Fission And Fusion


Fission is a special type of radioactivity which results in ‘splitting’ of the nucleus into 2 or more parts.

Now this process can happen spontaneously (by itself in accordance with half-life rule) or it can be induced (this is of interest to us). In the latter case fission is induced by a hitting neutron. The neutron adds up to the nucleus and makes in unstable and results in eventual ‘split up’. This is the basis for all nuclear reactors and nuclear weapons.


  • Nuclear fusion is the process of combining two or more nuclei to form a heavier nucleus and release high-energy radiation.
  • This process occurs naturally in stars, where the high temperature at the core allows for the nuclei to overcome the repulsive electromagnetic force and fuse together.
  • An example of fusion is the combination of four hydrogen nuclei to form helium, releasing energy in the form of neutrinos, gamma rays, and positrons.

Science behind Nuclear Fission (Weapon and Reactor)

Nuclear fission is a process that involves split or break up of nucleus of an atom into 2 or more parts. In the process a large amount of energy is released which is the basis on which both nuclear weapons and nuclear fission reactors work. In both cases nuclear fission is induced by hitting it with a neutron.

For nuclear fission reactors or weapons to work there must be continuous split of nucleus. This ‘split’ is induced by hitting the nucleus of some special atoms with a neutron. Accordingly, following conditions must be met to extract nuclear energy

  • Continuous supply of nucleus that can be split (Critical Mass)
  • Continuous supply of neutrons that induce the split (Chain reaction)

Chain Reaction

  • Continuous supply of neutrons to induce split in the nucleus is called a chain reaction.
  • Some special atoms, called fissile material, can split and release neutrons.
  • Fissile materials include Uranium-235, Uranium-233, and Plutonium-239.
  • When fissile material is hit with a neutron, it splits and releases 2-3 more neutrons, causing a chain reaction.
  • Uranium-238 and Thorium-232 are not fissile materials.
  • A chain reaction releases a lot of energy and is used in nuclear reactors and weapons.
  • 10 kg of U-235 completely splits in about 84 splits, while 10 kg of Pl-239 is completely split in 53 cycles.

Critical Mass

  • For a chain reaction to occur, there must be enough U-235 nucleus for the secondary neutrons to hit and sustain the reaction.
  • The minimum amount of fuel required to sustain the chain reaction is called the critical mass.
  • If there is not enough fuel, the neutrons will escape and the reaction will stop.
  • Maintaining the critical mass is crucial for nuclear reactors and weapons to function properly.

Difference between nuclear weapons and nuclear reactors

  • Weapons and reactors both involve the release of energy from the nucleus of an atom, but the difference lies in the amount and control of the energy release.
  • In nuclear weapons, the chain reaction is uncontrolled, resulting in an instant and massive release of energy until all the fissile material is consumed.
  • In nuclear reactors, a controlled chain reaction is desired to provide a constant and continuous supply of energy. This is achieved by allowing only one neutron to hit another nucleus and using materials called control rods to absorb additional neutrons and prevent an uncontrolled chain reaction.
  • The sustained chain reaction in nuclear reactors provides a steady supply of energy, while uncontrolled chain reactions in weapons result in a massive and instantaneous release of energy.


  • The speed of a neutron is an important factor in determining its ability to hit a nucleus.
  • Neutrons released in fission reactions are highly energetic and must be slowed down to increase the probability of them hitting another nucleus.
  • Moderators are used to slow down fast-moving neutrons. Moderators consist of atoms of light nuclei, which are smaller in size than the neutron and can slow it down through collisions.
  • Commonly used moderators include light water (normal water), heavy water (deuterium), and graphite.

Nuclear fuel

As mentioned above Uranium-235, Plutonium-239 and Uranium-233 are the only fissile material available for reactors. Out of them only Uranium-235 is naturally occurring, Pu-239 and U-233 are made in a special type of reactors called breeder reactors. Now let us look into the relevant details of these fuels.

Naturally occurring Uranium

  • Naturally occurring uranium consists of 0.3% U-235 (fissile) and 99.7% U-238 (not fissile).
  • U-238 is a fertile material that can be converted to fissile Plutonium-239 upon neutron absorption.
  • U-235 is fissile and undergoes fission upon neutron bombardment, releasing 2-3 neutrons that can induce further fission, setting up a chain reaction.
  • Naturally occurring uranium does not contain enough U-235 to sustain a chain reaction, so two strategies can be employed: uranium enrichment or slowing down the neutron.
  • Slowing down the neutron increases the probability of it hitting U-235, which is achieved by using moderators, materials that can slow down neutrons without absorbing them.
  • Neutron reflectors are inserted in the core of the reactor to slow down the fast neutrons and make more of them available to hit the little fissile U-235.

Enriched Uranium

  • Enrichment is a process to increase the amount of U-235 nucleus available for fission to sustain the chain reaction.
  • The level of enrichment depends on the amount and speed of energy needed to be released.
  • In a nuclear reactor, about 3% U-235 is sufficient, while in a bomb, it should be at least 80%.
  • The critical mass of U-235 is 3% and 80% for reactors and weapons, respectively.


  • U-235 is the only fissile material that is naturally occurring. Other two, Pu-239 and U-233, are made in reactors.
  • Plutonium-239 is a highly favored fissile material as it releases more energy than U-235.
  • U-238, when hit by a neutron, becomes highly radioactive U-239 and then neptunium-239, before turning into Pu-239.
  • Plutonium is produced by absorbing a neutron into U-238, then chemically separated.
  • U-238 tends to absorb slow-moving neutrons more readily than fast-moving neutrons, and Pu-239 undergoes fission readily if neutrons are slowed down.
  • Breeder reactors use this process to convert U-238 to Pu-239.


  • U-233 is a fissile material that is produced in reactors from Thorium-232.
  • U-233 releases 2-3 neutrons upon fission, setting in motion the chain reaction.
  • Fast-moving neutrons are more effective in bringing about fission in U-233 than in Pu-239.
  • Fast Breeder Reactors are not successful with Pu-239 as 35% of the neutrons get absorbed by Pu-239 to become Pu-240.
  • In contrast, 100% of the fast neutrons are used in splitting the nucleus in U-233.
  • India’s 3-stage nuclear program involves the use of U-233.
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