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Nuclear Fission vs. Nuclear Fusion

Both are nuclear processes by which atoms are changed to produce energy. The difference between nuclear fission and nuclear fusion is that a heavy nucleus splits into two small nuclei in nuclear fission while two heavy nuclei combine to form a heavy nucleus in nuclear fusion.

Key Differences

Nuclear fission remains to confine in lighter nuclei while nuclear fusion confines in larger nuclei.
Nuclear fission is an exothermic reaction while nuclear fusion is an endothermic reaction.
Samantha Walker
Jul 10, 2017
Nuclear fission is less expensive while nuclear fusion is more expensive.
A significant amount of energy is released in nuclear fission
Both nuclear fission and fusion processes are opposite to each other.
Starting elements in fission reaction have a higher atomic number than products of the reaction. For example, uranium breaks into strontium and krypton.
Aimie Carlson
Jul 10, 2017
Nuclear fission occurs naturally on earth. For example, spontaneous fission of uranium happens only when enough amount of uranium is present. Nuclear fusion does not take place It happens in stars.
Janet White
Jul 10, 2017
In a fusion reaction, the energy released is very greater than a fission
Samantha Walker
Jul 10, 2017
Products of fusion reaction have more neutrons and protons than starting elements. For example, hydrogen fuses with hydrogen to form helium.
Janet White
Jul 10, 2017

Comparison Chart


Splitting of a large atom into two or more small atoms is called nuclear fission.
Fusion of two or more small atoms into one large atom is called nuclear fusion.

Type of Reaction

It is a chain reaction
It is not a chain reaction


The high temperature is not required to initiate nuclear fission.
Very high temperature is required to start nuclear fusion.


It is occurred by bombarding the heavy nucleus with neutrons.
It is carried out by heating small nuclei at high temperature. Bombarding of neutrons is not required.
Janet White
Jul 10, 2017


Fission means breaking or splitting.
Fusion means combination or union
Samantha Walker
Jul 10, 2017


Nuclear fission is used in nuclear reactors because it can be controlled.
Nuclear fusion is not utilized to produce energy because it cannot be controlled.
Harlon Moss
Jul 10, 2017


Splitting of uranium
The combination of hydrogen nuclei to form helium nuclei, the hydrogen bomb.

What is Nuclear Fission?

Albert Einstein’s prediction that mass could be changed into energy made possible of nucleus division. He started experiments in 1939, and after one year Enrico Fermi built a nuclear reactor. Nuclear fission was discovered by German scientists, Lise Meitner, Otto Hahn and Fritz Strassmann. Nuclear fission takes place when high-speed particles, neutrons are bombarded on the unstable isotope. Neutrons are accelerated and slammed into isotopes which cause fission, break into small particles. During fission process, a neutron is accelerated and hits the target nucleus, which in the majority of a nuclear power reactor is Uranium. It splits the target nucleus and breaks it down into small isotopes, three high-speed neutrons and a large amount of energy. Produced energy is used to heat water in nuclear reactors and generate electricity. High-speed electrons become projectiles that initiate other fission reactions which are called chain reactions. Radioactive wastes are generated as a byproduct of fission reaction whose results take thousands of years to lose their dangerous level. Safeguards must be used with nuclear fission reactors for this waste and its transportation for storage.

What is Nuclear Fusion?

Fusion occurs when two low mass isotopes (mostly isotopes of hydrogen) combines under extreme temperature and pressure. Fusion reaction powers the sun. Atoms of Tritium and Deuterium combines under high temperature and pressure to produce a neutron and helium isotope. Besides this, an enormous amount of energy is released which is much greater than the energy of fission reaction. Researchers continue to work on controlling nuclear fission to make a fusion reactor to produce electricity. Scientists believe that fusion reaction produces less radioactive materials than fission reaction so has unlimited fuel supply which can be used for different opportunities. But it is tough to control reaction in a confined space, so this is a big challenge in its use. Nuclear fusion was achieved in the reaction of hydrogen bomb first time. It is also used in different experimental devices for the production of energy.

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