Nuclear energy

Nuclear Fission, What It Is, How It Works and Examples

Nuclear fission, what it is, how it works and examples

Nuclear fission is the physical-chemical reaction by which the nucleus of an atom is split. Through this operation a large amount of energy is obtained.

The nucleus of atoms are made up of other smaller subparticles: protons and neutrons. Depending on the element of the periodic table, the composition of these subparticles varies. Protons have a positive charge, therefore they tend to repel each other. On the other hand, neutrons are neutral, that is, they have no charge.

The subparticles 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 nuclear force, and allow the nucleons to separate.

After the fission of the atomic nucleus we obtain diverse fragments, two or three neutrons and the emission of a great quantity of energy. These fragments are known as 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

Where Does the Energy for Fission Reactions Come From?

In each fission process there is a loss of 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 = mc2 


  • 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 spontaneously due to isotope instability.

What Is the Chemical Element Used in a Nuclear Fission Reaction?

The material used as nuclear fuel has a very unstable atomic structure. Uranium and plutonium isotopes are generally used. The characteristics of these atoms are that they are very heavy, with a large number of 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 the entire structure and break it.

Nuclear Fission Chain Reactions

A chain reaction is a process by which neutrons that have been released in a first nuclear fission produce an additional fission in at least one more nucleus. This atomic nucleus fission and in turn releases more fast neutrons giving the opportunity to repeat the process.

Nuclear fission, what it is, how it works and examplesNeutrons are good projectiles to hit the nucleus because they have no electrical charge and the atomic nucleus does not reject them. 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 are those that 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 lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining and will stop.

The amount of critical mass of a fissile material depends on several factors: physical properties, nuclear properties, its geometry and its purity.

How Are Fission Chain Reactions Controlled?

Nuclear fission, what it is, how it works and examples

Neutron absorption elements are used to control the amount of free neutrons in the reaction space. Most nuclear power reactors are controlled by means of 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 through the use of 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 nuclear fissions or be absorbed by control rods.

Spontaneous Nuclear Fission

In spontaneous nuclear fission reactions, the absorption of a neutron is not necessary.

The spontaneous nuclear fission rate is the probability per second that a given atom fission spontaneously. That is, without any external intervention. 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:


Published: December 18, 2009
Last review: October 20, 2021