Nuclear fission is a nuclear reaction in which the nucleus of an atom splits into two or more smaller nuclei, along with the release of a significant amount of energy. This process is the opposite of nuclear fusion, where atomic nuclei combine to form a larger nucleus. Fission reactions are typically initiated by bombarding the nucleus of a heavy atom (e.g., uranium-235 or plutonium-239) with a neutron, which causes the nucleus to become unstable and break apart into smaller nuclei, as well as the emission of additional neutrons. These additional neutrons can go on to trigger more fission reactions in nearby nuclei, leading to a self-sustaining chain reaction. Nuclear fission is the process used in nuclear power plants and atomic bombs.
The release of energy in nuclear fission is primarily a result of the conversion of mass into energy, as described by Albert Einstein's famous mass-energy equivalence equation, $$E=mc^2$$
When an atomic nucleus undergoes fission, the total mass of the resulting smaller nuclei, along with the emitted particles (neutrons, protons, and electrons), is slightly less than the mass of the original nucleus. This mass deficit is converted into a tremendous amount of energy. In nuclear power plants, this energy is harnessed to heat water and produce steam, which then drives turbines to generate electricity. The energy released in nuclear fission is millions of times greater per unit mass than that released in chemical reactions
In the nuclear fission of uranium-235 (U-235), a neutron is absorbed by a U-235 nucleus, making it an unstable U-236 nucleus and causing it to split into two smaller nuclei, such as krypton-92 (Kr-92) and barium-141 (Ba-141). This process releases a large amount of energy in the form of kinetic energy of the fragments and high-speed neutrons, which can initiate further fission reactions, creating a chain reaction. This energy release is harnessed in nuclear power plants to generate electricity. Other products of the fission process include additional neutrons and gamma radiation.
$$^{235}_{92}U + ^1_0n \rightarrow ^{236}_{92}U \rightarrow ^{92}_{36}Kr + ^{141}_{56}Ba + 3^1_0n + energy$$Note: Throughout the nuclear fission process, the number of protons and the number of neutrons are conserved.
Nuclear fusion is a nuclear reaction in which two or more atomic nuclei combine to form a single, more massive nucleus. This process is associated with the release of a substantial amount of energy. It is the fundamental energy source of stars, including our Sun, where hydrogen nuclei (protons) fuse together to form helium, releasing an enormous amount of energy in the process. Scientists have been working on harnessing this process for practical energy production on Earth, as it holds the potential for providing a nearly limitless and cleaner source of energy compared to nuclear fission.
The release of energy in nuclear fusion is primarily due to the conversion of mass into energy, as explained by the equation $E=mc^2$. When atomic nuclei fuse and form a heavier nucleus, the mass of the resulting nucleus is slightly less than the sum of the masses of the original nuclei. This mass deficit is converted into energy, primarily in the form of kinetic energy of the particles produced during the fusion reaction. In practical fusion reactions, such as those involving isotopes of hydrogen (deuterium and tritium), the energy released is in the form of high-speed neutrons and other particles, which can be captured and used to heat water or another working fluid to produce steam and drive turbines to generate electricity.
Deuterium and tritium fusion is a nuclear process in which two isotopes of hydrogen, deuterium (D or $^2_1H$) and tritium (T or $^3_1H$), combine to form helium (He) and release a significant amount of energy. Deuterium contains one proton and one neutron, while tritium has one proton and two neutrons. When heated to extremely high temperatures and pressured, typically in excess of tens of millions of degrees Celsius, these isotopes can overcome the electrostatic repulsion between their positively charged nuclei and come close enough for the strong nuclear force to bind them together, resulting in the fusion reaction.
$$^2_1H + ^3_1H \rightarrow ^5_2He \rightarrow ^4_2He + ^1_0n + energy$$The fusion of deuterium and tritium is the most promising and well-studied approach to achieving controlled nuclear fusion for practical energy production and is the basis for experiments in fusion research, aiming to replicate the energy source of stars on Earth, offering a nearly limitless and environmentally cleaner energy solution.
Fusion reactions have several advantages, including the abundance of fuel sources (deuterium can be extracted from water, and tritium can be produced from lithium), minimal long-lived radioactive waste, and no greenhouse gas emissions, making it a promising candidate for sustainable energy production in the future. However, achieving and maintaining the extreme conditions required for controlled nuclear fusion on Earth remain significant technical challenges.
| Nuclear Fission | Nuclear Fusion |
|---|---|
| A heavy nucleus splits into lighter nuclei | Two or more light nuclei collide to form a heavier nucleus |
| A large amount of energy is released | More energy is released than in nuclear fission |
| Does not occur naturally | Occurs in stars such as the sun |
| Nuclear fuel is not easily available and is costly | Nuclear fuel is comparatively easier to source for and is cheaper |
| Chain reactions may occur | Chain reactions do not occur |
| Disposal of radioactive nuclear waste may be an environmental problem | Does not produce long-lived radioactive nuclear waste |