| Nuclear Fission | Nuclear Fusion |
| Definition | Fission is the splitting of a large atom into two or more smaller ones. | Fusion is the fusing of two or more lighter atoms into a larger one. |
| Natural occurrence of the process | Fission reaction does not normally occur in nature. | Fusion occurs in stars, such as the sun. |
| Byproducts of the reaction | Fission produces many highly radioactive particles. | Few radioactive particles are produced by fusion reaction, but if a fission "trigger" is used, radioactive particles will result from that. |
| Conditions | Critical mass of the substance and high-speed neutrons are required. | High density, high temperature environment is required. |
| Energy Requirement | Takes little energy to split two atoms in a fission reaction. | Extremely high energy is required to bring two or more protons close enough that nuclear forces overcome their electrostatic repulsion. |
| Energy Released | The energy released by fission is a million times greater than that released in chemical reactions, but lower than the energy released by nuclear fusion. | The energy released by fusion is three to four times greater than the energy released by fission |
| Nuclear weapon | One class of nuclear weapon is a fission bomb, also known as an atomic bomb or atom bomb. | One class of nuclear weapon is the hydrogen bomb, which uses a fission reaction to "trigger" a fusion reaction. |
| Energy production | Fission is used in nuclear power plants. | Fusion is an experimental technology for producing power. |
| Fuel | Uranium is the primary fuel used in power plants. | Hydrogen isotopes (Deuterium and Tritium) are the primary fuel used in experimental fusion power plants. |
