How do control rods in nuclear reactors work?
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The way neutrons and different materials behave together is an interesting science.
Hope this makes sense
When the uranium fissions, it generates a few fast moving neutrons. There are a few things that can happen to those neutrons. They can go right through some materials, can be absorbed into the nucleus of another atom, or they may bounce off the nucleus of other atoms. Different material have different probabilities for what happens when a neutron approaches it, and sometimes what happens is also affected by how fast that neutron is travelling.
In the reactor, the fuel pellets or rods are incased in material that appears essentially invisible to neutrons (zirconium alloy). The bundle is surrounded by material that neutrons tend to bounce around in, like billiard balls on a billiard table. This material is called a moderator, because it moderates, or slows down, the speed of the neutrons. Common material for this is water, or graphite. Finally, if the neutron is going the right speed and passes back into a fuel bundle and encounters a Uranium 235 nucleus, it will be absorbed, and that atom fissions, starting the whole process all over again.
The whole process is a balancing act of creating sufficient fissions to create sufficient neutrons, so that the reaction keeps occurring. Obviously, you don’t want it to run away on you, so that’s where control rods come into play. They are made of material that likes to simply absorb neutrons and hang on to them, no matter how fast the neutrons are travelling. If you insert enough control rods, there will be fewer and fewer neutrons available to find a uranium atom and cause another fission, and the reaction slows to a halt. As you withdraw the rods, the fission reaction speeds up. So, with lots of rods, you can control the fission reaction throughout the reactor.
As a side benefit, you can use Cobalt as your control material, and make Cobalt 60, which has many uses.
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