Control Rods
Definition and meaning of Control Rods in chemistry.
Control rods are rods made of strongly neutron-absorbing materials that are inserted into or withdrawn from a nuclear reactor core to regulate the rate of nuclear fission. By capturing free neutrons, they control the neutron flux and thus the reactor's power output.
In more detail
Each fission event releases several neutrons that can trigger further fissions; control rods remove some of these neutrons from the chain reaction before they cause additional splits. Inserting the rods further into the core absorbs more neutrons and reduces reactivity, while withdrawing them allows the chain reaction to proceed faster. Effective absorber materials, such as boron-10 and cadmium-113, are chosen for their very high thermal neutron capture cross sections. Control rods also serve as the primary emergency shutdown mechanism, rapidly halting the chain reaction when fully inserted.
Key facts
| Field | Inorganic Chemistry |
|---|---|
| Common absorber materials | B4C (boron carbide), Cd, Hf, Ag-In-Cd alloy |
| Function | Absorb neutrons to control the fission chain reaction |
| Key property | High thermal neutron capture cross section |
In a pressurized water reactor, control rods containing boron carbide (B4C) are lowered between the fuel assemblies; adjusting their depth keeps the fission rate steady and allows operators to shut the reactor down quickly if needed.
Frequently asked questions
Why is boron used in control rods?
The isotope boron-10 has an unusually high thermal neutron capture cross section, so boron carbide efficiently absorbs neutrons and slows the fission chain reaction.
What happens when control rods are fully inserted?
Full insertion absorbs enough neutrons to make the reactor subcritical, stopping the sustained chain reaction, as in an emergency SCRAM shutdown.