What is atomic energy?
        Atomic or nuclear energy is the inner energy of the nucleus, which is released during different changes in its structure, nuclear fission or fusion. The amount of the released energy is the result of a phenomenon called mass deficiency. Mass deficiency means that the total mass of the protons and neutrons is bigger that the mass of the nucleus:
                                Z*mp + N*mn – mnucleus = Δm  >0
The Δm mass deficiency according to Einstein’s formula is E=mc2, where E means the bond energy released during diverse nuclear reactions in the form of heat or radiating energy.
The heat produced during the nuclear reactions can be utilized in many ways. Nowadays, this heat is used to generate electricity in power plants, to drive submarines and flattops, and of course to build lethal weapons like the atomic bomb.
 Nuclear Fission
The fission is a reaction during which the heavy nucleus splits into two, almost equal-sized and lighter nuclei. The fission of atoms with bigger atomic mass is an exothermic reaction, in the course of which a great amount of energy is released in form of heat. Free neutrons, gamma ray and other corpuscles arise, like beta and alpha corpuscles, as by-products of the reaction.  
Fission is used for generating atomic energy (in nuclear power plants) or for detonating atomic weapons. This reaction can be an effective source of energy, because substances called nuclear fuels need neutrons during the reactions and release further neutrons which clash into further nuclei, and this way produce an automatic chain reaction.
 
In nuclear power plants regulating rods are used to keep the reaction under control. This is necessary because they absorb a part of the neutrons and this way slow down the nuclear reaction and the tempo of the energy release. In the case of an atomic bomb there’s nothing to stop the neutrons and the chain reaction, this is why the energy release becomes a destructive detonation.
***There is millions of times more energy in nuclear fuels than in the same amount of fossil or chemical fuels like gasoline or diesel oil, therefore it is a very tempting energy resource, nevertheless the nascent waste is highly dangerous, radioactive and it doesn’t dissolve for thousands of years. Along with this, another global problem emerged: the one of the nuclear waste.
Nuclear Power Plants
 
The nuclear power plant is a power house which produces energy in the form of heat and electrical energy. The basis of the energetic process is the heat gained from the atomic energy, in contrast with traditional power plants which use the chemical energy of fossil fuels.
The heat is assured by more reactors with an efficiency ranging between 400 and 2000 MWs.