How do control rods in nuclear reactors work?
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Real ELI5 answer, since most of the answer here are a bit lofty:
Reactors have a very angry metal inside of them. Think of the angry metal as clusters of billiards balls. Think of how the game billiards/pool works: you hit a ball into the rack and the balls explode in all directions. This isn’t a perfect analogy, but it gets us there.
Now think of a reactor as full of thin rods of this angry metal. Think in 2D, so that we’re on a pool table and we’re looking at “slices” of these thin rods. You have a very big billiards table full of multiple racks, all waiting to be hit by balls.
Now imagine that this is billiards table is also frictionless, and the racks somehow ADD energy to the system when they get split (unlike in real pool where the momentum is distributed over the balls so that over all they all sorta average out). The balls will go for a long ways and will hit other racks and split them and release more balls, causing a chain reaction. This can quickly get out of hand if none of the balls are removed from the table.
Besides balls just flying off of the table (leakage), or hitting other racks and not splitting them (not every absorption of a neutron in fuel causes a fission), there needs to be a way to “control” the amount of balls on the table. Now interspersed among all of these rods of billiards balls ready to be split, you have little pockets, and they can absorb a nearly infinite amount of billiards balls.
This is basically how a reactor works. Control rods keep the amount of neutrons in check, so that the reactor doesn’t go supercritical. Well, if the reactor is gaining power it is *technically* supercritical, so there’s another concept involved called “prompt criticality” that I can also ELI5 if anyone actually reads this or is interested, but there’s a difference between a nuclear reaction and a *nuclear reaction*, and that’s what prompt criticality is.
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