ignore either. There are three different common base processes that change the nucleus:
– **nuclear decay**: the nucleus is unstable for some reason and spits out something to change into something else plus a little energy. The net effect is that in the nucleus itself, either a proton turns into a neutron, a neutron turns into a proton, or 2 of each are lost.
– **fission (“splitting”)**: the atom gets hit, usually by a neutron, to make it immediately split into two or more smaller nuclei and maybe some extra neutrons. This releases energy only if the nucleus was heavy, having a lot of protons and neutrons; otherwise, it even needs energy to happen (provided by the neutron). If the right atoms are used, the number of released neutrons can then be used to hit more, creating exponentially more splittings and releasing quite a lot of energy in the process.
– **fusion (“merging”)**: two (or very rarely, more) nuclei are smashed together to form one or several products. Most often, the product(s) is a single larger nucleus plus again maybe some neutron(s). This releases energy if the atoms are not too big. You also need more energy the more protons are involved because they repel each other strongly. In general, anything humans do will only involve nuclei with at most 2 protons. Even the sun won’t do much more than that, only big heavy stars get further.
**Now lets get to the main course:**
A very common misconception is that a H(ydrogen)-bomb fuses simple hydrogen, called _protium_ (the nucleus is only a proton). It does not, instead it either fuses _deuterium_ (hydrogen, but the nucleus also has a neutron) in “D-D fusion”, or _tritium_ (hydrogen, but this time even two neutrons in the nucleus) in “D-T fusion”. Both create He-4 (helium, with 2 protons and 2 neutrons in the nucleus), and D-T also a neutron.
Why not use protium, the normal hydrogen without a neutron? It definitely is much easier to come by, about 99.985% of all hydrogen on Earth is like that. So, why is this not going to work?
Because it would be extremely slow to fuse; so slow that it would not make a bomb, not even a proper candle! It would not even become significantly warmer than the initial trigger reaction. The sun does do it, yet a cubic meter of its core produces only about as much energy as a few candles or a couple human beings! Yes, the sun’s core produces significantly less energy per volume than a human; even much worse if we count all of the sun! If you ever heard about the extreme amounts of energy the sun outputs, that may sound wrong to you. But this works out because the sun is just so absurdly big! The reactions only happen in the solar core deep within, but even that part is still large enough to produce all that energy despite the power per cubic meter being so low.
There is not just one but two reasons why protium (**P**) makes for bad fusion material:
(a) D-D and D-T each merge two things to create helium, and release energy. But **P**-based fusion has many steps: you merge two **P** to create **D** and a little bit of energy; then you merge **P** and **D** to create helium-3 (two protons, one neutron); and lastly, you merge two of the latter to create helium-4 and two single protons. The last step is by far the most energetic and what you want. Yet the entire thing is more complex.
(b) Two **P** smashing into each other is way less likely than with **D** or **T**. You can imagine it as them being smaller, but for complicated reasons it is even worse than that. Similarly, a **P** hitting a **D** is still quite less likely than D-D or D-T. In effect, this slows everything down to a crawl.
So, we really cannot use protium for a proper bomb. Nor a controlled reactor, those struggle keeping it going, them accidentally releasing to much energy is not a scenario they deal with. Hence we want that **D**, or **T**.
**D** is relatively simply to get, you can even chemically extract it from sea water. The cost is less than $1 per gram of _heavy water_ (with **D** instead of **P**). Not exactly cheap, but easily within the affordable price range for nuclear weapons or reactors; cheaper than the alternatives for sure.
**T** on the other hand is a bit tricky. It does almost not appear in nature as it decays after on average a dozen years, and this planet is way older than that. Instead, it is created as a side-product in nuclear fission (or fusion, if we ever get there) reactors. A stray neutron hits a **D** and turns it into **T**. To make this more likely, you would use heavy instead of normal water for cooling of the reactor.
So, quite annoying? Yeah, but it allows you to make larger bombs, and I think we both know how politicians and military think about having the bigger guns.
There is a third type of fusion that would be even better for some reasons: fusing two helium-3 nuclei, as in the last step of P-based fusion in the sun. But helium-3 is so rare on our planet, this has never even been used in a bomb. Our planet lost all of the stuff into space over time, as helium simply rises up the atmosphere until it is gone. The only reason we even have **He-4** around is that it is created by nuclear decay of many heavier elements (the decay mentioned above where the nucleus loses 2 protons as well as 2 neutrons as a single unit). But there are significant deposits of **He-3** on the moon; hence we might get that bomb some day. Or hopefully, rather a fusion reactor.
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