What Is a Nuclear Reactor?

A nuclear reactor is a facility capable of converting nuclear energy into thermal energy. The reactors can initiate, control, and maintain the nuclear chain reactions in this facility's core.

What is a nuclear reactor?

In a common nuclear power plant, nuclear reactors are used to produce thermal energy and generate water vapor. The way to create this amount of heat is through fission reactions (splitting atoms). The generated steam at a high pressure drives the turbines to produce electricity.

To build a nuclear reactor is necessary to have sufficient nuclear fuel, which we call critical mass. Having sufficient critical mass means having enough fissile material in optimal condition to sustain a nuclear fission chain reaction.

When the reactor's neutron production exceeds losses, characterized by increasing power level, it is considered "supercritical." When losses dominate, it is considered "subcritical" and exhibits are decreasing power.

The neutron absorbers and control rods' arrangement allows controlling the chain reaction and the shutdown and start-up of the nuclear reactor.

The nuclear chain reaction is produced and maintained in the core of the reactor to heat the water used to power the turbines of the plant.

There are several kinds of reactors, but the most used around the world are light water reactors:

  • Pressurized water reactors (PWR).

  • Boiling water reactors (BWR).

Only in the United States, there are 95 commercial nuclear reactors producing electricity.

What Are the Parts of a Nuclear Reactor?

A nuclear reactor is made up of the following components:

Nuclear Reactor Core

The reactor core is made up of fuel rods with a characteristic geometric shape. It is cooled by a fluid, usually water.

In some nuclear reactors, the core is located inside a water pool, about 10 to 12 meters deep, or inside a pressure vessel made of steel.

A reactor vessel is the pressure vessel containing the nuclear reactor coolant, core shroud, and reactor core.

Nuclear Fuel Rods

What is a nuclear reactor?

They are the physical place where nuclear fuel is confined. Some fuel rods contain uranium mixed in aluminum in the form of flat sheets. These sheets are separated by a certain distance that allows the circulation of heat sink fluid.

The sheets are located in a kind of box that serves as a support.

Nuclear Fuel

Nuclear fuel is a material with the ability to fission sufficiently to reach critical mass. That is, to maintain a nuclear chain reaction. It is positioned so that the thermal energy that produces this atomic reaction can be quickly extracted.

What is a nuclear reactor?Solid nuclear fuel is used in nuclear power plants. Nuclear fuels vary depending on the type of reactor, but generally, uranium fuel derivatives or plutonium are used.

Natural uranium is mined from uranium mines, but it is not radioactive enough to be used directly in a reactor. Natural uranium undergoes an enrichment process to obtain more unstable isotopes to increase the coefficient of reactivity.

Control Rods

Control rod bundles provide a quick means of monitoring chain reactions. These bars allow rapid changes in reactor power and its eventual shutdown in case of emergency.

The control rods are made of neutron-absorbing materials and are usually the same dimensions as the fuel elements. 

The fuel assemblies in the core are arranged in rows of 10 aligned along with linear support structures.

The core reactivity coefficient is increased or decreased by raising or lowering the control rods. By submitting or reducing them, the presence of neutron absorbing material in the nucleus is modified.

What is a nuclear reactor?

In typical operation, a nuclear reactor has the control rods fully or partially removed from the core.

The design of nuclear power plants is such that in the event of a failure in a reactor safety or control system, it always acts in the sense of maximum safety, fully inserting all the control rods into the reactor core.

This action brings the nuclear reactor to a safe shutdown in a few seconds.

Nuclear Moderator

The neutrons resulting from a nuclear fission reaction have high kinetic energy. The higher their speed, the less likely they are to fission other atoms, so it is convenient to reduce this speed to encourage new chain reactions.

The moderator's function is to reduce the neutrons' kinetic energy through the neutrons' elastic collisions with the moderator's nuclei.

Reactor Coolant

The reactor uses a coolant to take advantage of the thermal energy given off by nuclear fission reactions. The coolant's function is to absorb this heat energy and transport it to other plant elements and generate electricity.

The coolant must be anti-corrosive, with a high heat capacity, and must not absorb neutrons.

Neutron Reflector

A certain number of neutrons tend to escape from the region where they are produced in a nuclear chain reaction. This leakage of neutrons can be minimized with a reflector medium that redirects them within the reaction region.

The reflective medium that surrounds the nucleus must have a low effective capture section to reduce the number of neutrons and that as many as possible of them are reflected.

Nuclear Reactor Shield

When the reactor is in operation, a large amount of radiation is generated. Shielding is used to protect and isolate facility workers from radioactivity caused by fission products.


Published: July 18, 2012
Last review: January 31, 2021