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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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.
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