A nuclear reaction is a change in the composition of an atomic nucleus. This change occurs in the numbers of protons and neutrons. As a result of this, a new product (element) is formed with different atomic and mass numbers. Such reactions are either fission (the splitting of a nucleus into daughter nuclei) or fusion reactions ( the merging of nuclei to produce a large nucleus) with the release of an enormous amount of energy.

In fission reactions, the heavier nucleus splits into two smaller daughter nuclei and neutrons. The products of this reaction are radioactive in nature. While in fusion reactions, smaller or lighter nuclei fuse to give heavier nuclei. The products here are non-radioactive.

Difference between fission and fusion reactions

Fission Reaction Fusion Reaction
Heavier nucleus splits into lighter nuclei in fission reactions Lighter nuclei fuse to give a heavier nucleus in fusion reactions
It is a chain reaction, but it can be controlled It is not a chain reaction
It requires lower temperature
It requires extremely high temperature i.e. 1000000 oC
Fission reactions can be controlled by control rods Fusion reaction, once started, cannot be controlled
Released energy can be used for further process Released energy can not be used properly
The products of fission reaction are radioactive Their products of fusion reaction are non-radioactive
It is not a source of solar energy It is a source of solar energy
It produces nuclear waste It does not produce nuclear waste at all
Nuclear reactors and atomic bomb are examples of fission reaction CERN particle collider and hydrogen bomb are examples of fusion reaction

Process of Nuclear Fission

In 1939, two scientists Hahn and Strassmann discovered fission. They observed that a heavy nucleus splits into daughter nuclei when neutrons are bombarded on it. For example, uranium-235 absorbs a neutron and becomes a compound nucleus that is unstable. This unstable nucleus splits into daughter nuclei and releases further neutrons and a large amount of energy.

The splitting of a heavy nucleus into two or more daughter nuclei is called a fission reaction. The products that are formed as the result of fission are known as fission products.

For example, uranium 235 can undergo fission reaction in the following ways;

fusion vs fission

The above fission reactions show a minute decrease in the overall mass of products. This loss is about 0.2 amu per uranium atom. This is known as ‘mass defect’. This mass corresponds to the binding energy of the system. This lost mass converts into a great amount of energy that is about 2.5 million times greater than the energy produced by the same amount of coal.

Fission reactions and fusion reaction

Chain reaction (fission process)

When neutrons are bombarded on uranium-235, it breaks it into barium (Ba) and krypton (Kr) along with releasing three neutrons.

fission reaction of uranium

Each of these three neutrons strikes another uranium-235 nucleus which results in the emission of 9 neutrons. These 9 electrons then undergo similar reactions until a large bulk of neutrons is produced after some consecutive reactions. This process of propagation continues in powers of 3 like 3, 9, 27, 81, etc. This makes a long chain of reactions, known as, fission chain reaction.

It is not necessary that all the neutrons produced in the chain reaction are used to propagate further reactions. Most of them are lost in their surroundings. That is the reason why, to start a chain reaction, the sample must be contain fissioned material. Its size should be large enough to capture the neutrons internally. If the sample is too small, most of the neutrons would strike the surface and escape. As result, the chain reaction would break. The minimum amount of the mass of fissionable material required to sustain a chain reaction is called its critical mass. It varies for each reaction. For example, to start a fission chain reaction of U-235, the critical mass is about 10 kg.

chain reactions

A huge amount of energy is released when U-235 or Pu-239 undergo a chain reaction. This energy is called nuclear fission energy or nuclear energy.

Examples of fission reactions

Fission reactions can be accomplished by bombarding neutrons, alpha particles, protons, and deuterons on atoms. The positively charged species are excited to high kinetic energy with the help of a device called, a cyclotron. When the nucleus absorbs these particles, they become unstable and decompose to give the fission products along with energy.

Some examples of such reactions are shown here:

  • Bombarding of alpha particles on a beryllium atom creates carbon-12 and a neutron.

alpha particles bombared on beryllium

  • Bombarding of a proton on a lithium atom creates a beryllium atom and a neutron.

reaction of fisssion

  • Bombarding of deuteron on a sodium atom creates Na-24 (an isotope of sodium) and a proton.

fission reaction

  • Bombarding of neutron on nitrogen creates C-14 (an isotope of carbon) and a proton.

reaction of fission

Properties of the fission reactions

Fission reactions have the following basic properties:

  1. A heavy nucleus break into two or more nuclei.
  2. Fission produces two or more neutrons from each nucleus.
  3. By using a small mass of fissionable material, a vast amount of energy can be produced.
  4. The resultant fission products are radioactive. They release beta and gamma radiations.
  5. An uncontrolled fission reaction is named an ‘atomic bomb’.
  6. Boron (B) and cadmium (Cd) rods are used in fission reactors as neutron absorbers, which limits the reaction.
  7. A controlled fission reaction is used to produce energy of all sorts. For example, nuclear reactors are a major necessity these days for producing electricity.
  8. After fission reaction, the fission products i.e. radioactive smaller nuclei are impossible to get rid of. There is nowhere we can dump them off. So, radioactive waste disposal is a major concern for authorities.
  9. Uranium (U235) and Plutonium (Pu239) are most commonly used for fission reactions because they are easier to initiate and control.
  10. Almost 200 MeV energy is produced in one fission event.

Process of Fusion reaction

In a fusion reaction, two daughter nuclei combine or fuse to form a heavy nucleus. For example, two nuclei of deuterium undergo a fission reaction to give a heavy nucleus of helium-3. This reaction takes place at a very high temperature of 100 million degrees Celsius.

fusion reactions

fusion reaction equation

The collective mass of the reacting nuclei is greater than the mass of the nucleus that is formed. It is because some mass is converted into a high amount of energy which is released in the process. For example,

fusion reaction with mass lost

The total mass of reactants in the above example is 4.02277 amu, while that of the product is 4.00260 amu. The difference is 0.02017 amu which corresponds to the released energy along with binding energy used to make a larger nucleus.

Mechanism of fusion reaction

Let’s take the example of deuterium. At extremely high temperatures, about 100 million degrees Celsius or more, the deuterium atoms are completely exposed to the orbital electrons. This results in a collection of positive nuclei and electrons, called plasma. In such a condition, the kinetic energy becomes very high and overcomes the electrostatic force of repulsion between the same charged particles. The nuclei collide, however, only with great force to give a fused heavy nucleus.

Related topics

Examples of fusion reaction

  • Two hydrogen nuclei, fuse to give a deuterium nucleus along with positron emission.

examples of fusion reactions

  • The deuterium nucleus and tritium nucleus combine to produce a helium-4 nucleus and a neutron.

fusion reaction examples

Properties of fusion reactions

Fusion reactions have the following basic properties:

  1. Daughter nuclei merge to give a heavy nucleus.
  2. A huge amount of energy is released as a product.
  3. It can be used as a good source of energy for many purposes when the starting material is much cheap.
  4. Fusion fuel can be obtained in abundance from heavy water.
  5. Fusion products are not radioactive which gives it an edge over fission reactions.
  6. Although fusion reaction seems more useable, its starting energy requirements are very difficult to bear. They are almost impossible without a very large source of energy, like a fission reaction.
  7. Once this activation energy barrier is removed, fusion reactions will take over, all energy creation systems and there will be no further need to make expensive energy.
  8. Fusion reactions are the reason all stars, including the sun, are burning. The energy released as fusion energy reaches earth and other planets and makes life possible via heat and light.
  9. As fusion reaction already occurs on Sun, it can be regarded as a natural source.
  10. The most common application of fusion reaction on earth is the CERN hadron collider.

Key Takeaway(s)

fission vs fusion the key difference

Concepts Berg

What is the difference between fusion and fission?

The main difference between fusion and fission is their occurrence. Fission does not occur naturally but fusion occurs in stars. Fission requires a low temperature as compared to a fusion reaction. Fission products are radioactive but fusion products are not radioactive in nature. Moreover, fission can be controlled by using moderators but fusion can never be controlled.

Why fusion doesn’t produce energy, yet?

It is very difficult to start fusion and the energy released by fusion is also uncontrollable yet.

What are the effects of fusion on the environment?

Fusion reactions have no effects on the environment. Obviously, every great discovery has a silver lining and in this case, it is a hydrogen bomb. Otherwise, it is beneficial for humans and the environment because it can give us a large amount of energy without any hazardous and radioactive waste.

Which is safer fission or fusion?

In terms of handling, fission is safer as compared to fusion because it is easier to handle. In terms of radioactive waste, fusion reactions are our only option to use.

How do fission and fusion bombs work?

In fission reactions, the heavy nucleus breakdown into two daughter nuclei. While in a fusion reaction, daughter nuclei combine to give a larger nucleus. These processes work with a release of a very large amount of energy. If that energy is uncontrolled, it turns into a bomb i.e. atomic bomb, hydrogen bomb, etc.

Nuclear fusion is safer than nuclear fission, how?

Fusion waste materials are not radioactive as in fission. So, if the energy could be handled somehow, the effect of radioactivity on the environment can be decreased by using nuclear fusion reaction only.

Was Chernobyl fission or fusion?

Chernobyl was a nuclear reactor so, everything that happened was a result of, fission reaction.

Does fission or fusion involve the transmutation of elements?

Transmutation is a process in which one element converts into another element. So both fission and fusion reactions are involved in transmutations.


  • Energy From The Nucleus: The Science And Engineering Of Fission And Fusion edited by Gerard M Crawley (University of South Carolina, USA)