NUCLEAR REACTORS
Nuclear reactor is a principal component in the process of nuclear power generation. It is the main unit in which heat is generated, transferred to cooling medium like water and steam is produced which is used to run turbine. Turbine coupled to electrical alternator produces electrical power.
Principal Parts of a Nuclear Reactor :
It is a plant in which nuclear chain reaction is sustained (kept going on) at a steady controlled rate so that heat is produced continuously. The main function of the reactor is to control the emission and absorption of neutrons. For this to happen, the number (neutron density) as well as the speed of the neutrons has to be controlled. This is affected by absorption of some neutrons by 'control rods' and retarding the neutron speed by moderators. Nuclear Reactor Components is shown in figure below.
The essential components of a nuclear reactor (Figure) are
(1) Active core
(2) Moderator
(3) Control rods
(4) Thermal shield
(5) Biological shield
1. Active core : It contains mainly the fissionable material (nuclear fuel), For better control of reaction, the fuel may be diluted with non-fissionable material. For instance, uranium is clad with aluminium, stainless steel or zironium to prevent its oxidation and thus have better control over reaction.
The reactor fuel may be in solid form such as rods, plates or spheres; or in liquid form as a slurry. The minimum sire of the reactor for a self sustaining reaction is called 'critical size'.
2. Moderator : The function of moderator is to slow down the last fission neutrons travelling at 4200 km/s to about 1.5 km/s before subsequent fission occurs. The type of moderator used depends upon the type of fuel used. For example, 'Heavy Water' is used as a moderator when nuclear fuel is uranium. Heavy water looks and tastes like ordinary water but it contains Deuterium atom instead of hydrogen atom, chemically known as deuterium oxide. Ordinary water cannot be used as moderator since it absorbs too many neutrons to sustain a chain reaction. Graphite or Carbon is other type of moderator.
3. Control rods : These are meant to absorb as many of the neutrons impinging upon them as possible. Rods made of boron-steel alloy and cadmium are most commonly used as control rods. Fission is built up by pulling the rods up the reactor; and operation stops when the rods are pushed down.
4. Thermal shielding : It surrounds the entire reactor core and absorbs some of the radiations in the form of cc-rays and p-particles. It absorbs escaping neutrons produced by fission. It is usually made of iron. It gets heated but prevents the reactor wall from getting heated. Coolant flows through the shielding to take away the heat. Coolant may be circulated in tubes throughout the reactor to remove heat produced in the process of fission. The coolants generally used are air, water, carbondioxide, helium, hydrogen fused salts, organic fluids, sodium, potassium, lead and.bismuth alloys etc.
5. Biological shield : The whole of the reactor is enclosed in a biological shield to prevent the leakage of fast moving neutrons, slow neutrons , α - rays, β - particles etc. These radiations are much harmful to living organisms. This shield is made of iron or built with dense concrete.
Advantages of Reactors :
• Electric power generation by heat produced in the reactors.
• Production of radio isotopes of various elements which are useful in the fields of Physics, Biology, Medicine, Agriculture and Industry.
• Plutonium is produced which is used in atom bomb.
• Fast neutrons are produced which are used in fast reactors.
Main Products of a Reactor :
The main products of the reactor are
• Thermal energy (i.e., heat)
• Neutrons
• Radioactive fission fragments
• Fissionable materials
• Gamma quantas and other radiations
• Radio isotopes which are used for commercial purposes.