Nuclear Fission vs Nuclear Fusion- Definition, 12 Differences, Examples

Nuclear Fission Definition

Nuclear fission is a type of nuclear reaction or a radioactive decay process where the nucleus of a heavy atom divides into two or more roughly equal nuclei.

  • Nuclear fission might occur spontaneously or might be induced by the excitation of the nucleus.
  • Nuclear fission results in the release of a large amount of energy, radioactive products, and several neutrons.
  • The resulting fragments after nuclear fission have a combined mass that is less than the original mass as some of the mass is converted into nuclear energy.
  •  The energy released from nuclear fission can occur in the form of electromagnetic radiation as well as kinetic energy.
  • Nuclear fission is a type of nuclear transmutation as the resulting fragments from the fission are different from the original atom.
  • Most nuclear fissions are binary fissions producing two equal halves; however, some fissions result in three or more fragments.
  • Such fission reactions can result in a chain reaction where a large number of nuclei undergo fission in a sequence as a result of the emitted neutrons from the previous fission.
  • The nuclear chain reactions produce an enormous amount of energy which can be used to power communities when controlled but can also result in an explosion if left uncontrolled.
  • The products of nuclear fission are primarily unpredictable, which distinguishes the process from other similar reactions like alpha decay and cluster decay, where the same products are produced each time.
  • Nuclear fission can occur spontaneously in the form of radioactive decay or as nuclear reaction as a bombardment process.
  • Common heavy elements that undergo both spontaneous fission and induced fission include Uranium, thorium, and plutonium.
Nuclear Fission vs Nuclear Fusion
Nuclear Fission vs Nuclear Fusion

Nuclear Fusion Definition

Nuclear fusion is a type of nuclear reaction where two or more light atomic nuclei combine to form heavier nuclei and subatomic particles.

  • Nuclear fusion can either release large amounts of energy to absorb energy depending on the resulting nuclei. Usually, if a fusion process produces nuclei lighter than iron or nickel, energy is released.
  • Thus, the nuclear fusion of lighter nuclei is an exothermic reaction, whereas that of heavier nuclei results in an endothermic reaction.
  • There is a difference in the combined mass of the lighter nuclei and the produced heavier nuclei as some mass is released in the form of energy.
  • The release of energy in the case of lighter nuclei is due to the interplay of two opposite forces; nuclear force between the protons and the neutrons and the Coulomb force between protons.
  • Lighter nuclei are usually small, and thus the nuclear attraction can overcome the Coulomb force repulsion, resulting in extra energy from the net attraction.
  • Nuclear fusion occurs in stars and most of the elements in the process known as nucleosynthesis.
  • In stars like Sun, most of the energy is generated by nuclear fusion between the hydrogen nuclei to form helium.
  • The energy resulting from nuclear reactions like nuclear fusion is greater than the chemical reactions as the binding energy between nuclei is greater than the energy holding the electrons to a nucleus.
  • Fusion, like fission, can be induced and requires high temperature and high-pressure conditions. 
  • Nuclear fusion is of two types; nuclear fusion involving preserving the number of protons and neutrons, and nuclear fusion involving conversion between protons and neutrons.

12 Major Differences (Nuclear Fission vs Nuclear Fusion)

Characteristics Nuclear Fission Nuclear Fusion
Definition Nuclear fission is a type of nuclear reaction or a radioactive decay process where the nucleus of a heavy atom divides into two or more roughly equal nuclei. Nuclear fusion is a type of nuclear reaction where two or more lighter atomic nuclei combine to form heavier nuclei and subatomic particles.
Type of Reaction Nuclear fission reactions involve the breakdown of heavier nuclei into smaller fragments. Nuclear fusion reactions are combination reactions forming heavier nuclei.
Occurrence Nuclear fission reactions are not common in nature. Nuclear fusion reactions are common in stars like Sun.
Requirement Nuclear fission can be induced in the presence of high-speed neutrons. Nuclear fusion can be induced under high temperature and high-pressure conditions.
Release of Energy Fission reactions produce a high amount of energy. Fusion reactions can be endothermic or exothermic, depending on the size of the nuclei.
Chain reactions Nuclear fission can induce a chain reaction. Chain reactions are not involved.
Control Nuclear fission can be controlled under the induced condition. Nuclear fusion reactions are difficult to control.
Products A large number of radioactive products are produced as a result of nuclear fission. No radioactive products are produced in nuclear fusion.
Nuclear Waste Nuclear fission produces nuclear waste. No nuclear waste is left after nuclear fusion.
Raw Material The raw materials for nuclear fission are not easily available and are generally costly. The raw materials for nuclear fusion are comparatively cheap and readily available.
Nuclear Weapon Nuclear fission is the mechanism involved in the atomic bomb. Nuclear fusion is the mechanism involved in the hydrogen bomb.
Examples The fission of uranium-235 is an example of nuclear fission. The fusion of deuterium and tritium to form helium-4 is an example of nuclear fusion.

Examples of Nuclear Fission

Fission of Uranium-235

  • Uranium-235 (235U) is an isotope of Uranium that can be broken down by the bombardment of high-speed neutrons.
  • Fission of uranium-235 occurs in the form of a chain reaction beginning with an α-decay followed by both α and β-decay.
  • The fission of uranium-235 results in thorium-231, which is further broken into Palladium-105. The process occurs in the form of a chain reaction with final products like iodine, cesium, strontium, xenon, and barium.
  • The fission of one atom of uranium-235 releases about 202.5 MeV of energy. The energy produced from Uranium has been used to run heavy water reactors as well as some graphite-moderated reactors.
  • Uranium-235 has also been used in explosives, with the most well-known example being the Little Boy gun-type atomic bomb dropped on Hiroshima during World War II.
  • These days, uranium-235 is used as fuel for nuclear power plants as well as to power artificial satellites.

Examples of Nuclear Fusion

Fusion of Deuterium and Tritium

  • The fusion of hydrogen atoms on the Earth’s surface is the classic example of nuclear fusion. As the gravity of the Sun is very high, it traps hydrogen from the atmosphere, which fuels the fusion reaction on the surface.
  • The fusion occurs between deuterium (2H) and tritium (3H), which results in the formation of helium-4 and a neutron while releasing 17.59 MeV in the form of kinetic energy.
  • The hydrogen gas in the Sun’s core becomes plasma, where the negatively charged electrons of the atoms become separated from the positively charged nuclei.
  • The hydrogen nuclei then collide at high speed as a result of gravitational force, which overcomes the natural electrostatic repulsion between the charges.
  • As a result, heavier helium atoms are formed with the release of energy and a neutron.



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