The main part of the nuclear reactor is called the reactor core. It consists of the following parts:
a) Fuel rods
The fissionable material used in the reactor is called fuel. The fuel used is enriched uranium 235 (in the form of U2O3). The solid fuel is made into rods or pellets, which are shielded by placing in stainless steel tubes.
b) Control rods
To control the fission process, rods made of cadmium or boron are suspended between the fuel rods.
These rods can be raised or lowered and control the fission process by absorbing neutrons. That is why they are called 'control rods'. Controlling of neutrons is based upon the fact that cadmium and boron can absorb neutrons to form the corresponding isotopes, which are not radioactive.
c) Moderator
The speeds of the neutrons produced in the fission have to be slowed down so that they are easily captured by the fuel and the fission process can take place most effectively. This is done by surrounding the fuel rods with heavy water (D2O). The material used to slow down the neutrons (without absorbing them) is called a moderator. Graphite is also used as a moderator sometimes.
d) Coolant
To carry away the heat produced during fission, a liquid is circulated in the reactor core. This liquid enters the base of the reactor core and leaves at the top. The heat carried by the outgoing liquid is used for producing steam. The liquid cools down and is recycled again. This liquid is called the coolant. Usually heavy water is used as coolant so that it may also act as moderator.
e) Shield
To prevent the losses of heat and to protect the persons operating the reactor from the radiation and heat, the entire reactor core is enclosed in a heavy steel or concrete some called the shield.
In nuclear power plants, the large amount of energy which is released in the form of heat during controlled fission process is converted into electrical energy. Twelve such nuclear power plants have been set up in our country.
