The nucleus of U-235 isotope is highly unstable. Even a slow stray neutron colliding with it can rupture its nucleus completely, resulting in the formation of more stable smaller nuclei. During nuclear fission reactions high energy radiations such as gamma rays are also produced along with tremendous amount of heat.

Another element which undergoes fission easily in Pu-239.

Why is U-235 unstable?
Inside the nucleus there are protons and neutrons. Between the protons exist the electrostatic force of repulsion pushing the nucleus apart. Between the protons and neurons exist the nuclear force of attraction holding the nucleus together. In heavy atoms since the number of protons and neutrons is large, the electrostatic force of repulsion is stronger then nuclear force. Hence the atom is unstable.
A chain reaction may be defined as a reaction in which the particle that initiates the reaction is also produced in greater number so that the process is repeated.

When a nuclear fission reaction takes place in such a way that the number of atoms undergoing fission remains constant with time the reaction is said to be
critical. The minimum amount of fissile material required so that the chain process continues is called critical mass.

The main parts of a reactor are:
1.Fuel Rods: The fissile material used in a reactor is called nuclear fuel. Enriched u-235 in the form of rods or pellets is used as fuel.

2.Control Rods: Rods made of Cadmium or Boron is suspended between the fuel rods. Control rods absorb the neutrons and hence control the rate of the reaction.

3.Moderator: The speed of the neutrons produced in the fission has to be slowed down so that the neutrons can be effective for the chain reaction. A moderator slows the fast moving neutrons. Heavy water or graphite is used as moderator.

4.Coolant: A coolant like liquid sodium or heavy water carries the heat produced during the fission process to a heat exchange unit.

5.Shielding: The entire reactor core is encased in heavy concrete or steel to protect workers from dangerous adiations.

6.Generation of electricity: The heat generated in the core is carried by the coolant to the heat exchange unit where it converts water to steam. The steam operates a steam turbine. The used steam is sent to a condenser and pumped back to the heat exchanger.

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