Nuclear Power Plant : Components , Advantages and Disadvantages
Introduction to Nuclear Power Plant:
Nuclear energy is a recent entrant in the field of power production. The heat produced by nuclear fission in the atoms is utilized in a special heat exchanger for the generation of steam which is used to drive turbo-generator.
Any atom can be made to undergo fission for energy release. In practical case, we take such a fuel which can sustain a chain reaction.
Certain heavy atoms are in a precarious state of internal balance, to the extent that by inducing a neutron to enter their nuclei they will fission into two small nuclei, eject two or three neutrons and the fragments emit rays such as α, β, and ƴ rays.
or each splitting atom, about 2.5 new neutrons are emitted. These free atoms have very high velocity of the order 1.5 × 107 m/s. U238 will also undergo fission but the reaction is not a sustained chain reaction. To sustain a chain reaction in natural Uranium, fast neutrons must be moderated in order to slow down to thermal speed. U238 is transformed to Pu239 which is highly fissionable and contributes to heat production.
Functions of Nuclear Power Plant:
The functions of nuclear reactors are as follows:
(a) To provide neutrons with enough energy so that the same may be absorbed by the nuclei of certain heavy atoms causing fission so that a large amount of energy is liberated.
(b) To utilize the energy liberated in the form of kinetic energy.
(c) To maintain a self-sustaining chain reaction of fission so as to provide a continuous source of heat energy at any desired temperature.
Components of Nuclear Power Plant:
following are the main components of a nuclear power plant:
i. Fuel core or fissionable material
v. Reactor vessel
vi. Control rod
i. Fuel Core:
It is made up of fuel rods or assembles containing natural uranium or plutonium or U235 or thorium232. This must be clad with a corrosion-resisting metal. Fissioning makes the metal hot and produces a high temperature. The resisting material should be stainless steel or zirconium.
It consists of a material such as graphite packed around the fuel to slow down the fast neutrons to thermal speed (2200 m/s). The fission process produces neutrons of high velocity which must be slowed down. The moderator may be graphite, light water, or heavy water (D2O, deuterium). The moderator should not absorb the neutron produced.
The fission results in the emission of α, β, and ƴ rays. All these are dangerous to human life. Thus, a nuclear reactor is enclosed within a biological shield which reduces the emission passing through it. A shield of several meters of heavy barite concrete may be used.
It usually surrounds the reactor core within the thermal shield. It bounces back most of the neutrons that escape from the fuel core. Beryllium can be used as a reflector but it is toxic and therefore must be handled with care.
v. Reactor Vessel:
It completely encloses the reactor core with its reflector and thermal shield. The reactor vessel provides the passage for directing the coolant flow through the nuclear core.
vi. Control Rod:
It absorbs excess neutrons released during the fission process. The rod is usually of boron. It absorbs neutrons instantly and can be moved in and out of holes in the reactor core assembly.
The main problem in a reactor is to remove the heat produced. The coolant used may be H2O, D2O, helium, sodium, or sodium-potassium.
Types of Nuclear Power Plant:
Nuclear power plant are majorly classified as:
(a) Boiling water reactor (BWR)
(b) Pressurized water reactor (PWR)
(c) Liquid metal cooled reactor (LMCR)
(a) Boiling Water Reactor:
In a boiling water reactor, the water acts as a coolant as well as moderator. The water circulates upward through the fuel core and downward over the thermal shield. Steam is formed by the dissipation of heat of the fuel and the steam produced is used to run the steam turbine. In this reactor, the pressure of steam is just the working pressure of steam used by the turbine.
In this reactor, there is no separate steam generator and there will be only one steam circuit. The steam produced by the reactor is radioactive. Steam piping and casing of turbine requires proper shielding to protect the persons working in the power station.
The condensate is directly sent to the reactor as feed water. The steam produced in this case is radioactive. Hence, proper care must be taken. This is the simplest form of nuclear power plant.
(b) Pressurized Water Reactor:
In a pressurized water reactor, there are two circuits of water. One is primary circuit in which water is directly passed through the fuel core and becomes radioactive. Steam is produced in the secondary circuit which consists of heat exchanger which acts as boiler, as in a steam turbine.
The pressure in primary circuit should be high so that the boiling of water takes place at high pressure. A pressurizing tank keeps the water at about 100 bar, so that it will not boil. The water acts both as a coolant as well as a moderator. The pressurized water reactor is shown in Fig
(c) Liquid metal cooled reactor:
The layout of the liquid metal cooled reactor is shown in Fig. The setup has used two coolant loops, primary and secondary loops, and there are two heat exchangers to form the loop.
In the first loop, sodium (Na) is used as the coolant. Sodium is opted because it can be operated at high temperature (about 700°C) at atmospheric pressure of 1 bar and boiling and freezing temperatures are 880°C and 75°C, respectively. Sodium is first melted by electrical heater and pressurized to 7 bar pressure and then it is circulated through reactor vessel and gets heated by fission heat of reactor and then it is cooled in primary heat exchanger and goes back again to reactor vessel after transferring heat to the alloy of sodium and potassium (Nak) in the secondary loop.
In the secondary loop, the Nak in liquid form is used as coolant. The liquid Nak gets heated in the primary loop heat exchanger after taking heat from hot sodium. The heat received by sodium-potassium is transferred to feed water for the generation of steam.
In this case, low-cost graphite can be used as moderator. Sodium reacts with water and air and hence, heat exchanger must be made leakproof. Comparing to other coolant, leakage of sodium is very dangerous. Utmost care should be taken during the operation.
Advantages of Nuclear Power Plant:
As the amount of fuel consumption is very small compared to the conventional type of power plant, there is saving in the cost of fuel and transportation.
i. A nuclear power plant requires less space compared to other power plants.
ii. Besides producing large amount of power, a nuclear power plant produces valuable fissionable material.
Disadvantages of Nuclear Power Plant:
i. Inspite of utmost care, danger of radioactive radiation always sustains.
ii. The working place is always harmful to health.
iii. The disposal of nuclear waste is difficult.
iv. The capital cost is always high.
v. The maintenance cost of a nuclear power plant is high due to high salary paid to workers.
vi. A nuclear power plant does not respond to load fluctuations.
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