The breaking up of nuclei of heavy atoms (U235 or Th232) into two nearly equal parts with the release of huge amount of energy is known as nuclear fission. The release of huge amount of energy during fission is due to mass defect i.e. the mass of the final product comes out to be less than the initial product.
This mass defect is converted into heat energy according to Einstein’s relation, E = mc2.
Nuclear fission is done by bombarding Uranium nuclei with slow moving neutrons. This splits the Uranium nuclei with the release of huge amount of energy and emission of neutrons (called fission neutrons).
These fission neutrons cause further fission. If this process continues, then in a very short time a huge amount of energy will be released which may cause an explosion. This is known as the explosive chain reaction.
But in a reactor, a controlled chain reaction is allowed. This is done by systematically removing the fission neutrons from the reactor. The greater the number of fission neutrons removed, the lesser is the intensity (i.e., fission rate) of energy released.
Nuclear Power Plant Working Principle
Nuclear power plants consist of a nuclear reactor in place of a furnace, in which heat is generated by splitting atoms of radioactive material under controlled conditions.
The heat energy thus produced is used in generating steam at high temperature and pressure. This steam drives the steam turbine which converts steam energy into mechanical energy. The turbine spins the alternator which converts mechanical energy into electrical energy.
The most amazing feature of a nuclear power plant is that huge amount of electrical energy can be produced from a small amount of nuclear fuel.
Elements of Nuclear Power Plant
- Nuclear Reactor,
- Coolant and Coolant pump,
- Heat Exchanger,
- Steam Turbine, Condenser, Generator.
Nuclear Reactor: It is the apparatus in which controlled nuclear fission chain reaction is carried out for practical utilization of the released energy. It is constructed in the form of a cylinder or sphere from 10 to 15 cm thick steel plate and contains the fuel elements, the neutron control devices and a coolant.
One type of nuclear reactor is shown in the Figure. It consists of a large number of uranium rods placed in a calculated geometrical lattice between the layers of pure graphite (moderator) blocks. To prevent oxidation of uranium, the rods are covered by close-fitting aluminum cylinders.
The control rods are so inserted in the lattice that they can be raised or lowered between the uranium rods whenever necessary. The whole reactor is surrounded by a concrete shield. A modern reactor has following important parts:
Fuel: The fissionable material, known as fuel, plays a key role in the operation of a reactor. Uranium enriched with the isotope U235 of Pu239 is used as fuel.
Moderators: They are substances which when introduced into radioactive fuel mass, are capable of slowing down the neutrons. The slow neutrons are more effective in triggering fission in natural uranium than the fast neutrons. Heavy water, light water, beryllium and graphite are generally used as the moderator.
Control Rods: The control rods are made of cadmium and are inserted into the reactor. Cadmium is strong neutron absorber and thus regulates the supply of neutrons for fission. The intensity of chain reaction and hence heat production can be controlled with the help of control rods.
Coolant: It is the medium through which the heat produced in the reactor is transferred to the heat exchanger for production of steam.
- Gas Coolants — Air, helium and CO2,
- Liquid Coolants – Light and Heavy water,
- Metal Coolants – Molten sodium and Lithium.
Shield: Various types of intense rays are emitted from the reactor which may be injurious to the people working near the reactor. To protect them from this radiation, thick concrete walls are erected around the reactor.
Safety Devices: In case of an accident or any other emergency, a special set of control rods, known as ‘shut-off rods’ enter the reactor automatically. They immediately absorb the neutrons so that the chain reaction stops entirely.
Working of Nuclear Reactor
The actual operation of the reactor is started by pulling out the control rods so that they do not absorb many electrons. Then the stray electrons, which are always present in the reactor, start fissioning the U235 nuclei. In each fission, two or three fast electrons are produced. Then these neutrons also start fissioning the U235 nuclei.
Thus a chain reaction of fission starts. The increasing number of neutrons is controlled by pushing the cadmium rods into the reactor. These rods absorb some of the neutrons. Thus the energy produced is kept under control to avoid an explosion.
The coolant, say CO2 gas is pumped through the reactor to carry away the heat generated by the fission of uranium nuclei. The hot CO2 passes through a heat-exchanger and converts cold water into steam. This steam is used to derive turbines for generating electricity.
Working of Nuclear Power Plant
Stepwise working of a nuclear power plant is as under:
- Nuclear fuel is loaded into the nuclear reactor and chain reaction is started.
- The coolant is constantly circulated through a closed loop linking with the reactor and heat exchanger with the help of coolant pump. This circulating coolant carries heat produced in the reactor to heat exchanger.
- In the heat exchanger, circulating coolant gives its heat to the water circulating in another closed loop and converts it into the steam.
- Now, this steam is fed to the steam turbine where it drives the alternator and generates electricity.
Exhausted steam from steam turbine is condensed and converted into the water and fed again to the heat exchanger.This cycle is repeated again and again till plant is working.
Advantages and Disadvantages of Nuclear Power Plant
- It reduces the demand for depleting resources of energy (coal, oil and gas).
- The problem of transportation of fuel is also reduced.
- They require lesser space as compared to any other type of power plant of the same capacity.
- Very economical for bulk production of electricity.
- They can be located near the load centers.
- Nuclear fuels are available in abundance, all over the world, ensuring the operation of such plants for thousands of years.
- Capital cost is high.
- Highly skilled staff is required.
- Disposal of radioactive wastes is a big problem.
- High degree of safety measures is required.
- Fuel cost is high.
- Power generation cost is high.
Thanks for reading about nuclear power plant working principle.
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