During fission an atom is split apart into other things including neutrons. I know this starts a chain reaction because more neutrons are produced, but how is the first neutron (or group of neutrons) obtained and launched?
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Radioactive atoms are unstable by nature. When one of these enters fission spontaneously, it liberates one or more neutrons. If one of these neutrons, by luck, collide with another radioactive atom’s nucleus, the chain reaction begns.
Nuclear reactors use what’s called a startup neutron source and nuclear weapons use what’s called a neutron initiator. The mechanism of both is similar.
In a Startup neutron source in a nuclear reactor, there are a few types of neutron sources. The first is just some radioactive isotope that undergoes spontaneous fission and emits neutrons by itself, such as californium-252. Second is a mixture of radioisotopes that emit alpha particles (such as plutonium-238, americium-241, polonium-210, and radium-226) and isotopes of light elements such as beryllium, carbon or oxygen. In this mixture, the alpha particles from the alpha emitters hit the lighter atoms and knock of neutrons, providing the initial neutrons to start the chain reaction. A third way is to use a radioisotope that emits gamma rays to knock neutrons off beryllium-9 or hydrogen-2 (deuterium). A final way is to use antimony or beryllium that has absorbed neutrons by being inside the reactor already. When stable atoms absorb neutrons, they become unstable and re-emit those neutrons later.
In nuclear weapons, a neutron initiator is used. In older nuclear weapons, the initiator was similar to the second type mentioned above – the mixture of an alpha emitter and beryllium, that was placed at the center of the plutonium or uranium core. For example, the fat man bomb design (the bomb that was dropped on Nagasaki), the initiator was a small pellet with a beryllium core, a thin polonium layer, and an outer beryllium layer, which was placed at the center of the plutonium sphere. The explosion from the conventional explosives compressed the Plutonium sphere and in turn the pit. This compression mixed the polonium and beryllium in the initiator, which caused the alpha particles from the polonium to knock neutrons off the beryllium atoms, and those neutrons initiated the fission process.
Modern nuclear weapons use external neutron sources that are inside the warhead but not inside the plutonium core. These neutron generators are essentially tiny particle accelerators. They accelerate deuterium ions into a hydrogen-3 (tritium) target. This causes the deuterium ions and tritium atoms to fuse, and in the process, emit neutrons. These neutrons are then channeled into the center of the plutonium pit at the moment of detonation.
You use some external neutron source. It can be a material that spontaneously emit neutrons from radioactive decay. It can also be from [https://en.wikipedia.org/wiki/Spontaneous_fission](https://en.wikipedia.org/wiki/Spontaneous_fission) Natural Th-232, U235 and U-238 can spontaneously go trough fission you do not need to hit them with neutrons, it just does not happen that often
A way is to use an element that emits alpha radiation and let it hit an element that when hit by it releases nurone radiation. You can for example use polonium that you can make or naturally find in uranium ore ( pitchblende). The target can be beryllium, boron, or lithium, This is how neurons was discovered back in 1931,
If you have a pellet with beryllium separated from polonium with layers of nickel and gold you have something that will emit neutrons when you crush it and the material mix. This is the general idea of how you generate neutrons at the exact right moment in time in early nuclear bombs.
https://en.wikipedia.org/wiki/Modulated_neutron_initiator#Urchin
Another way is by using fusion. Fusion is not hard to achieve you can accelerate ionized hydrogen with electrical field and high deuterium or tritium and the result is fusion with release neutrons. This was fist shown in the 1930 and was how fusion was shown to work. You can do that is quite small device, the simplest Neutristor fits in your closed fist, and something similar are used to trigger modern nukes. It is not a way to make a power plant because the electrical energy required is more than the energy the fusion energy
https://en.wikipedia.org/wiki/Neutron_generator
There is other way too to you can read about it at
https://en.wikipedia.org/wiki/Neutron_source
Awesome so I’ve actually explained this to a child before! [This](https://lh3.googleusercontent.com/-uloKbDZ53sA/XBNSYhmyhNI/AAAAAAABaEs/3jRUDIOzkJEdU4SuUaplaRUlu11HumiGACHMYCw/w1200-h630-p-k-no-nu/demon-core-slotin-26?imgmax=1600) is a Demon Core and the guy playing with this wonderful toy is Louis Slotin. The Demon Core was used in the Manhattan Project to develop those good ol WW2 atomic bombs. So here we go: the core is a plutonium sphere a bit larger than a softball, along with two large half-spheres of beryllium that have a cutout in them perfectly matching that of the plutonium ball. It’s important to note that plutonium is extremely radioactive, so it’s already gonna be spitting out neutrons everywhere Willy nilly. Pretty much, this beryllium half sphere on the top would be moved closer and closer to completely surrounding the plutonium. The point at which the two half-spheres lock around that plutonium would be supercriticality, which means that instead of those neutrons spraying all over the room, they’re gonna be bouncing off of that beryllium dome, and reflecting into each other a billion times per nanosecond. This is what actually manipulates the radiation into having a supercritical chain reaction. My boy Louis Slotin in that picture is actually holding the top dome open with a screw driver because he’s a crazy dude (super nelk guy). One day, that screwdriver slipped and the dome enclosed that demon core, causing it to reach criticality a mere foot away from Slotin’s manhood
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