0We get the energy to do all our work in our daily life from different sources. Do you know what the source of energy for the plant life and animal life on our planet is? Yes, you are right! It is the sun which gives out light and heat for all the lifeforms on EARTH. But from where the SUN gets its energy? It is obtained from the atoms present in the sun mainly from HELIUM and HYDROGEN. The nuclear reactions of the atoms of these two elements continuously produce the necessary energy for the sun to survive for millions of years in the Milky Way Galaxy. These reactions are called NUCLEAR FUSION.
In nuclear fusion, two NUCLEI of HYDROGEN fuse together to form a HYDROGEN nucleus. A large amount of heat and light energy will be emitted, and that is being radiated into space, in this process. Our planet earth receives that energy and all the life forms survive.
Before we understand the process of nuclear fusion reactions that take place in the sun and distant stars, let us understand the process of a nuclear reaction that takes place in our planet, that is NUCLEAR FISSION.
Fission
After the discovery of the neutron. Enrico Fermi found that when neutrons bombard various elements, new radioactive elements are produced. However, when a neutron was bombarded on a uranium target, the uranium nucleus broke into two nearly equal fragments releasing a great amount of energy. An example of such a reaction is
n + U 235 ---> U 236 ---> Ba 144 + Kr89 + 3n
Fission does not always produce barium and krypton. A different
pair can be produced, for example,
n + U235 ---> U236 --->Sb133 + Nb 99 + 4 n
Still another example is
n + U235 ---> Xe 140 + Sr94 + 2 n
The fragment nuclei produced in fission are highly neutron-rich and unstable. They are radioactive and emit beta particles in succession until each reaches a stable end product.
The energy released (the Q value ) in the fission reaction of nuclei like uranium is of the order of 200 MeV per fissioning nucleus.
Energy can be released if two light nuclei combine to form a single larger nucleus, a process called nuclear fusion. Some examples of such energy liberating reactions are
1H + 1H ---> 1H 2 + e+ v + 0.42 MeV
1H + 1H ---> 2He + n + 3.27 MeV
To generate a useful amount of energy, nuclear fusion must occur in bulk matter. What is needed is to raise the temperature of the material until the particles have enough energy – due to their thermal motions
alone – to penetrate the Coulomb barrier. This process is called thermonuclear fusion.
Thus, for thermonuclear fusion to take place, extreme conditions of temperature and pressure are required, which are available only in the interiors of stars including the sun. The energy generation in stars takes place via thermonuclear fusion.
In the sun the fusion reaction is a multi-step process in which hydrogen is burned into helium; hydrogen is the ‘fuel’ and helium the ‘ashes’. This is called a proton-proton (p, p) cycle.
The first thermonuclear reaction on earth occurred at Eniwetok Atoll on November 1, 1952, when USA exploded a fusion device, generating energy equivalent to 10 million tons of TNT (one ton of TNT on explosion releases 2.6 × 1022 MeV of energy).
A sustained and controllable source of fusion power is considerably more difficult to achieve. It is being pursued vigorously in many countries around the world (including India) because fusion reactor is regarded as the future power source.
