Nuclear Reactor Based on Nuclear Fission

Nuclear reactors are known to be a complete system in which several nuclear chain reactions take place, right from its initiation. The energy yield can be contained for several usage applications. In general, there are different types of nuclear reactors and could even mean a device that provides vast scope for research and development of radioactive isotopes. They are:

1. In Terms of General Usage

• Nuclear Power Reactors

• Nuclear Research Reactors

2. In Terms of Fuel Usage

• Uranium fueled 

• Plutonium fueled

3. In Terms of Other Types

• Pressurized Water Reactor

• Boiling Water Reactor

• Pressurized Water 

• Advanced Gas-Cooled Reactor

• Light water Graphite-moderated Reactor

Spread across all corners of the world, it's the nuclear fission reactor that produces more than 11% of the world's gross electricity. It takes place because of the nuclear fission reaction in which the splitting of atoms in a nucleus takes place, which is otherwise bound together by the most potent force in nature. Therefore, elements like Uranium (the heaviest natural element) can be split apart on bombardment with a neutron and release a lot of energy because of the splitting. 

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The nuclear reactors contain such fission reactions that release enough energy to heat the water to more than 271 degrees Celsius, which can help in the spinning of turbines and thus, generating electricity. 

Working of a Nuclear Reactor

Driven by the nuclear fission reaction process, when a neutron is fired at an atom, it causes them to disintegrate into small, excited states that are continuously emitting neutrons, photons, and other subatomic particles. Such a vast release of energy causes these particles to trigger fission in its colliding atoms and set off a chain reaction by yielding more neutrons that also generates massive heat energy. When this generated heat is isolated from the system via heating water and produces steam that can further propel turbines, electricity production begins by the generator process. 

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In an atom bomb, such chain reactions help in the natural fission of elements and create massive energetic explosions. However, in a nuclear reactor, such comprehensive processes require a detailed study on the precise temperature controls for slowing or accelerating reactions, which can be done via control rods. The control rods are made up of neutron-absorbing elements like silver and boron that can help in quick absorption of neutrons and, thus, maintaining and controlling the chain reactions that follow. As much as 85% of the total energy released from a fission reaction occurs early on the equation and can take place in the right amount of time. In comparison, the rest of the 15% of the energy is yielded via the radioactive decay of the element once the neutrons are emitted. Radioactive decay of an element is the process in which the atoms acquire a stable state, occurring over long time periods. 

Chain Reaction

The nuclear fission reaction is a self-enriching process in which the rate of neutrons created is regulated can help the reactor stay on a critical scale. In the pre-starting of a nuclear core, the number of neutrons remains equivalent to zero. Still, as the reactor gets started with the removal of control rods from the core, the reactor goes into the supercritical scale, where the number of neutron populations increases rapidly over time, causing the energy generated to be improved. Once the threshold power scale is reached, the control rods are re-introduced to keep the neutrons in balance, taking the reactor in its steady-state operation. For a reactor to shut down completely, the control rods are entirely placed into the core, slowing down the fission and taking it back to the subcritical state where the rate of fission drops down zero.

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Reactivity

It is a widely used parameter that describes the measure of the reactor's state in terms of its undergone change in the core. The reactivity is generally represented as:

δk = (k − 1)/k, where δk = 0, when the reactor is in the critical, and positive, negative values in case the reactor is supercritical and subcritical, respectively. 

These factors are controlled in three ways: 

• By regulating the absorber elements introduced

• Addition or removal of fuel

• Changing the ratio of neutrons that leak out and contain in a system.

In general, making changes in the neutron leakage is completely self-driven since the increase of power generated in the core causes the density of the coolant to drop and get boiled. Such a massive decrease can enhance the mass neutron leakage from the system and decrease the reactor's overall reactivity. 

It takes an abrupt timing of one picosecond (which is 10^-12 of a second) for a fission reaction to occur and be responded accordingly by the reactor. 

Fissile Elements

There are only a handful of elements with heavy nuclides that can undergo fission reactions consistently via low-energy neutrons. Such elements are called fissile elements. Some of them are uranium-235, plutonium-239, and plutonium-241. Apart from the enriching of uranium-235, certain nucleotides can transform into fissile elements called fertile materials like thorium-232.