How do control rods in nuclear reactors work?
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Nuclear fission works by a neutron striking a nucleus of uranium-235.
When a neutron with the right amount of energy (not too little, not too much) hits it, it causes the nucleus to disintegrate, producing some smaller nuclei, lots of energy and typically three neutrons.
Each of these neutrons could go off and cause another U-235 nucleus to disintegrate (so 3 becomes 9, becomes 27, becomes 81, 243, …) this is a run-away chain reaction and causes the reactor to overheat and melt down, which is typically not ideal.
To prevent this, control rods (usually made of boron) are inserted and they absorb some of the neutrons, trying to make it so that, on average, each U-235 disintegrated produces one viable neutron to disintegrate another U-235 atom, keeping the reaction – and heat produced – in check.
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