ANS:A - Cobalt

Cobalt is not typically used as the control rod material in a nuclear reactor. Control rods are an essential safety feature in nuclear reactors. They are inserted into or withdrawn from the reactor core to control the rate of the nuclear reaction by absorbing neutrons. The choice of material for control rods is crucial as it determines the effectiveness and stability of reactor control. Common materials used for control rods include:

1. Boron: Boron is widely used in control rods because of its high neutron absorption cross-section, meaning it effectively absorbs neutrons, thereby controlling the reactor's power output.
2. Cadmium: Cadmium is another material commonly used in control rods due to its high neutron capture cross-section, making it effective in regulating reactor power.
3. Hafnium: Hafnium has also been used in some types of nuclear reactors as a control rod material due to its excellent neutron-absorbing properties.
4. Cobalt: While cobalt is a material that can absorb neutrons, it is not commonly used as a control rod material in nuclear reactors. This is primarily because cobalt has other important applications in nuclear engineering, such as being a component of certain alloys used in reactor components or as a radiation source in medical and industrial applications.