Refractory compound:
The refractory compound boron carbide (B4C) has been used as a control material either alone or as a dispersion in aluminum (boral). Those materials suffer from burnup limitation. A preferred control rod material for boiling-water reactors is boron carbide. Long stainless-steel tubes containing the powdered boron carbide merged into assemblies along with cruciform cross sections make up the control rods. A Control rod of this nature has been used in BWRs, PWRs, and HTGRs and has been proposed for use in quick breeder reactors employing oxide fuels. Since of its ability to withstand high temperatures, boron carbide (possibly mixed along with graphite) will possibly be the control material in future gas-cooled reactors operating at high temperatures.
Further to its use in control elements, boron is hugely used in PWRs for control of reactivity changes over core lifetime through dissolving boric acid in the coolant. While this scheme is used, the movable control elements have a reactivity worth enough to go from full power at operating temperature to zero power at operating temperature. At the starting of life, sufficient boric acid is added to the coolant to permits the reactor to be just critical with all rods nearly done withdrawn. As fuel burnup takes place by power operation, the boric acid concentration within the coolant is decrease to hold criticality. Additional boric acid is added to compensate for the reactivity added as the moderator cools if a cold shutdown is required. This method is commonly referred to as chemical shim control.
Boron might also be used as a burnable poison to compensate for the modification in reactivity along with lifetime. Within this scheme, a small amount of boron is incorporated into the fuel or special burnable poison rods to decrease the beginning-of-life reactivity. Burnup of the poison causes a reactivity rise which partially compensates for the decrease in reactivity because of fuel burnup and accumulation of fission products. Difficulties have commonly been encountered while boron is incorporated directly along with the fuel, and most applications have used divided burnable poison rods.