Construction and working principle of Nuclear based thermal power plant!

  

    One of the most demandable sources in the world is electricity. This electricity is generated by numerous power plants across the world. These power plants use various sources to generate electricity. These sources are classified as conventional and non-conventional sources. Conventional source uses fossil fuels to generate electricity whereas non-conventional uses renewable sources like sun, wind, and water to generate electricity. Conventional sources offer more efficiency than non-conventional sources. One of the best conventional sources, that satisfy’s the 60% of the world’s needs is, the coal-based thermal power plant.  On our previous topic, we have seen about the construction and operation of the coal-based thermal power plant. Please visit the article for a detailed explanation of the coal-based thermal power plant.

https://sciencetopic03.blogspot.com/2021/10/how-thermal-power-plant-works-coal-based.html

    The coal-based thermal power plant uses coal as its main fuel to generate electricity. Here the coal is burnt in a giant boiler to generate intense heat. This heat is used to convert the water in a chamber into steam. This steam is made to drive a steam turbine through which the generator is made to rotate to generate electricity. But one of the main disadvantages of the coal-based thermal power plant is, that it emits a huge amount of harmful gases while burning, which causes air pollution. But there is also another kind of thermal power plant, called a nuclear-based thermal power plant, which causes no air pollution. This nuclear power plant uses atomic fuel to generate electricity. Even it does not create air pollution it creates another waste, which is also considered as very dangerous. Now let’s see what that’s waste is and how a nuclear power plant works.

Credit: By Reetesh Chaurasia - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=61854707

    As in the case of a coal-based thermal power plant, where coal is used as the main fuel; the nuclear power plant uses Uranium (U) as its main fuel. Uranium is so powerful that 1kg of Uranium is equivalent to 4500 tons of high-grade coal or 1700 tons of oil. Also, the average production cost for the production of electricity through nuclear power is 1.2/kWh, while in gas, it is 2.27/kWh. There are also other materials that are used as fuel in the nuclear power plant like Plutonium (PU) and Thorium. But Uranium is primarily used. The Uranium fuel used in nuclear power plants exists in three forms, namely, Uranium-238 ( ₉₂U²³⁸ ), Uranium-235 ( ₉₂U²³⁵ ), Uranium-234 ( ₉₂U²³⁴ ). Among these three elements, the Uranium-235 is the primary one. There are other materials such as U-233 and PU-239. These are called secondary fuels and they are artificially produced by the U-238 and thorium-232. The upward number is called the mass number and the downward number is called the atomic number. The mass number denotes the combination of the number of protons and neutrons in the atomic nucleus whereas the atomic number denotes the number of protons in the atomic nucleus. In the case of Uranium atoms, there are 92 protons and 143 (235-92) neutrons in the nucleus.

Atomic and mass number of Uranium 235

      Even though there are lot of materials available as fuel for nuclear power plants, a pure amount of U-235 is required for a successful nuclear reaction in the power plant. When the uranium is cultivated, only 0.7% of U-235 is present. The remaining elements are isotopes of uranium (U-235). In order to get an efficient nuclear reaction, a pure amount of U-235 is required. To get the Uranium U-235, enrichment is done to separate it from the uranium element. This enrichment is done in anyways to obtain a pure amount of Uranium (U-235) to be used in the nuclear power plant.

    First of all, the nuclear process can be classified into two main processes, namely, nuclear fission and nuclear fusion. In short to say, nuclear fission is the process of splitting a single atom into two, which is commonly done in the nuclear power plants we see today, whereas nuclear fusion is the process of joining two atoms into a single atom, happening in the universal stars. Both processes release a tremendous amount of energy. But nuclear fusion is not used in nuclear plants as it requires a heavy pressurized atmosphere to conduct it.

Difference between Nuclear fission and Nuclear Fusion

    After enrichment was done, Uranium U-235 is received as small pellets. These pellets are loaded into the fuel rods. Each pellet is capable of generating several kilowatts of electricity. These uranium pellets are 1cm in diameter. Several pellets are aligned in a straightway to form a fuel rod.  Several fuel rods are made as such and a fuel assembly is made. And again several fuel assemblies are placed inside the nuclear reactor’s core with proper arrangements.

Credit (Uranium Pellets): By Chemolunatic - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=86461112

Uranium pellets, Fuel Rod, Fuel Assembly

    The nuclear reactor is a specially constructed dome for the nuclear reaction to happen. It has several parts including control rods, moderators, and coolants. It has to be safely maintained and progressed to avoid dangerous accidents. The important point to be considered in the nuclear reactor is, that the fusion of nuclear elements creates a huge amount of radioactive waste. Radioactive means, an element that is in an unstable state. An atom is said unstable when the electro repulsive force between the protons and neutrons in the atom gets disturbed and can’t remain in a normal state. During this unstable state, the atom emits radiation to return back to its normal state (radioactive decay). These radiations vary from alpha rays to various harmful rays. Harmful rays mean, they can knock off the electron in our body cell or alter the DNA structure of our body, leading to loss of life. The particles emitting those radiations can come to halt at different life stages. Some atoms take hours to days to decay while others take millions of years to decay. Nuclear elements like Uranium, take several million years to decay. So these elements are very much harmful, when it comes in contact with the human body. So the nuclear fission is properly done and the radioactive wastes are properly disposed of. If anything is mishandled in the nuclear power station it could take millions of people’s life. One of the best examples of the nuclear power plant accident is the Chernobyl accident in Ukraine. It happened on April 26, 1986. This place is remained closed till now for radiation exposure. Now let’s see how a nuclear fusion works and how it is transferred to produce heat and electricity.

    As the nuclear fuels (U-235) are loaded into the reactor, a slow-moving neutron is made to fire over the uranium atoms. When one uranium atom gets collided with the slow-moving neutron, it becomes unstable and transforms into U-236. Through this condition, it split up into two different atoms (Barium and Krypton). This splitting of atoms is called a fission reaction. This reaction also creates an excess of two to three excess neutrons, other particles, and a tremendous amount of energy in the form of heat equivalent to 200MeV. The nuclear fission reaction is explained by the below equation.

Fission of Uranium235 when collided with a slower-moving neutron

    These three neutrons are expelled from the fission reaction at a very high-speed rate. But in order to get a successful chain reaction, the neutrons must collide with the uranium atoms at a certain speed. To slow down the speed of the neutrons, moderators are used. These things are used in either solid or liquid conditions, to successfully reduce the speed of the neutrons. After the neutrons are slowed down, it further collides with another set of uranium atoms which further releases two to three neutrons each, to be collided with further atoms. Thus this process creates a successful chain creation and an immense amount of heat is generated. But if this reaction goes in this multiple manner, a heavy explosion will possibly occur, leading to radiation exposure. Thus to make it a controlled reaction, some neutrons must be absorbed from the reaction process. This process is done by the Control rods. These control rods are in motion and lowered down when the neutrons must be absorbed for a controlled reaction. 

Continuous Fission process

    The amount of generated energy is captured by the surrounding coolant or the moderator itself depending upon the type of reactor. This coolant is then transferred out by a pump to generate electricity. There are numerous types of nuclear reactors used worldwide for the production of electricity. The most used reactor is the light water type reactor. These reactors use light water as their moderators whereas other reactor uses Graphite rods as its moderator. Light water is the same as the normal water we drink. The opposite of the light water is the hard water which is quite different.  Light water contains two hydrogen atoms and an oxygen atom; whereas hard water consists of two Deuterium atoms (an isotope of Hydrogen) and one oxygen atom. Heavy water is used in reactors like CANDU reactor which does not use enriched fuel (100% pure U-235). So the fission reaction would not be efficient and the neutrons will not be that much colliding with the U-235 atoms. So hard water is used to increase the fission reaction; as the hard water atoms have an excess neutron. There are majorly two types of light water reactors, namely,

• Boiler Water Reactor (BWR)
• Pressurized Water Reactor (PWR)

Boiler Water Reactor

    Boiler Water Reactor is widely used across the United States. It uses a simple construction. The nuclear reactor is constructed with fuel rods, control rods, and moderators. This moderator gets directly converted into steam without the steam generator. The steam is then pumped through a steam turbine and condensed back and fed to the nuclear reactor. The steam turbine then drives the Generator for generating electricity.

Working principle of Boiler Water Reactor (BWR)

Pressurized Water reactor

    The PWR is the most used Nuclear Reactor Worldwide. It houses the fuel rods, control rods, moderators. The hot coolant from the reactor is fed through a steam generator where the heat is transferred to the water. By capturing the heat, the water gets converted into steam and passed to the steam turbine and condensed back and circulated again. The coolant on the other side is fed back into the reactor for capturing the heat again.

 

Working Principle of Pressurized Water Reactor (PWR)