How do control rods in nuclear reactors work?
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Uranium 235 needs a neutron added to it with a lot of energy to cause fission. When it splits, it emits 2-3 neutrons with enough energy to cause more fission. If left uncontrolled, the reactor will get got enough that a nuclear meltdown will occur, and liquid uranium will burn its way out of the reactor.
Control rods are made of materials that are really good at absorbing neutrons. That absorption means those neutrons can’t go on to cause more fission, thus slowing down the reaction.
Subcritical – lots of control rods, the nuclear reaction is slowing down. If left untouched, it will eventually stop
Critical – a balanced amount of control rods, the nuclear reaction is maintaining its rate. If left alone, it will continue reacting at the same rate
Supercritical – few control rods, the nuclear reaction is speeding up. If left alone, it will eventually go into meltdown. Supercritical is a necessary step to getting the reactor up an running
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