What Is Happening During Fission?

Nuclear fission is the process by which a heavy atomic nucleus splits into two or more lighter nuclei, releasing vast amounts of energy in the process. Fission was discovered in 1938 by German scientists Otto Hahn, Lise Meitner and Fritz Strassman, although the process had been first theorized in 1934 by physicists Ida Noddack, Enrico Fermi and others.

The discovery of nuclear fission opened up the possibility of nuclear power generation via controlled fission reactions. It also led to the development of atomic bombs based on uncontrolled fission reactions. Nuclear fission continues to be profoundly important today, providing around 10% of the world’s electricity from nuclear power reactors. Fission also remains at the heart of modern nuclear weapons.

Nuclear Fission Process

Nuclear fission is the process by which the nucleus of a heavy atom splits into two or more lighter nuclei. This occurs when a neutron strikes the nucleus of an atom like uranium-235 and is absorbed. This collision destabilizes the nucleus and causes it to split into two smaller nuclei, releasing energy and more neutrons in the process.

Before fission, the uranium-235 nucleus contains 92 protons and 143 neutrons. When it absorbs an extra neutron, it becomes unstable and starts to vibrate and stretch in shape. Eventually, the nucleus splits into two lighter nuclei, such as barium-141 and krypton-92, along with 2 or 3 extra neutrons.

Diagram of nuclear fission reaction
a uranium nucleus absorbs a neutron and splits into two lighter nuclei during nuclear fission.

In the diagram above, a uranium-235 nucleus absorbs a neutron and splits into a barium-141 nucleus, a krypton-92 nucleus, and 3 neutrons. These fission products have less combined mass than the original uranium nucleus, with the lost mass converted into energy according to Einstein’s equation E=mc^2.

The neutrons released can strike other uranium nuclei, causing a chain reaction. This releases massive amounts of energy which can be used to generate electricity or build nuclear weapons.

Binding Energy

Binding energy refers to the energy required to hold together the protons and neutrons inside an atomic nucleus. The more protons and neutrons within the nucleus, the stronger the binding energy that holds them together. However, as more and more nucleons are added, the binding energy per nucleon (binding energy divided by the number of nucleons) reaches a peak around iron-56, then decreases for heavier elements.

This creates a binding energy curve that looks like a hill, with lighter elements like hydrogen up the left side, iron-56 at the peak, and heavier elements like uranium down the right side. The higher up the curve, the more energy would be required to break those nuclei apart through fission.

Elements toward the bottom of the curve, like U-235, have the lowest binding energy per nucleon. This makes them more likely to split apart than high-binding-energy light nuclei. The fragments produced from splitting a heavy nucleus end up more tightly bound than the original. This releases energy according to Einstein’s equation E=mc2.

In summary, the binding energy curve explains why heavy isotopes like uranium and plutonium are prone to fission, while light isotopes require energy input to fuse together. The high potential energy of heavy unstable nuclei can be tapped and converted into useful energy through fission.

Chain Reactions

During nuclear fission, neutrons collide with fissile atoms like uranium-235 or plutonium-239, causing them to split into smaller atoms called fission fragments. This process also releases 2-3 more neutrons. If enough of these neutrons go on to cause subsequent fissions, it can lead to a self-sustaining chain reaction.

For a chain reaction to be sustained, each fission must on average trigger at least one more fission. The minimum amount of fissile material required to sustain a chain reaction is called the critical mass. When critical mass is achieved, even the smallest addition of extra material can cause a rapid exponential increase in fissions, leading to a nuclear explosion. Nuclear reactors use moderators and control rods to carefully regulate the number of neutrons and prevent reaching critical mass.

Chain reactions release enormous amounts of energy from relatively small amounts of fuel. Each fission releases approximately 200 MeV of energy. Since uranium-235 has over 80 quadrillion atoms per kg, 1 kg of uranium undergoing complete fission would release 16 billion kWh of energy, millions of times more than burning 1 kg of coal.

Energy Released

Nuclear fission releases an enormous amount of energy relative to chemical reactions. Per fission event, around 200 MeV (megaelectronvolts) of energy is produced. To put this in perspective, burning a mole of methane in a chemical reaction releases 802 kJ of energy. Converting units, 200 MeV is equivalent to 3.2 x 10-11 kJ per fission event. Given Avogadro’s number is 6 x 1023 particles per mole, burning a mole of methane releases around 130 billion times more energy than a single fission event. However, nuclear fuels have far greater energy density than chemical fuels. Just one kilogram of uranium-235 can produce around 80 terajoules of energy if fully fissioned. This is equivalent to burning over 2,000 tons of coal. So while individual fission events release relatively little energy, the incredible density of nuclear fuel leads to far more energy production overall.

Fission Products

Fission produces radioactive daughter nuclei called fission products. When an atom undergoes nuclear fission, it splits into two or more lighter nuclei. These daughter nuclei have too many neutrons to be stable, so they undergo radioactive decay. About 200 different isotopes of 36 elements are created as fission products from 235U.

The daughter nuclei or fission products are often radioactive. This is because the fission process leaves these nuclei with unstable ratios of protons to neutrons. To become more stable, fission products undergo radioactive decay. This releases energy in the form of ionizing radiation like beta particles and gamma rays. The initial fission products have very short half-lives, but as they decay, longer-lived isotopes accumulate. Some fission products remain radioactive for hundreds or thousands of years after being produced by fission.

Neutron Absorbers

Neutron absorbers are used to control the rate of nuclear fission. By absorbing excess neutrons, they prevent the neutrons from causing further fissions. This allows the rate of the chain reaction to be slowed or stopped.

Control rods, made of neutron absorbing material like boron or cadmium, are a common type of neutron absorber. They are inserted into the reactor core to absorb neutrons. When the control rods are lowered deeper into the core, more neutrons are absorbed, slowing down the reaction. When the control rods are raised and more neutrons can travel freely, the rate of fission increases.

Other neutron absorbing materials are also used, like boron salts in the coolant/moderator. The concentration of these neutron absorbing materials can be adjusted to control the rate of fission. Neutron absorbers are vital to regulating nuclear chain reactions and safely extracting power from fission.

Fission Applications

There are two main applications of nuclear fission:

Nuclear Power Generation

Nuclear fission is used to generate electrical power in nuclear power plants. In a nuclear reactor, a controlled nuclear chain reaction is induced to release enormous amounts of heat energy. This heat is used to boil water into steam, which then drives turbine generators to produce electricity.

Nuclear power accounts for about 10% of the world’s electricity production. It produces large amounts of baseload power without emitting greenhouse gases. However, there are concerns about managing radioactive waste and potential nuclear proliferation risks.

Nuclear Weapons

Uncontrolled nuclear fission reactions can lead to nuclear explosions. Nuclear weapons utilize such explosive fission chain reactions to create massive blasts. When a critical mass of fissile material like uranium-235 or plutonium-239 is brought together rapidly, it can result in an exponentially growing fission reaction and nuclear detonation.

Nuclear weapons based on fission were developed during World War II. They derive their explosive energy from splitting atomic nuclei, in contrast to chemical explosives that involve the rearrangement of valence electrons. Nuclear weapons are the most destructive devices ever invented.

Fission vs Fusion

While fission and fusion are both nuclear processes that release energy, there are some key differences between the two:

Fuel Source: Fission uses heavy radioactive elements like uranium or plutonium that have large atomic nuclei. Fusion uses light isotopes of hydrogen like deuterium and tritium that have smaller atomic nuclei.

Process: In fission, a heavy nucleus splits apart into lighter nuclei, releasing energy. In fusion, light nuclei are combined to form a heavier nucleus, releasing energy.

Products: The products of fission reactions are lighter elements and neutrons. Fusion creates heavier elements up to iron on the periodic table.

Conditions: Fission occurs readily under normal conditions. Fusion requires incredibly high temperatures and pressures to overcome electrostatic repulsion between nuclei.

Energy Released: Fission releases energy on the order of 200 MeV per reaction. Fusion releases energy on the order of 17 MeV per reaction.

Control: Fission chain reactions are easily controlled and sustained. Fusion reactions are much harder to sustain for any length of time.

Applications: Fission is used in nuclear power plants and atomic bombs. Fusion is not yet used on an industrial scale, but could provide an abundant energy source if controlled fusion can be achieved.

While fission technology is mature, fusion remains an ongoing research challenge. Both processes produce huge amounts of energy from small amounts of fuel, but have different mechanisms, applications, and technical challenges.


Nuclear fission is a complex process, but the key points are:

  • Fission occurs when a heavy nucleus splits into lighter nuclei, releasing energy.
  • The fission process is driven by binding energy differences between the original and daughter nuclei.
  • Fission releases neutrons that can cause a chain reaction if conditions are right.
  • Fission reactions release large amounts of energy that can be harnessed for electricity and other applications.
  • The fission products created have various impacts and uses.

Understanding nuclear fission is critical, as it is the basis for nuclear reactors which provide a significant portion of the world’s electricity. Fission also powered the first atomic bombs. While there are concerns surrounding nuclear power, fission remains an important discovery that shaped science and society in the 20th century and beyond.

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