The control effectiveness of such alloys in water-moderated reactors can approach that of
hafnium and is the control material commonly used in pressurized-water reactors. The alloys
(generally 80% silver, 15% indium, 5% cadmium) can be readily fabricated and have adequate
strength at water-reactor temperatures. The control material is enclosed in a stainless steel tube
to protect it from corrosion by the high-temperature water.
Boron is a useful control material for thermal (and other) reactors. The very high thermal-
absorption cross section of 10B (boron-10) and the low cost of boron has led to wide use of
boron-containing materials in control rods and burnable poisons for thermal reactors. The
absorption cross section of boron is large over a considerable range of neutron energies, making
it suitable for not only control materials but also for neutron shielding.
Boron is nonmetallic and is not suitable for control rod use in its pure form. For reactor use,
it is generally incorporated into a metallic material. Two of such composite materials are
Stainless-steel alloys or dispersions with boron have been employed to some extent in reactor
control. The performance of boron-stainless-steel materials is limited because of the 10B (n,a)
reaction. The absorption reaction is one of transmutation, 10B + 1n 7Li + 4a, with the a-particle
produced becoming a helium atom. The production of atoms having about twice the volume of
the original atoms leads to severe swelling, hence these materials have not been used as control
rods in commercial power reactors.
The refractory compound boron carbide (B4C) has been used as a control material either alone
or as a dispersion in aluminum (boral). These materials suffer from burnup limitation. The
preferred control rod material for boiling-water reactors is boron carbide. Long stainless-steel
tubes containing the powdered boron carbide combined into assemblies with cruciform cross
sections make up the control rods. Control rods of this nature have been used in PWRs, BWRs,
and HTGRs and have been proposed for use in fast breeder reactors employing oxide fuels.
Because of its ability to withstand high temperatures, boron carbide (possibly mixed with
graphite) will probably be the control material in future gas-cooled reactors operating at high
In addition to its use in control elements, boron is widely used in PWRs for control of reactivity
changes over core lifetime by dissolving boric acid in the coolant. When this scheme is used,
the movable control elements have a reactivity worth sufficient to go from full power at
operating temperature to zero power at operating temperature. At the beginning of life, enough
boric acid is added to the coolant to allow the reactor to be just critical with all rods nearly
completely withdrawn. As fuel burnup takes place through power operation, the boric acid
concentration in the coolant is reduced to maintain criticality. If a cold shutdown is required,
additional boric acid is added to compensate for the reactivity added as the moderator cools.
This method is generally referred to as chemical shim control.