Nuclear Power Stations :
These convert nuclear energy into electrical energy.
Nuclear power reactor :
Nuclear power reactor is basically that part of nuclear power plant where energy released as a result of nuclear fission of radioactive material is utilized to heat the coolant which may in turn generate steam or be used in a gas turbine. The nuclear reactor may thus be regarded as a substitute for the boiler fire box of steam plant or combustion chamber or a gas turbine plane. The steam or the gas may be used as working fluid in nuclear power plant. The nuclear power plant may be of steam driven turbine or gas driven turbine as per the choice of the fluid.
The following functions are associated with the working of nuclear reactor:
(i) Producing a chain reacting or critical system,
(ii) Controlling the level of power release from the system,
(iii) Using spare neutrons to convert fertile into fissile material,
(iv) Protecting personnel from harmful radiations emanating from the core.
(Nuclear Power Station)
The purpose of the moderator in the reactor core is to moderate or reduce the neutron speeds to a value that increased the probability of Fission occurring. The moderator is a material having low atomic number and small neutron absorption cross-section which slows down the fast neutron by elastic collision. Three commonly used moderators are:
(i) Graphite
(ii) Heavy water
(iii) Beryllium.
A control rod in a nuclear power reactor is used to initiate the nuclear chain reaction when starting the reactor from cold; and to maintain the chain reaction at a steady value during the operation of the reactor. Also control rod shuts down the reactor automatically under emergency conditions. All this is being done without melting of fuel rods disintegration of coolant and destructive of reactor as the amount of energy released is enormous.
The materials used for control rods must have very high absorption capacity for, neutrons. The commonly used materials for control rod are:
Cadmium, boron
Three different types of fuel used in nuclear reactors are:
(i) Uranium 92 U 235
(ii) Secondary Uranium 92 U 235
(iii) Plutonium 92 U 235
Classification of nuclear reactors :
Nuclear reactors may be classified on the basis of neutron energies as follows:
1. Fast Nuclear reactors :
Here fusion occurs with high energy neutrons, in the absence of moderator.
2. Intermediate reactors :
With the use of some moderator, the reactors are known as intermediate reactors.
3. Thermal Reactors :
When the energy of neutrons is reduced to low electron voltages i.e. thermal range, the reactors are known as thermal reactors.
On the basis of fuel reproduction characteristics, the nuclear reactors may be classified as
A. Non regenerative type Nuclear Reactors :
These reactors do not create an appreciable amount of replacement fuel as the fuel is burned. Reactors using highly enriched fuel containing 90% or more of U235 in the fuel are of non-regenerative type.
B. Regenerative type :
In these reactors fuel is slightly enriched. These reactors do not replace the used fuel fully.
C. Heterogeneous reactors :
Here fuel is in the form of rods, plates and is placed in the matrix of moderator.
D. Fast Breeder reactor :
These reactors use highly enriched uranium or plutonium as fuel. A small core of few cubic meters is sufficient to give a heat output of about 100 MW. The core is surrounded by a blanket or ordinary fertile Uranium 238 or thorium which can be converted to fissile plutonium or Uranium 233, It is possible to get more new atoms of fissile material in the blanket than are destroyed by fission in the core. Such a reactor is known as breeders type. The major problem in such reactors is the removal of from the core. Liquid sodium is used as the coolant. A primary and secondary coolant circuit is used to transfer heat. Foolproof arrangements are incorporated to guard against the failure of coolant circuit. Even a momentary stoppage of the blow of coolant will result in disastrous rise in temperature.
(Fast Breeder Reactor)
Terms associated with nuclear reactions are given below :
Reflector in nuclear reactor:
In order to keep the size of the reactor small, and hence the amount of the fashionable material, it is necessary to conserve the neutrons. For this purpose the reactor core is surrounded by a material which reflects the escaping neutrons back into the core. This material is known as reflector.
Shielding in nuclear reactor:
The intensity of gamma and neutron radiation coming from the reactor core is far greater than the human body can tolerate. Hence it is necessary to surround the reactor with a shielding material to reduce the radiation intensity to the levels which are not harmful to personnel.
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