The energy source of a nuclear power plant is fission reaction. In fission reaction we start with an unstable atom (uranium-235) which splits apart into two small more stable atoms. When we go from something that is really very unstable (uranium-235) to thing that is more stable (barium and krypton), energy is released.

Now to split the uranium atom we need to aggravate it, what we do is we strike a neutron into this big uranium atom, which converts it into uranium-236 from uranium -235.

Uranium-236 is super unstable atom and splits up into krypton-92 and barium-141, which are way more stable. Other than release of energy, 3 free neutrons are also released in the process.

These neutrons further strikes another 3 uranium atoms and starts up a chain reaction, helping in getting a great amount of thermal energy.

Working

It all starts with a thermal neutron striking the uranium pellet and starting the chain reaction, with release of 3 new high energy neutrons as by-products of this exothermic reaction. These neutrons strike another 3 uranium atoms, and this chain reaction keeps on increasing. Heavy water is feed into the high pressure vessel which absorbs the heat generated by the chain reaction. This water doesn’t really boil as its being heated under high pressure. This is not the ordinary water that is feed into high pressure vessel; it’s 10 times heavier than the ordinary water. The main function of this water is to slow down the high energy neutrons to the level of thermal neutron or to modify the speed of neutron, that’s why it is called moderator. When neutrons travels in heavy water (deuterium oxide) it strikes with each and every molecule of heavy water thus transferring its energy to moderator, and slows down to an optimum speed.

In nuclear power plant, the chain reaction so started is controlled by the controls rods mainly boron or cadmium rods. These rods absorb the extra neutrons thus stopping the chain reaction. Generally these rods are put in and out of pressure vessel every 10 seconds, to control the power output according to the requirement or load at the turbine.

Heated water or moderator in the pressurized vessel is then pumped to the heat exchanger. Keeping in mind moderator cannot leave the circuit as it is radioactive so the water for the heat exchange is pumped from a water source (river) and this water absorbs the heat of moderator and converts into highly pressurized steam.

This pressurized steam is first feed in a high pressure turbine which converts its pressure energy into mechanical energy, but this steam still have enough pressure energy left in it to turn a low pressure turbine, so this combined mechanical energy from both the turbines is used to turn the armature of a generator thus producing electricity.

This electricity is then stepped-up to high voltage by step-up transformer, and transmitted to the nearest power grid through transmission lines.

The steam that leaves the turbines is then condensed in a condenser. The condenser is connected to the cooling tower. The cold water from the cooling tower takes the heat from the condensed water of the condenser. The hot water in cooling tower is sprayed in air thus cooling it down, to be again pumped back to the condenser.