so nuclear reactors generate power by splitting atoms but what actually splits these atoms?
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Radioactive atoms have a nucleus which is unstable. The nucleus can rearrange itself spontaneously and this rearrangement will release energy, and often a particle as radiation.
The common types of radioactive decay are alpha (a cluster of 2 protons and 2 neutrons gets kicked out of the nucleus), beta (a neutron turns into a proton and electron, and the electron gets kicked out) and gamma (the protons and neutrons in the nucleus rearrange into a more compact shape releasing energy as “light”).
However, those are not the only ways. There is a very rare way called fission, where the nucleus splits into two large chunks which then recoil away from eqhx other with massive energy. This does happen very occasionally in some atoms like uranium, but it is not usually significant.
However, you can make new atoms by firing neutrons at existing nuclei. If a neutron hits a nucleus, there is a chance that the neutron will get sucked in and become a new part of the nucleus. In some cases, the new atom is radioactive, potentially extremely radioactive and the new nucleus decays very quickly.
It turns out that if you hit a natural uranium 235 atom there is a high chance that the neutron will get captured, and the new energised and wonky uranium nucleus will almost instantly decay by fission. This strange and rare property means that it is possible to trigger fission by neutrons, and therefore fission is controllable.
The other interesting thing is that although the main product of fission is two big chunks of nucleus, there are also a number of little shrapnel pieces, which are almost always neutrons.
So, not only is fission controllable, fission can trigger more fission, in other words, a chain reaction.
By changing how many neutrons are available to continue the chain reaction (by moving uranium atoms apart so that the neutrons miss, or by adding other atoms which capture neutrons but don’t fission) the rate of the chain reaction can be controlled.
This means that all you need to get a chain reaction going is to put enough concentrated uranium together in one block, that when one atom fissions, more than 1 neutron from the fission triggers another fission. This is called a critical mass. Below a critical mass, too many neutrons escape out of the block into the air and the chain reaction doesn’t chain. Keep adding uranium and the chain reaction will start once a spontaneous fission occurs.
Practical control is a bit more difficult. Nuclear reactors use other atoms to control the speed of neutrons (slow neutrons are better for triggering fission) and to absorb neutrons. Essentially, they have a combination of uranium and neutron absorbers packed into metal cans. The core is built with more than a critical mass of uranium, but with more absorbers than needed to stop the chain reaction. Then to start, the cans with the absorbers in are very carefully pulled out of the core. Once enough absorbers are removed, then the chain reaction starts. Put the absorbers back and the chain reaction stops.
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