Important Components of Nuclear Reactor and its Significance

The importance of Nuclear Energy is a very important topic of discussion in recent times. This topic is also considered important from exam point of view as this is a part of current affairs and also science and technology. In this article we are going to discuss the mechanism of nuclear reactors and important components needed in nuclear reactor for the production of energy

Nuclear Fission :

• In nuclear physics, fission is the radioactive decay process in which the nucleus splits into smaller parts [light nuclei].
  
• The fission process often produces free neutrons and gamma photons [gamma rays] and releases large amounts of energy [exothermic reactions].
  
• Exothermic reaction = release of heat during the reaction. [CaCO₃ (calcium carbonate or lime) + H₂O (water) → Ca(OH)₂ (calcium hydroxide) + CO₂ + HEAT]
  
• The fission process can occur spontaneously or can be induced by exciting the nucleus with electromagnetic radiation in the form of various particles (neutrons, protons, deuterons, or alpha particles) or gamma rays.  
  
• During nuclear fission, radioactive products are formed and some neutrons are released.
  
• These neutrons trigger fission in the core near the fissile material, releasing more neutrons and setting off a chain reaction.
  
• Fissile material → Material that can undergo a nuclear fission chain reaction. Fissile → may undergo controlled or self-sustaining nuclear fission chain reactions.
  
• Controlled in a nuclear reactor, such a chain reaction can be used to generate electricity. An uncontrollable [Nuclear Bomb] can cause a large explosion.
  
• Uranium is the fissile material most commonly used in nuclear reactors and nuclear weapons.
  
• The Uranium isotopes in natural Uranium are Uranium 238 or U-238 or 238U (99.27%) and Uranium 235 or U-235 or 235U (0.72%). Uranium-235 fission only when exposed to slow neutrons.
  
• Since fast neutrons cannot be captured, the neutrons must be moderately moderated to increase the capture probability in fission reactors.
  
• Another isotope that undergoes fission under slow neutron irradiation is Uranium-233.
  
• Uranium-238 fission only when exposed to fast neutrons.
  
• U-238 has a low natural fission probability and a low fission probability when exposed to fast neutrons, but it is useless as a nuclear fuel source. The nuclei of other heavy elements, such as thorium, are also fissile but have fast neutrons.

Main Components of a Nuclear Reactor:

1. Fissile material :

• Uranium-233, Uranium-235, Plutonium-239, Plutonium-241, etc. are commonly used fissile materials.
  
• Natural uranium consist of 00.72% U-235 (fissile isotope), 99.27% ​​U-238, and traces of 0.0055% U-234. 00.72% U-235 is not enough to initiate significant autonomous chain reactions.
  
• For light water reactors, the fuel should be enriched to 2.5-3.5% U-235. Plutonium-239 can be produced from Uranium-238 by “breeding”.
  
• Thorium-232 is a fertile material that can absorb neutrons and transform them into uranium-233, a fissionable nuclide that forms the basis of the thorium fuel cycle.
  

2. Uranium Enrichment

• Natural Uranium contains only 0.7% of the fissile isotope U-235. The remaining 99.3% is non-fissionable U-238.
  
• For use in light water reactors, Uranium is typically enriched to 2.5-3.5% U-235. A centrifuge is used for uranium enrichment.
  
• The enriched uranium fuel used in fission reactors cannot be used to create bombs.
  
• Enrichment of 90% or higher is required to obtain the fast chain reaction required for weapons applications.
  
• Enrichments of 15-30% are common in breeder reactors.

3. Nuclear Reactor

• A nuclear reactor is a system that contains and controls a sustained nuclear chain reaction.
  
• The fuel [enriched uranium-235 or plutonium-239] is put into the reactor vessel along with a small neutron source.
  
• The neutrons start a chain reaction in which each atom that splits releases more neutrons that split the other atom.
  
• Each time an atom splits, a large amount of energy is released in the form of heat. Heat is removed from the reactor by the coolant. The coolant is mostly pure water. The coolant heats up and goes to the turbine to turn the generator or drive shaft.
  
• Coolant is the substance that flows through the core and transfers heat from the fuel to the turbine. These include water, heavy water, liquid sodium, and helium.
  
• Turbines convert heat from the coolant into electricity, much like fossil fuel power plants.
  
• The containment is a reinforced concrete structure that separates the reactor from the environment. 

4. Coolant

• A reactor coolant (usually water or molten salt) circulates through the core to absorb the heat generated.
  
• Heat is removed from the reactor and used to generate steam.
  

5.  Moderator

• A neutron moderator is a medium that slows down fast neutrons and converts them into thermal neutrons that can sustain a nuclear chain reaction.
  
• A heavy nucleus splits into two or lighter nuclei (fission products), releasing kinetic energy, gamma rays, and free neutrons. Some of these neutrons can later be absorbed by other fissile atoms, causing further fission events that release even more neutrons. This is known as a nuclear chain reaction.
  
• To control such nuclear chain reactions, neutron poisons and neutron moderators can also change the fraction of neutrons that cause nuclear fission.
  
• Commonly used moderators are ordinary (light) water (74.8% of reactors worldwide), solid graphite (20% of reactors), and heavy water (5% of reactors).

6. Control rods

• Reactor power is adjusted by controlling the number of neutrons that can produce more fission.
  
• Control rods made of neutron poison are used to absorb neutrons. Moderator slows down neutrons and control rods absorb neutrons.
  
• Absorbing more neutrons in the control rods means fewer neutrons are available to cause fission. Therefore, pushing the control stick deeper into the reactor will decrease power, and pulling the control stick out will increase power.
  
• Control rods are composed of chemical elements such as Boron, Silver, Indium, and Cadmium.
  

7. Critical mass

• Critical mass is the minimum amount of fissile material required for a sustained nuclear chain reaction.
  
• The critical mass of fissile material depends on its nuclear properties, density, shape, enrichment, purity, temperature, and environment.
  
• When a nuclear chain reaction is self-sustaining within a mass of fissile material, the mass is in a critical state where there is no increase or decrease in power, temperature, or neutron numbers.