Fission is the process in which a heavy nucleus (an atom with a very high mass) disintegrates. The process of disintegration breaks the original atom, converting it into a lighter element. This disintegration process produces other smaller particles and some residual energy in the form of photons. There are two types of fission, namely induced fission, which is currently used in nuclear technology, and spontaneous fission, a type of radioactive decay.
The fission process, Manuel R. Camacho – StudySmarter Originals
What is induced fission?
Induced fission is what we can call “a forced decay”. During induced fission, slow-moving neutrons are ejected towards a nucleus of heavy radioactive material, such as uranium-235 or plutonium-239. The uranium and plutonium used for these reactions are called fissile, and the breakaway of the heavy elements will produce two lighter elements named fission products. The reaction will also emit other neutrons and high-energy photons.
Neutron-induced fission
During an induced fission reaction, neutrons force instability at the heavy nuclei. The disintegration reactions of the fissile materials will produce two of three neutrons with a high velocity. A moderator is used to slow the neutrons. The moderator is necessary for the neutrons released by the breakaway to start another breakaway in another fissile atom.
The process in which a fissile atom breaks down, producing more neutrons that will start other fission reactions, is known as a chain reaction. In the case of uranium-235, the process of accepting another neutron will convert it into uranium-236.
The chain reaction of uranium-236 begins at 1 when it decays and produces products and neutrons. The released neutrons will be trapped by other atoms of uranium-235 at 2, converting into uranium-236. The process repeats as the new isotopes are highly unstable. Manuel R. Camacho – StudySmarter Originals
The energy measured in the electrons during fission processes reaches an average of 200 terajoules per kilogram, which is equivalent to 6.6% of the light velocity in a vacuum.
The moderator, which works as an energy dissipator mechanism, is used in many nuclear reactions where fission is present in water or graphite. Fast-moving neutrons will collide with the atoms of water or graphite and transfer part of their energy in the collision to the other particle. This reduces their velocity.
Slow neutrons are necessary as fast-moving electrons aren’t likely to be captured by other heavy nuclei. Remember that to be absorbed by the nucleus, the neutron has to pass close to the nucleus and be attracted to it (by the strong nuclear force).
The energy released during the induced fission process
During a fission process, energy is released in two forms:
- Kinetic energy, which is given to the neutrons produced and the products released.
- High energy photons (gamma rays).
The high-energy photons and the products are then converted into residual energy as they slow down. The residual energy of these reactions and collisions is then used to produce work. In this case, the residual energy is heat used in nuclear reactors to create high temperatures and pressurised steam.
On average, uranium-235 after fission produces 83 terajoules per kilogram, the same as 23.05 gigawatt-hour per kilogram of fissile material.
Products formed by the fission process
The products released by the fission process will depend on each reaction: the velocities of particles and energies will not be the same. However, it can be estimated that the products formed will split with almost the same mass, and these products will be heavy elements and radioactive. In some cases, the reaction will produce two smaller products (radiation and neutrons) and alpha particles or tritium.
The products formed will also undergo a spontaneous disintegration process, releasing more particles and energy. In nuclear reactors, the products released are known as nuclear waste.
What is spontaneous fission?
Spontaneous fission is when a heavy nucleus disintegrates without being forced to do so by absorbing an additional neutron. During this event, the atom splits into two lighter nuclei of almost identical mass and residual energy as photons.
Spontaneous fission occurs naturally in elements of heavy mass (such as uranium-235 and uranium-238) because they are naturally unstable. The example of the reaction in uranium-235 is shown below. In this reaction, the atom breaks apart without needing to capture a neutron, and it divides into thorium and an alpha particle, which is a helium nucleus.
\[^{235}_{92}Uranium \rightarrow ^{231}_{90}Thorium + ^{4}_{2}He\]
\[^{4}_{2}He = \text {Alpha Particle}\]
The rate of decay in these events is related to the half-life of the radioactive element, which is the average time it takes for a certain amount of the radioactive material to decay to half of its original mass.
See our explanation on Half Life for more info.
Fission and nuclear reactors
The process of fission is used in nuclear reactors to produce electrical energy, also known industrially as nuclear energy. Let’s very briefly look at how this works:
- Nuclear fission will produce high-velocity particles and photons. Their slowing down and transmission will produce heat.
- The heat is transmitted from the reactor’s core to a fluid (the fluid can be water). The water, which possesses a high pressure and temperature, moves to a pipe where it exchanges heat with other fluids.
- The new fluid (which can be water again) is then converted into steam, moving through a system of pipes. The pipes deliver the steam with high pressure to a turbine, and the turbine uses the impulse given by the high-pressure steam to move its blades.
- The turbine will rotate at high speed, and through a shaft, it will transmit its kinetic energy to an electrical generator. The generator will then produce electricity.
A nuclear power plant
The future of nuclear fission
Nuclear fission is one of the leading technologies used to produce energy in certain countries. France, the UK, Japan, Russia, China, the USA, and Canada continue to develop new technologies that can make safer, more efficient, and more advanced nuclear reactors.
Newer designs improve fission by reducing the amount and risk of nuclear waste. They also improve the amount of energy produced and are designed to disable the possibility of being used to produce nuclear weapons.
There are instances where fission has been achieved naturally. One of these examples happened in Gabon, where traces of a natural reactor were found. In a natural reactor, radioactive material starts a fission reaction that self-sustains by its particular conditions.
In these conditions, minerals with a high concentration of uranium start to decay naturally, and the deposit needs groundwater to act as a moderator. The water then boils, stopping the reaction since the neutrons cannot be slowed down. When the natural reactor cools, the water returns, and the process starts again.
According to researchers, natural reactors are a very rare occurrence, and so far, only one has been confirmed to have existed.
Induced Fission - Key takeaways
- Induced fission is what we can call “a forced decay”.
- The radioactive decay will produce products (lighter elements), other neutrons, smaller particles such as alpha particles, and high-energy photons (gamma rays).
- The fission reaction can be controlled using a moderator, and in most cases, water is used as a moderator.
- Spontaneous fission is when a heavy nucleus disintegrates without being forced to do so by absorbing an additional neutron.
- The process of fission is used in nuclear reactors to produce electrical energy, also known industrially as nuclear energy.
References
- https://unsplash.com/photos/C82jAEQkfE0
Explanations 
Textbooks 
Exams 
Magazine 
