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A nuclear reactor is a device in which the nuclear fission reaction takes place in a self sustained and controlled manner. The first nuclear reactor was built in 1942 at Chicago USA.

Depending on the purpose for which the reactors are used, they may be classified into research reactors, production reactors and power reactors. Research reactors are used primarily to supply neutrons for research purpose and for production of radio isotopes. The purpose of production reactors is to convert fertile (non-fissile but abundant) material into fissile material. The power reactor converts nuclear fission energy into electric power. The power reactors can be further classified into boiling water reactor, pressurised water reactor, pressurised heavy water reactor and fast breeder reactor depending upon the choice of the moderator and the coolant used. Numerous reactors of different designs have been constructed all over the world for a variety of purposes, but there exists a number of general features common to all the reactors. The schematic diagram of a nuclear reactor is shown in visual.

Essential parts of a nuclear reactor
(i) Fissile material or fuel
The fissile material or nuclear fuel generally used is Uranium 235. But this exists only in a small amount (0.7%) in natural uranium.
In the pressurised heavy water reactors (PHWR) built in our country, natural uranium oxide is used as fuel. In the pressurised light water reactors (PWR), low enriched uranium is used. In the fast breeder test reactor (FBTR) at Kalpakkam, a mixture of the carbides of uranium and plutonium is used as fuel. The fuel of the prototype fast breeder reactor (PFBR) that is being built at Kalpakkam, is a mixture of oxides of plutonium and uranium. Kamini [Kalpakkam mini reactor] is the only operating reactor in the world which uses Uranium 233 as fuel. In this reactor, the fuel is an alloy of uranium and aluminium and is in the form of plates.

(ii) Moderator
The function of a moderator is to slow down fast neutrons produced in the fission process having an average energy. Ordinary water and heavy water are the commonly used moderators. A good moderator slows down neutrons by elastic collisions and it does not remove them by absorption. The moderator is present in the space between the fuel rods in a channel. Graphite is also used as a moderator in some countries.

(iii) Neutron source
A source of neutron is required to initiate the fission chain reaction for the first time. A mixture of beryllium with plutonium or radium or polonium is commonly used as a source of neutron.

(iv) Control rods
The control rods are used to control the chain reaction. They are very good absorbers of neutrons. The commonly used control rods are made up of elements like boron or cadmium.

(v) The cooling system
The cooling system removes the heat generated in the reactor core. Ordinary water, heavy water and liquid sodium are the commonly used coolants. A good coolant must possess large specific heat capacity and high boiling point. The coolant passes through the tubes containing the fuel bundle and carries the heat from the fuel rods to the steam generator through heat exchanger. The steam runs the turbines to produce electricity in power reactors. In fast breeder reactors, liquid sodium is used as the coolant. Being a metal substance, liquid sodium is a very good conductor of heat and it remains in the liquid state for a very high temperature as its boiling point is about 1000 degree Celsius.

(vi) Neutron reflectors
Neutron reflectors prevent the leakage of neutrons to a large extent, by reflecting them back. In pressurised heavy water reactors the moderator itself acts as the reflector. In the fast breeder reactors, the reactor core is surrounded by depleted uranium (uranium which contains less than 0.7% of Uranium 235 or thorium 232 which acts as neutron reflector). Neutrons escaping from the reactor core convert these materials into Plutonium 239 or Uranium 233 respectively.

(vii) Shielding
As a protection against the harmful radiations, the reactor is surrounded by a concrete wall of thickness about 2 to 2.5 m.