A nuclear chain reaction is a process in which nuclear reactions repeat in a chain. For example, when a nucleus of uranium-235 is hit by a neutron, it splits into two small nuclei barium (Ba) and krypton (Kr), and produces three neutrons.

nuclear chain reaction equation

Each of the neutrons produced in the above reaction hits another nucleus of U-235, which results in further nine such neutrons, and then it proceeds to twenty-seven reactions. These reactions further give rise to more and more nuclear reactions. This process of continuous production of three neutrons and an exponential increase in reactions is called a nuclear chain reaction.

nuclear chain reaction

The nuclear chain reaction continues until all of the nuclear material is fissioned. All the neutrons produced in this reaction are used to speed up (increase) the chain reaction. Some neutrons are also lost to the surroundings, therefore, for maintaining a chain reaction, it is necessary to keep emitted neutrons hitting other unstable nuclei.

The fissionable material should be enough to capture the neutrons for a chain reaction to continue as well. For example, most of the neutrons escape before striking the surface of the sample and break the chain reaction, if this sample is too small. This is why a critical mass is set for every material, for a chain reaction to take place.

The minimum amount of mass of fissionable material required to maintain a chain reaction is called critical mass. The value of critical mass is different for each reaction. For example, the critical mass value for the uranium-235 fission reaction is about 10 kgs.

Chain reaction as a source of energy

There is a production of a large amount of energy when a fission reaction takes place. However, a chain reaction consists of various numbers of fission reactions that produce a very large amount of energy. For example, uranium-235 or plutonium-239 produce a vast amount of energy when undergoing a nuclear chain reaction. The energy released by fission chain reaction is called nuclear fission energy or nuclear energy.

The fission chain reaction of U-235 or Pu-239 occurs immediately. This produces a huge amount of energy in the form of heat and radiation. This energy can be useable in controlled ways. For example, In a controlled chain reactor, electricity can be produced. However, if the chain reaction is uncontrolled, it is an atomic bomb.

Types of Nuclear Chain Reactions

There are two main types of nuclear chain reactions:

  • Uncontrolled or fast chain reactions
  • Controlled or slow chain reactions

Uncontrolled chain reactions

This type of nuclear chain reaction is used in atomic (nuclear) bombs. This is an uncontrol way to produce an enormous amount of energy which then vaporizes everything it gets in touch with. There is a specific design of a bomb to control such kind of energy. This contains two subcritical masses of fissionable samples such as U-235 or Pu-239.

It is designed in such a way that it has separate packets. A pocket contains a measured mass of trinitrotoluene (TNT). When trinitrotoluene is detonated, it directs one critical mass of uranium into another critical mass. This results in achieving a supercritical mass and in this way, an uncontrolled chain reaction occurs that explodes with the release of a huge amount of heat energy. The temperature inside the bomb can reach about 10 million degrees Celcius which is the temperature of the sun’s core.

nuclear chain reaction, subcritical masses of uranium

Besides heat and energy, it also releases gamma rays and radioactive nuclei. These radioactive materials adhere to the dust particles which then spread over a wide range and become a source of radioactive hazards for a long period of time.

On 6 August 1945, the first-ever atomic bomb, Little Boy was used against Hiroshima (city of Japan) during World War II. It exploded with the force of thirteen thousand tons of TNT. The second atomic bomb was Fat Man. It was dropped on Nagasaki (another city in Japan) on 9 August 1945. The fuel of Fat Man was plutonium which is even more explosive than uranium. Fat Man generated a force that was equal to twenty kilotons of trinitrotoluene. Little Boy and Fat Man are considered the worst and most massive killings in the history of human beings.

Controlled Chain Reactions

The nuclear chain reaction can be controlled in nuclear reactors. The energy obtained from the nuclear reactor is mainly used for power generation i.e. electricity production. Nuclear reactors are specially designed power plants for controlled fission chain reactions.

Nuclear reactors are composed of several components:

Fuel rods

These rods are usually composed of uranium-235. When the fission reaction of U-235 occurs, it produces neutrons that lead to a chain reaction. These rods are the source of nuclear reaction for nuclear reactors.


They are used to slow down the fast-moving neutrons. For example, heavy water and graphite are used for this purpose. The neutrons slow down due to losing energy when they collide with the moderators.

Control rods

These rods are used to control the rate of nuclear reactions. They are composed of boron-10 or cadmium and work by absorbing the slow-moving (emitted) neutrons. This way, they control the energy production and take control of the nuclear reactions, so that they do not go uncontrolled (atomic bomb).


When a fission reaction occurs, it produces a large amount of heat energy in the core. This heat is removed by coolant which keeps on changing with the ongoing nuclear reaction. For example, ordinary or hard water is used as a coolant.

Concrete shield

The outer boundary or shield of a nuclear reactor is made necessarily of concrete. It is a very strong shield and serves many purposes.

  • Concrete is a good radiation absorber, thus it protects the outer environment from the harmful effects of continuously released radiations from a nuclear reactor.
  • In case, if any accident happens, it protects (up to some extent) the operating employees and equipment from destruction.

Nuclear Power plants

Most nuclear reactors that work today are light water reactors. These reactors are basically power plants. They are designed in such a way that fuel rods are dipped in water as it has the power to act as a coolant and a moderator. There are spaces between the fuel rods which are filled by control rods (boron-10 or cadmium).

When a fission chain reaction occurs. It generates a large amount of heat. This heat energy goes to the fuel core where it is absorbed by the coolant (water). After some time, the water gets heated up to about 300 °C. This water (steam) then goes to an exchanger where it transfers heat to a large water body e.g. seawater, which gets converted into vapors phase (steam). This steam is used to drive the turbines which generate electricity.

light water nuclear reactors

The waste products of nuclear reactors are very radioactive. For example, barium-139 and krypton-92. They keep emitting harmful radiations for several years and therefore, are dangerous for living beings. That is why nuclear waste is the only disadvantage and the main concern regarding nuclear power plants because there is no place we can dump this radioactive waste.

Almost 19% of electricity in the USA is generated by nuclear reactors.

Additional articles:

Concepts Berg

How did the first nuclear reactor come to be?

Chicago Pile-1 (CP 1) was the first artificial nuclear reactor. It was manufactured in 1942 by scientist Enrico Fermi who was awarded by Nobel prize for his achievement.

How can a nuclear reaction become a chain reaction?

Neutrons break the nucleus of uranium into two daughter nuclei with a release of a large amount of energy. There are also three further neutrons produced which in turn react with the other three nuclei and so on. This creates a chain reaction.

What are the types of nuclear chain reactions?

  • Controlled chain reactions
  • Uncontrolled chain reactions

What is an example of a chain reaction?

The breakdown of the uranium nucleus into barium and krypton is the best example of a chain reaction.

What properties make a nuclear chain reaction self-sustaining?

The nuclear chain reaction can be self-sustained until the number of neutrons produces by fission becomes equal to the number of neutrons lost by escaping. The speed of neutrons is also a factor for nuclear reactions to be self-sustaining.

How are nuclear chain reactions controlled?

A nuclear chain reaction can be controlled by:

  • Control rods
  • Moderators
  • Coolant

How is boron used in nuclear reactors?

Control rods usually composed of boron-10 are employed in nuclear reactors as control rods. They are adjustable/moveable. They are used between the fuel rods of uranium to control the rate of the fission reaction.

What is a fission reaction?

When a large nucleus breaks down into two small daughter nuclei along with the production of a large amount of energy, it is called a fission reaction.

What is a fusion reaction?

When two small nuclei come together to form a large nucleus, along with the production of a very large amount of energy, it is called a fusion reaction. Fusion reactions require a large amount of energy to initiate.


  • Chemistry | Fifth edition, by Steven S. Zumdhal and Susan A. Zumdhal (University of Illinois, Urbana Champaign, IL, USA)
  • Essential of Physical Chemistry: 2nd edition By B.S Bahl (Gurdaspur, India) and Arun Bahl (RSC, UK) and G.D. Tuli (Delhi University, India)
  • Nuclear chain reactions (atomicarchive.com)