How does a control rod work?

Answer 1

Control rods need to contain material that is a strong absorber of neutrons, so that rapid shutdown can be achieved when necessary. Two such materials are Cadmium and Boron. Both have been used in different types of reactor. The elements may be used as alloys or as chemical compounds, and will be encased in stainless steel to avoid corrosion in the reactor.

Answer 2

Control rods absorb neutrons, reducing the reactor's neutron flux. As the neutron flux declines the fission rate drops. As the fission rate drops the reactor's thermal power drops and its temperature drops.

Remove control rods undoes all that.

They are the "valve" that the operators use to adjust reactor power generation.

Answer 3

They are made of materials that are good at absorbing neutrons, such as boron. They can be lowered into a nuclear reactor to slow or stop the reaction, for example they are designed to drop in automatically if the reactor overheats. They can also be used to speed up the reaction if they are raised.

The reason they can slow or speed up a nuclear reaction is because in nuclear reactions a fuel such as uranium or plutonium undergoes nuclear fission. Nuclear fission is where the nucleus of an atom splits. This can happen in spontaneous fission, but in nuclear reactors it happens in neutron-induced fission. In neutron-induced fission, a neutron is fired at an unstable nucleus, such as that of uranium, which is unstable because it has too much energy and mass. The fired neutron adds to the excess of energy and mass, so it makes the nucleus even more unstable. As a result, the nucleus splits releasing energy, and products are two smaller daughter nuclei, gamma radiation, and two or more neutrons. These two or more neutrons can then go off to add excess mass and energy to more nuclei, causing more nuclei to split - this is a chain reaction. Therefore, the control rods work by absorbing these neutrons so that the reaction doesn't go out of control which would result in a nuclear bomb. The idea is that to keep the reaction going at a steady rate, an average of one neutron resulting form a fission should go on to cause another fission.

Answer 4

The control rods in a nuclear reactor are made of neutron-absorbing material and are used to start a nuclear reactor (by pulling them out), control a reactor (by pulling them enough to set an "operating point" in the core), and to shut the reactor down (by driving or dropping them into the core).

Answer 5

Carbon rods are used. Boron-10 is particularly good, so sometimes boron carbide made from boron-10 is used. Hafnium is used where it can be afforded.