The control rod has the ability to stop fission reactions. Once a control rod is inserted next to a reactor core (adjacent to it) it will stop the fission reaction and start decreasing the temperature in the fission reactor.

If you're really keen on setting up redstone, you can wire control rods up to a sticky piston system so you can control a reactor's temperature with the flick of a lever, or through the use of a thermometer block.
Recipe control rod

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