Main components of a nuclear reactor and their functions
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Fuel Rods
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The fuel used in a nuclear reactor is enriched U-235. The solid fuel is made into pellets or rods which are shielded by placing it in stainless steel tubes.
Note : Enrichment is a process by which the percentage of fissionable U-235 is increased and is the process of converting U-238 into fissionable U-235.
Control Rods
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To control the fission reaction, rods made of cadmium or boron is suspended between the fuel rods. These rods can be raised or lowered and thus check the reaction by absorbing the right number of neutrons. Hence they are called control rods. When the control rods are completely inserted into the fuel, i.e., inside the reactor all the neutrons are absorbed and the reaction does not start.
These rods are then slowly raised till they absorb the right number of neutrons and thus leaving behind just enough neutrons to sustain the chain reaction. That is, when the control rods absorb two neutrons leaving behind one neutron to bring about further fission reaction we say that the reactor has attained criticality. At this stage number of atoms getting fissioned in one second is constant which means that energy is generated at a constant rate.
Moderator
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The speed of the neutrons produced during the fission reaction has to be slowed down so that the neutrons are in contact with the fuel rods for sufficient time to bring about fission reaction. This is done by surrounding the fuel rods with graphite or heavy water. The material used to slow down the speed of neutrons is called a moderator. A moderator should absorb as few neutrons as possible and also slow down the neutron.
Coolant
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The heat released during the fission reaction is carried away by liquid sodium or heavy water. The coolant enters from the base of the reactor core and leaves at the top. The heat carried away by the coolant is used for producing steam and the coolant is pumped back into the reactor core.
Shielding
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The whole reactor core is covered with a thick coat of radiation absorbent material like lead and is enclosed in a heavy concrete or steel dome. This is done to protect the workers from dangerous radiations.
Nuclear Reactor
Generation of Electricity
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When a neutron hits the enriched U-235, energy is released and this heat generated in the reactor is carried away by heavy water or liquid sodium. This energy is used for generating steam in the heat exchanger. The steam thus produced is used by the turbines and these are in turn connected to the coils of an a.c. generator. The spent steam is condensed and pumped back to the heat exchanger.
Atomic Bomb
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Explosive fission reaction is the basic principle of an atom bomb. Two pieces of a fissionable material like U-235 (each with a sub critical mass) are brought together by means of a conventional explosion. The two masses together constitute super critical mass. The chain reaction is started by a stray neutron. A tremendous amount of energy is released at a very fast rate and this results in a violent explosion.
Safety Measures
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In 1986 at Chernobyl, Russia an experiment at the nuclear power station went wrong. An explosion blew open the reactor and released radiation into the atmosphere. Over 6 tonnes of nuclear material escaped causing release of radiation. Radiation, if absorbed by an organism can cause damage. A large amount of iodine 131 was released in the Chernobyl accident.
Nuclear Power Station - Chernobyl
Apart from these nuclear accidents radioactive nuclear wastes are produced by nuclear reactors. These are classified as Nuclear wastes, intermediate - level wastes and high level wastes. Laundry water from workers uniforms, towels and hospital wastes form low level wastes. They must be carefully bundled up and put where they can cause no harm. Low level wastes are not very radioactive and the products have short lives. A lot of low level wastes are often burnt in sealed containers and the liquids are diluted and released into sea.
Intermediate - level waste contains reactor parts and chemicals. This waste is often covered in concrete and it is buried under ground.
High level waste is mainly liquid from fuel reprocessing. This is highly radioactive and have long half life periods. High level waste is virtified and trapped in special kind of glasses. It is sealed in steel containers and stored underground.
The fuel rods contain a lot of material that can be recycled. By reprocessing a large amount of uranium is recovered. This still leaves wastes which is bombared with neutrons and transmuted to a different isotope.
