Nuclear Physics

Nuclear fission chain reactions : definition and examples

Nuclear fission chain reactions : definition and examples

Nuclear fission is the physical-chemical reaction by which the nucleus of an atom is split into two smaller nuclei. Through this process, we can obtain a large amount of energy.

The nucleus of atoms comprises other smaller sub-particles: protons and neutrons. Depending on the element of the periodic table, the composition of these sub-particles varies. Protons have a positive charge; therefore, they tend to repel each other. On the contrary, neutrons are neutral, meaning they have no charge.

The sub-particles of the atom are held together thanks to a nuclear force that holds them together. The nuclear force is a hundred times stronger than the electromagnetic force.

The purpose of a nuclear fission process is to alter this balance of forces, break this atomic force, and allow the nucleons to separate.

After the fission of the atomic nucleus, we obtain diverse fragments, two or three neutrons, and a  significant quantity of energy emission. These fragments are fission products that, having changed their proton composition, are different chemical elements.

Fission should not be confused with nuclear fusion, which is a way of obtaining energy from the fusion of two light atoms. The reactions that take place in the Sun are of nuclear fusion

Nuclear fission energy and Einstein’s equation

In each fission process, there is a loss of the total mass: the sum of the masses of the fission products is less than the original mass of the atom. The missing mass is converted into energy according to Einstein's equation:

E = m·c


  • “E” is the energy obtained.

  • “m” is the "lost" mass.

  • “c” is a constant, the speed of light, which is 299,792,458 m / s2.

The energy resulting from a fission reaction is in the form of heat.

Nuclear fission can occur when a nucleus of a heavy atom captures a neutron or naturally without any interaction due to isotope instability. Nuclear reactors use free neutrons to undergo fission reactions. Controlling the number of neutrons inside the reactor, they control its power.

What is the chemical element used in a nuclear fission reaction?

The material used as a nuclear fuel has a very unstable atomic structure. Uranium and plutonium isotopes are ideal for this purpose because they arecumbersomey, with many positively charged protons in the nucleus.

Isotopes are atoms of the same element but with different numbers of neutrons. For example, the isotope of uranium-235 is more unstable than natural uranium.

Having so many positively charged protons, the nucleus has a hard time maintaining the force bonds to hold them together. For this reason, the collision with a single neutron is enough to destabilize and break the entire structure.

What is a chain reaction in nuclear fission?

A chain reaction in nuclear fission is the repetitive process by which neutrons released in a first nuclear fission bump into other atoms and produce additional fissions. This atomic nucleus fission and, in turn, releases more fast neutrons giving the opportunity to repeat the process.

Nuclear fission chain reactions : definition and examplesNeutrons aresuitable projectiles to hit the nucleus because they have no electrical charge, and the atomic nucleus does not reject. However, fast neutrons can become slow neutrons when they collide with particles in a moderator. Slow neutrons are more likely to hit the nucleus of another fuel atom.

These chain reactions can be controlled or uncontrolled.

  • Controlled reactions occur in a nuclear reactor to generate electrical energy.

  • Uncontrolled reactions cause the detonation of an atomic bomb.

What is the critical mass?

Critical mass is the minimum amount of fissile material for a nuclear chain reaction to take place.

Although two to three neutrons are produced in each nuclear fission, not all neutrons are available to continue the fission reaction; some are lost. If the neutrons released by each nuclear reaction are lostfastere than they are formed by fission, the chain reaction will not be self-sustaining and will stop.A fissile material's critical massl depends on several factors: physical properties, nuclear properties, geometry, and its purity.

How are fission chain reactions controlled?

Nuclear fission chain reactions : definition and examples

Neutron absorption elements are used to control the number of free neutrons in the reaction space. Most nuclear power reactors are controlledusingf control rods made of a material that has the property of absorbing free neutrons, for example, boron or cadmium.

In addition to the need to capture neutrons, neutrons often have a lot of kinetic energy. The speed of these fast neutrons is reduced by using a neutron moderator, such as heavy water and running water.

Some nuclear reactors use graphite as a moderator, but this design has several problems. Once fast neutrons have slowed down, they are more likely to produce more atomic fissions or be absorbed by control rods.

Spontaneous nuclear fission

In spontaneous nuclear fission reactions, a neutron's absorption is unnecessary.

The spontaneous nuclear fission rate is the probability per second that a given atom fission without any interaction; it means, without any external intervention. For example, plutonium 239 has a very high spontaneous fission rate compared to the spontaneous fission rate of uranium 235.

Examples of nuclear fission

Here are some examples of nuclear fission reactions:

  • In a nuclear power plant to generate electricity. These types of power plants take advantage of the heat generated to create steam to drive steam turbines connected to an electrical generator.

  • In the propulsion of a nuclear submarine, they can use the same principle as in a nuclear power station. However, in this case, they use the turbines' mechanical energy to drive helices.

  • At the detonation of an atomic bomb. The heat emitted is enough to initiate a fusion reaction in the so-called H-bomb.

  • Nuclear industry can obtainin plutonium atoms artificially from a uranium nucleus. This process takes place in nuclear reactors because after the splitting of the atom, one of the possible atoms generated can be plutonium.

Publication Date: December 18, 2009
Last Revision: October 20, 2021