Control rods

Rods containing strong neutron absorbers, such as boron or hafnium, that operators insert or withdraw to change a reactor's reactivity: inserting them lowers the chain reaction's growth rate, withdrawing them raises it. A full, fast insertion to shut the reactor down is called a scram or trip. Several small reactor designs use rotating control drums with an absorbing face instead of rods, and pressurised water reactors also adjust boron dissolved directly in the coolant for slower, longer-term control.

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concept:control-rods

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