Control rods in a nuclear reactor

The control rods in nuclear power plants are the elements that regulate the reactor power and are essential for the safety of the reactor. Without them, the reactor core power would increase uncontrollably.

The nuclear reactions inside a fission reactor are chain reactions, i.e. each time an atom hits a neutron it splits and emits one or two more neutrons. The released neutrons can hit other atoms so that more and more reactions would occur per unit of time.

Control rods are cylindrical tubes made of neutron-absorbing material, such as boron carbide or alloys of silver, indium, and cadmium.

The dimensions of the control rods are the same as those of the nuclear fuel rods.

What are the control rods used for in a nuclear reactor?

Control rods in nuclear reactors allow the frequency of nuclear reactions to be controlled as the absorption of neutrons prevents further fissioning of fuel elements by neutrons.

The effectiveness depends on the proportion of the rods in contact with the reaction zone. The further inside the reactor core, the more neutrons are absorbed, and therefore fewer reactions occur.

How is the power of a nuclear reactor regulated?

Control rods are one of the mechanisms for regulating the status of a nuclear reactor. Often, the rods are the most important element in a quick shutdown of the reactor (so-called emergency shutdown or scram).

In pressurized water reactors, the rods hang in the reactor and can function as a safety system. They are held up by an electromagnet and in the event of an unexpected loss of control power, they automatically drop into the reactor. In this way, the nuclear fission chain reaction is stopped.

In other reactor types, such as the boiling water reactor, the rods protrude from the bottom into the reactor. To activate, they must be actively pushed into the reactor core.

In reality, the reactor control process is more complicated: the course of the reaction also depends on the temperature and cooling of the reactor and the presence of certain debris from the fission process (which sometimes absorbs many neutrons).

When the control rods go in completely (e.g. during an emergency shutdown) the nuclear chain reaction will stop almost immediately and the reactor is said to become sub-critical. However, the radioactive material continues to emit heat for some time, which must continue to dissipate.