Difference Between Nuclear Fission and Fusion

Main Difference – Nuclear Fission vs Fusion

Nuclear fusion and nuclear fission are chemical reactions that take place in the nucleus of an atom. These reactions release a very high amount of energy. In both reactions, the atoms are altered, and the end products would be completely different from the initial reactants. Nuclear fusion releases a higher energy than that of nuclear fission. Although nuclear fission reactions are not that much found in the environment, nuclear fusion is found in stars such as the sun. The main difference between nuclear fission and fusion is that nuclear fission is the division of an atom into smaller particles whereas nuclear fusion is the combination of smaller atoms to form a large atom.

Key Areas Covered

1. What is Nuclear Fission
      – Definition, Mechanism, Examples
2. What is Nuclear Fusion
      – Definition, Mechanism, Examples
3. What is the Difference Between Nuclear Fission and Fusion
      – Comparison of Key Differences

Key Terms: Deuterium, Half-Life, Neutron Bombardment, Nuclear Fission, Nuclear Fusion, Nucleus, Radiation, Radioactive Decay, Tritium

What is Nuclear Fission

Nuclear fission is the splitting of a nucleus into smaller particles. These smaller particles are called fragments. Often, the products of nuclear fission include neutrons and gamma rays. A nuclear fission reaction can release a high amount of energy. This reaction may occur in two ways as below.

Neutron Bombardment

This is a nonspontaneous reaction where a large, unstable isotope is bombarded with high-speed neutrons. These accelerated neutrons cause the isotope to undergo fission. First, the neutron combines with the nucleus of the isotope. The new nucleus is more unstable; thus, it undergoes fission reaction. The fission produces more neutrons that can induce other isotopes to undergo nuclear fission. This makes it a chain reaction. This is called “nuclear chain reaction.”

Mechanism – Binary Fission

The nuclear fission occurs through a special mechanism called binary fission. The nucleus of an atom gets a spherical shape due to the presence of nuclear forces between sub-atomic particles (neutrons and protons). When the nucleus captures the accelerated neutron, the spherical shape of the nucleus is deformed. This causes the formation of a shape with two lobes. This lobe formation causes the sub-atomic particles to separate from each other. If the speed of the bombardment is enough, the two lobes can get separated completely, forming two fragments because the nuclear forces are now not enough to hold the lobes together. Here, a very high amount of energy is released. This energy comes from the nucleus, where the strong nuclear forces between sub-atomic particles are converted into energy.

Figure 01: The stages of binary fission of nucleus. Here, the two fragments are considered to be the same size. But, one product is actually smaller than the other product.

Radioactive Decay

This is a spontaneous process. Unstable isotopes undergo radioactive decay. In this process, sub-atomic particles of the nucleus of isotopes are converted into different forms, resulting in a different element. The product is more stable, and the unstable isotopes undergo radioactive decay until all atoms get stable.

In this process, unstable isotopes lose energy by emitting radiation. Radioactive decay may result in radiation composed of alpha particles and beta particles. The decay of radioactive material is measured through a term called “half-life.” The half-life of a material is the time taken by that material in order to become half of its initial mass.

Figure 2: A Nuclear Fission Reaction

The above image shows a nuclear fission reaction that occurs due to neutron bombardment. The neutron hits the Uranium-235 isotope and forms a Uranium-236 atom. It is very unstable. Thus, it is split into Barium-144, Krypton-89, and more accelerated neutrons along with a high amount of energy.

What is Nuclear Fusion

Nuclear fusion is the combination of two smaller atoms to create a large atom, releasing energy. This happens under high temperature and pressure conditions. Sometimes, the nuclei combination will result in more than one large atom. When calculated, there is a mass difference between reactants and products. This missing mass is converted into energy. The difference of mass arises due to the difference in nuclear binding energies.

Nuclear fusion reactions are most commonly found in the sun. The energy released from the sun is a result of nuclear fusion reactions taking place inside the sun. The nuclear binding energy is the energy required to hold protons and neutrons together inside the nucleus. Since protons are positively charged and repel each other, there should be a strong attractive force to hold them together. When it comes to tiny nuclei, there are a less number of protons present; hence, less repulsion occurs. Attraction forces here are higher. Therefore, the binding of nuclei will release extra energy due to the high attraction between two nuclei. But for larger nuclei combinations, no energy is released. This is because there are more protons that cause a high repulsion between two nuclei.

Due to the presence of more protons causing a repulsion between nuclei, nuclear fusion between heavier nuclei are not exothermic. But due to the high attraction forces between protons, lighter nuclei undergo nuclear fusion reactions that are highly exothermic.

Figure 3: Nuclear Fusion Reaction in the Sun

Sun is a star. It produces a high amount of energy in the form of heat and light. This energy comes from the fusion reactions that occur in the sun. The fusion reaction involves the fusion of nuclei of Deuterium and Tritium. The end products given by this reaction are Helium, neutrons and a lot of energy.

Difference Between Nuclear Fission and Fusion

Definition

Nuclear Fission: Nuclear fission is the splitting of a nucleus into smaller particles, releasing a high amount of energy.

Nuclear Fusion: Nuclear fusion is the combination of two smaller atoms to create a large atom releasing energy.

Natural Occurrence

Nuclear Fission: Nuclear fission reactions are not common in nature.

Nuclear Fusion: Nuclear fusion reactions are common in stars such as the sun.

Requirements

Nuclear Fission: Nuclear fission reactions may require high-speed neutrons.

Nuclear Fusion: Nuclear fusion reactions require high temperature and high pressure conditions.

Energy Production

Nuclear Fission: Nuclear fission reactions produce a high energy.

Nuclear Fusion: Nuclear fusion reactions of light nuclei produce a very high energy whereas nuclear fusion reactions of heavy nuclei may not release energy.

Examples

Nuclear Fission:  Neutron bombardment of Uranium-235 and radioactive decay in unstable isotopes are examples of nuclear fisson. 

Nuclear Fusion: Nuclear fusion reactions are most commonly found as the fusion between Deuterium and Tritium.

Conclusion

Nuclear fission and nuclear fusion reactions occur when the nucleus of an atom undergo changes in either spontaneous or non-spontaneous ways. These reactions cause the creation of new elements rather than the initial element. The difference between nuclear fission and fusion is that nuclear fission is the division of an atom into smaller particles whereas nuclear fusion is the combination of smaller atoms to form a large atom.

References:

1.”Nuclear fusion.” Wikipedia. Wikimedia Foundation, 28 July 2017. Web. Available here. 31 July 2017. 
2.”Nuclear Fission.” Hyperphysics concepts. N.p., n.d. Web. Available here. 31 July 2017. 

Image Courtesy:

1. “Nuclear fission” (Public Domain) via Commons Wikimedia
2. “Nuclear fusion” By Someone – Someone (CC BY-SA 3.0) via Commons Wikimedia