Nuclear fission and Fusion Upsc

What are two types of Nuclear Reaction?

Basically, there are two types of reactions in nuclear energy production they are Fission and Fusion reactions.

Nuclear fission

German scientists, Otto Hahn, and F.Strassman in 1939 discovered that when Uranium Nucleus is bombarded with a neutron, it splits into two smaller Nuclei among with emission of neutrons and energy.

Nuclear fission is the phenomenon of splitting the heavier nucleus into two smaller nuclei with the release of an enormous amount of energy and a few neutrons are called Nuclear Fission.

Example: Nuclear fission of a Uranium Nucleus (U235).

92 U 235  0  n  à56 Ba 141 + 36 Kr 92 + 3 0 n 1  + Q (Energy)

The average energy released in fission is 3.2 x 10 -11 J.

We can perform nuclear Fission at room temperature. Alpha, Beta, and Gamma radiations are emitted. Fission reaction that emits Gamma radiation, creates mutation in the human gene and causes genetic diseases.

Fissionable Materials

A Fissionable material also called Fissile Material, is a radioactive element, that goes through fission in a constant process when it is hit by a neutron. Example: U235, Plutonium (Pu 239, Pu 241 ). All isotopes of Uranium do not go through Nuclear fission when hit by a neutron.

Example U235 is fissionable, U 238 is not fissionable.

We can convert some radioactive material into fissionable material. They are called Fertile Material. Example of Fertile material is Uranium-238, Thorium-232, Plutonium-240.

Chain Reaction

A Uranium nucleus U-235, when bombarded with a neutron, goes through fission by producing three neutrons. These three neutrons again react with the three other nuclei of Uranium producing nine neutrons. Then these nine neutrons react with the

Uranium nucleus similar producing 27 Neutrons. In process continuous and rapidly multiplying Neutrons in a geometrical progression. This process is called a Chain Reaction. A Chain reaction is a process in which the number of neutrons multiplies rapidly.

There are two types of chain reactions. One is a Controlled Chain Reaction, and Another one is an Uncontrolled Chain reaction.

Controlled Chain Reaction

In the controlled chain reaction, Only one neutron is allowed to release. Only one neutron is allowed to do the chain reaction. The excess neutron produced in the nuclear reaction is absorbed by the Neutron absorber.

Thus the chain reaction is controlled. This controlled chain reaction is used for a constructive purpose such as Nuclear Power Generation.

Uncontrolled Chain Reaction

In the Uncontrolled Chain Reaction, it does not control the neutrons released. There the Neutron release allowed to multiply rapidly with no control. As a result, enormous Neutrons were produced and continued to multiply at a very rapid rate resulting in the release of an enormous amount of energy.

We employ this process in Atom Bombs.

Critical Mass

During the fission process, it releases about 2 to 3 neutrons. Not all the neutrons are released to go for a chain reaction. Some neutrons are lost called leakage of neutrons, and some are absorbed by the non-fissionable materials. If it loses these neutrons, the chain reaction cannot continue further.

To maintain the chain reaction the number of neutrons produced must be greater than the number of neutrons lost. This can be done only when the size of the fissionable material is at a certain optimum range. This value is called Critical Mass (Mc).

As per the above statement, Critical Mass is the minimum mass of the fissile material to maintain the chain reaction. If the fissile material is less than the Critical Mass, we call it Subcritical.

If the fissile material is more than the critical mass, we call it Supercritical.

Atom Bomb or Nuclear bomb

Atom Bomb is the classical example of an uncontrolled chain reaction that happens in a small interval of time leading to a huge explosion. The Nuclear bomb dropped in Hiroshima is called the Little Boy of Gun Type bomb that used Uranium Core.

The Nuclear bomb dropped in Nagasaki was called Fat Man, it was an explosion-type bomb that used Plutonium core.

Nuclear Fusion

If two lighter nuclei joined to create a heavier nucleus. This phenomenon is called Nuclear Fusion. Example: 1 H 2 + 1 H 2 à 2 He 4 + Q (Energy). 1 H 2 represents an isotope of hydrogen called ‘Deuterium’. The average energy released in each fusion reaction is 3.84 x 10 -12 J.

Alpha rays, positrons, and Neutrino are emitted. It emits only light and heat energy. The mass of the daughter nuclei formed during fission and fusion reaction is less than the sum of the masses of the two-parent nuclei.

This difference is mass is called Mass Defect. This mass is converted to energy, as per the mass-energy equivalence. Einstein proposed this concept in 1905. This can be converted into energy and vice versa.

The relation between energy and mass proposed by Einstein is E = MC2.

Where M–Mass, E–Energy, C–Velocity of light (3 x 10 8 ms -1 ).

Conditions Necessary for Nuclear Fusion

Nuclear fusion only takes at high temperature in the order of 107 to 109 K and high pressure to push hydrogen nuclei closer to fuse. It is also called a Thermonuclear reaction. The two charges are positive. According to the electrostatic theory, when two nuclei come together it gets repelled.

This repulsive force will be overcome by the Kinetic energy of the nuclei at high temperatures at an order of 107 to 109.

Stellar Energy

The fusion reaction takes place in the sun and star results in the release of enormous light and heat which is called ‘Stellar Energy’. This high surface temperature is enough to induce the fusion of the hydrogen nuclei.

Thus, nuclear fusion or thermonuclear reaction is the source of light and heat energy in the Sun and stars.

Hydrogen Bomb

A hydrogen bomb is working on the principle of Nuclear Fusion. There is an inbuilt atom bomb that explodes that creates high temperature and pressure that is required for a fusion reaction.

Once the atom bomb explodes, then the fusion reaction takes place in the hydrogen core. The energy released by the Hydrogen bomb (Nuclear Fusion) is much higher than the atom bomb (fission bomb).

* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.
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