Rod Control Systems

Control Rods

All reactors have some form of control rods that can be inserted to shutdown the reactor. The rods generally have one of 2 designs:

• rod control cluster assemblies of 16-20 rods that can be inserted into selected fuel assemblies, or
• cross or H shaped rod designs that can be inserted between a number of fuel assemblies.

Typically, PWRs use rod control clusters and BWRs use the cross rod design, as shown in the figures below..

Courtesy Westinghouse	<<<== Rod control cluster
Cross rod design ===>>>	Courtesy General Electric

The control rods contain the neutron "poison" (i.e. prevents neutrons from causing fission). Materials as boron carbide, alloys of silver-indium-cadmium, and hafnium are used. The neutron absorber material is typically surrounded by a stainless steel shell.

PWRs typically use the silver-indium-cadmium alloy and BWRs use boron carbide.

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