Nuclear Fusion

Nuclear fusion is the phenomenon of combining or fusing two or more lighter nuclei to form a more stable heavy nucleus with the liberation of vast amounts of energy.
                   
                                A huge amount of energy is required for the fusion of nuclei.High temperature(millions of degrees)and high pressure(millions of pascals)are needed to bring about fusion.

   Examples

(1)Two deuterons can join together to form a Helium-3 nucleus and a neutron.the nuclear reaction can be represented as

Such nuclear reactions are the source of energy in the sum other stars.

The source of energy in both nuclear fusion and nuclear fission is the same,i.e,disappearance of a certain mass in the process which appears in the form of energy. the sum of masses of the products is somewhat less than the sum of the masses of the reactants.This difference in masses before and after the nuclear reaction is called the mass defect(,m).It is this mass defect which appears in the form of nuclear energy according to Einstein's mass-energy relation: 

                                                                                           E= mc²
Where C is velocity of light in vacuum.
This famous equation was derived by Albert Einstein in 1905.

Nuclear fission

Nuclear fission is the phenomenon of splitting of an unstable nucleus of a heavy atom(e.g.-uranium,plutonium or thorium,U-235)into two medium weight nuclei with liberation of an enormous amount of energy.The fission is carried out by bombarding the heavy nuclei with suitable projectiles,usually slow neutrons.
                                                       
                                                   The phenomenon was discovered in the year 1939 by two German scientists Otto Hahn and Fritz Strassman. The bombarded U-235 with low energy neutrons, called thermal neutrons.Just as a glass ball when hit with a hammer may shatter in different sizes,U-235 nucleus splits up broadly into two groups of nuclei:
  (1) a heavy group of nuclei,with mass number in the range A=130 to A=149.
  (2) a light group of nuclei,with mass number in the range A=85 to A=104.
     
           out of these, the most prominent products and Barium with mass number 139 and Krypton with mass number 94.three neutrons(or in some cases two neutrons)are produced along with enormous amounts of energy in the form of gamma rays ; heat energy and light energy.Fig. 1 represents line diagram of the fission of U-235.As the fission produces two neutrons or three neurons,the third neutron is shown with a light shade.

Figure. 1

             
                           In the form of a nuclear reaction,we represent nuclear fission of U-235 as follows :

The materials which can be fissioned even with low energy neutrons are called fissile materials.

These materials are used as fuels for nuclear power plants and also for production of atom bombs.

  The amount of energy produced in nuclear fission is enormous.One kilogram of U-235,on fission,produces as much energy as 2500 tons of coal produces on burning.