How do control rods in nuclear reactors work?
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All atoms contain two main parts, the nucleus made up of protons and neutrons and the electrons that orbit around it.
The particles within the nucleus want to repel each other but are held together with a strong force. That means there’s a lot of energy contained within the nucleus. Imagine you had a load of springs in a box. As soon as you open it all of the springs will bounce out. The nucleus is the same, if you can get rid of the strong force the nucleus will split apart and release energy.
The isotopes used for nuclear reactions are very unstable. Imagine a glass that’s completely full of water, it only takes a tiny bit more water for the glass to overflow. It’s similar with things like Uranium. If you introduce another neutron to the nucleus it “overflows” and splits apart releasing energy.
The most important part is that when the nucleus splits it releases more neutrons. These then impact nearby nuclei and cause them to split as well leading to a chain reaction. In the case of Uranium-235 each split releases three neutrons. That means that every time a nucleus splits it’ll then cause three more to split. If you let this happen unrestricted then the number of splits (and therefore energy output) will increase exponentially to a massive amount, that’s basically a nuclear bomb.
What control rods do is safely absorb neutrons before they can cause another split. If you put a control rod into the reactor it’ll absorb a certain percentage of the free neutrons. As an example let’s say one control rod absorbs 1/3 of the neutrons. If you add one control rod then each split will only lead to two further splits, rather than three. This means that the energy output will increase more slowly. If you add a second rod each split will only lead to one more split, that means that the energy output will remain constant. If you have three rods all of the neutrons will be absorbed and the reaction will stop.
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