Nuclear Fission Definition and Examples

What Is Nuclear Fission
Nuclear fission is a nuclear reaction where a heavy nucleus breaks into two or more smaller nuclei, releasing energy.

Nuclear fission is a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into two or more smaller, lighter nuclei. This process often produces gamma photons and releases a significant amount of energy. The term “fission” comes from the Latin word fissio, which means “a cleaving” or “splitting.”

History of the Discovery

The phenomenon of nuclear fission was discovered in the late 1930s by German physicists Otto Hahn and Fritz Strassmann. Hahn and Strassmann proved that the products from bombarding uranium with neutrons were isotopes of barium, lanthanum, and other elements that are lighter than uranium. Lise Meitner and Otto Frisch coined the term “fission” to describe the disintegration of a heavy nucleus into two fragments of approximately equal size. The discovery of fission led to the Atomic Age and the development of both nuclear power and atomic weapons.

Nuclear Fission vs. Nuclear Fusion

Nuclear fission is the reverse of nuclear fusion. While fission involves splitting a heavy, unstable nucleus into two lighter nuclei, fusion is a process where two light atomic nuclei combine to form a heavier nucleus. Both are forms of transmutation, in which one element changes into another.

In nuclear fission, the nucleus of a heavy atom, such as uranium or plutonium, splits into two or more smaller nuclei, along with a few neutrons and a significant amount of energy. Conversely, nuclear fusion involves two light elements, typically isotopes of hydrogen (deuterium and tritium), merging under conditions of extremely high temperature and pressure to form a heavier nucleus, releasing energy in the process.

Spontaneous Fission and Induced Fission

There are two types of nuclear fission: spontaneous fission and induced fission.

Spontaneous fission, as the name implies, occurs naturally. It is a form of radioactive decay found in only the heaviest of isotopes, including certain isotopes of uranium and plutonium. The probability of spontaneous fission happening is generally quite low, and it occurs alongside other forms of decay, such as alpha or beta decay. An example of spontaneous fission is the decay of californium-252 into xenon-140, ruthenium-108 and 4 neutrons.

Induced fission, on the other hand, occurs when a nucleus absorbs of a neutron (or sometimes another particle). The additional energy from the neutron triggers the already unstable nucleus to split. This process is utilized in nuclear reactors and nuclear weapons. An example of induced fission is the reaction where plutonium-239 absorbs a neutron and breaks into xenon-134, zirconium-103, and 3 neutrons.

Fission Chain Reaction

A chain reaction in nuclear fission is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. A fission chain reaction is self-sustaining because one single reaction initiates multiple other reactions.

For example, consider a chain reaction involving uranium-235 (U-235), a common isotope in nuclear reactors.

  1. A U-235 nucleus absorbs a neutron, forming an excited uranium-236 (U-236).
  2. The excited U-236 nucleus undergoes fission, splitting into two smaller nuclei (fission fragments), for example, barium-141 (Ba-141) and krypton-92 (Kr-92), along with three new free neutrons and a significant amount of energy.
  3. These newly released neutrons can then be absorbed by other U-235 atoms, causing them to also undergo fission and release more neutrons. Whether or not this happens depends on whether or not there are enough neighbor uranium atoms.

The reaction is:

U-235 + n → Ba-141 + Kr-92 + 3n + energy

In a nuclear power plant, the chain reaction is carefully controlled to maintain a steady rate of fission, while in a nuclear weapon, the chain reaction proceeds at an explosive rate.

Key Properties of Fission

Nuclear fission is characterized by a mass difference between the reactants and products. This is due to the principle of mass-energy equivalence, famously outlined in Einstein’s equation E=mc2. When a nucleus undergoes fission, the combined mass of the resulting particles is less than the original mass. This “missing” mass converts into energy, which is released during the fission process.

The energy produced in a fission reaction primarily comes from the kinetic motion of the fission products and the photons in the form of gamma radiation. A single fission event can release around 200 MeV (million electron volts) of energy, which is roughly a million times more than the energy released by a typical chemical reaction.

Fissionable vs Fissile

Two commonly confused terms related to fission are “fissionable” and “fissile.” A fissionable nuclide is one capable of undergoing fission after capturing a low or high energy neutron (even if the reaction only occurs rarely). A fissile nuclide is a fissionable nuclide that has a high probability of fission after absorbing low energy neutrons. U-238 is fissionable, but not fissile. U-235 is fissionable and fissile.

Uses of Nuclear Fission and Its Safety

Nuclear fission is most commonly known for its role in nuclear power plants and atomic weapons. In nuclear power plants, the heat generated from a controlled fission chain reaction produces steam, which then drives turbines to generate electricity.

However, the utilization of nuclear fission does not come without risks. There are substantial concerns regarding the safe management of radioactive waste produced in nuclear power plants. Additionally, the potential for nuclear accidents, such as the Chernobyl and Fukushima disasters, raise safety and environmental concerns.


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