Working Principle:
A nuclear power plant differs from a conventional steam power plant only in the steam generating part. There is no change in the turbo-alternator and the condensing system.
The nuclear fuel which is at present in commercial use is uranium. Scientists say that 1 kg of uranium can produce as much energy as can be produced by burning 4500 tonnes of high grade coal.
To understand how nuclear fission works, refer the below image.
Uranium exists in the isotopic form of U235 which is unstable. When a neutron enters the nucleus of U235, the nucleus splits into two equal fragments and also releases 2.5 fast moving neutrons with a velocity of 1.5*10^7 meters/sec producing a large amount of energy, nearly 200 million electron-volts. This is called “nuclear fission”.
Chain reaction:
The neutrons released during the fission can be made to fission other nuclei of U235 causing a chain reaction. A chain reaction produces enormous amount of heat, which is used to produce steam.
The chain reaction under uncontrolled conditions can release extremely large amounts of energy causing “atomic explosion”.
Energy liberated in chain reaction, according to Einstein law, is E=mv^2, where E=energy liberated, m=mass in grams, v= speed of light = 3*10^10 cm/sec.
Out of 2.5 neutrons released in fission of each nuclei of U235, one neutron is used to sustain the chain reaction, 0.9 neutron is converted into fissionable material Pu239 and 0.6 neutron is absorbed by control rod and coolant moderator.
Function of the moderator is to reduce the energy of neutrons evolved during fission in order to maintain the chain reaction. The moderators which are commonly used are ordinary water and heavy water.
Main components of a nuclear power plant
Nuclear reactor:
A nuclear reactor may be regarded as a substitute for the boiler fire box of a steam power plant. Heat is produced in the reactor due to nuclear fission of the fuel U235. the heat liberated in the reactor is taken up by the coolant circulating through the core. Hot coolant leaves the reactor at top and flows into the steam generator (boiler).
Radiation hazards and shielding:
The reactor is a source of intense radio-activity. These radiations are very harmful to human life. It requires strong control to ensure that this radio-activity is not released into the atmosphere to avoid atmospheric pollution. A thick concrete shielding and a pressure vessel are provided to prevent the escape of these radiations to atmosphere.
Types of reactors:
● Pressurized water reactor
● Boiling water reactor
● Heavy water-cooled reactor.
Steam generator:
The steam generator is fed with feed water which is converted into steam by heat of the hot coolant. The purpose of the coolant is to transfer the heat generated in the reactor core and use it for steam generation. Ordinary water or heavy water is a common coolant.
Turbine
The steam produced in the steam generator is passed to the turbine and work is done by the expansion of steam in the turbine.
Coolant pumps and feed pump
The steam from the turbine flows to the condenser where cooling water is circulated. Coolant pump and feed pump are provided to maintain the flow of the coolant and feed water respectively.
Advantages of nuclear power plant
● It can be easily adopted where water and coal resources are not available.
● The nuclear power plant requires very small quantity of fuel. Hence fuel transportation cost is less.
● Space requirement is less compared to other power plants of equal capacity.
● It is not affected by adverse weather conditions.
● Fuel storage facilities are not needed as in case of the thermal power plant.
● Nuclear power plant will conserve the fossil fuels (coal, petroleum) for other energy needs.
● Number of workmen required at nuclear plant is far less than thermal plant.
● It does not require large quantity of water.
Disadvantages of nuclear power plant
● Radio-active wastes, if not disposed carefully, have adverse effect on the health of workmen and the population surrounding the plant.
● It is not suited for varying load conditions.
● It requires well-trained personnel.
● It requires high initial cost compared to hydro or thermal power plants.