Control Rods
Movable rods composed of high-neutron-capture elements (like cadmium, boron, or hafnium) inserted into a nuclear reactor core to regulate chain reaction rates.
Governing formula
Neutron Capture Reaction: ¹⁰B + n → ⁷Li + α
SI unit
Reactivity Control (pcm)
In depth
Inserting control rods increases neutron absorption, reducing the effective neutron multiplication factor k (subcritical, k < 1) to shut down or lower reactor power. Withdrawing rods increases reactivity (critical, k = 1).
Examples in the real world
Boron carbide control rods adjusting reactor power output or executing automatic scram emergency shutdowns.