Nuclear Fusion

NUCLEAR FUSION

You have learnt that energy can be produced when a heavy nucleus is split up into two smaller nuclei. Similarly, energy can be produced when two ighter nuclei combine to form a heavier nucleus. This phenomenon is known as nuclear fusion.

1. Definition

The process in which two lighter nuclei combine to form a heavier nucleus is termed as 'nuclear fusion'.

E.g.: ₁H² + ₁H²  → ₂He⁴ + Q (Energy)

Here, ₁H² represents an isotope of hydrogen known as 'deuterium'. The average energy released in each fusion reaction is about 3.84 × 10^-12 J. Figure 6.5 represents this.

The mass of the daughter nucleus formed during a nuclear reaction (fission and fusion) is lesser than the sum of the masses of the two parent nuclei. This difference in mass is called mass defect. This mass is converted into energy, according to the mass-energy equivalence. This concept of mass-energy equivalence was proposed by Einstein in 1905. It stated that mass can be converted into energy and vice versa. The relation between mass and energy proposed by Einstein is E = mc² where c is the velocity of light in vacuum and is equal to 3 × 10⁸ ms^–1.

2. Conditions necessary for nuclear fusion

Earth’s atmosphere contains a small trace of hydrogen. If nuclear fusion is a spontaneous process at normal temperature and pressure, then a number of fusion processes would happen in the atmosphere which may lead to explosions. But, we do not encounter any such explosions. Can you explain why?

The answer is that nuclear fusion can take place only under certain conditions.

Nuclear fusion is possible only at an extremly high temperature of the order of 10⁷ to 10⁹ K and a high pressure to push the hydrogen nuclei closer to fuse with each other. Hence, it is named as 'Thermonuclear reaction'.

3. Stellar Energy

The stars like our Sun emit a large amount of energy in the form of light and heat. This energy is termed as the stellar energy. Where does this high energy come from? All stars contain a large amount of hydrogen. The surface temperature of the stars is very high which is sufficient to induce fusion of the hydrogen nuclei.

Fusion reaction that takes place in the cores of the Sun and other stars results in an enormous amount of energy, which is called as 'stellar energy. Thus, nuclear fusion or thermonuclear reaction is the source of light and heat energy in the Sun and other stars.

4. Hydrogen Bomb

Hydrogen bomb is based on the principle of nuclear fusion. A hydrogen bomb is always designed to have an inbuilt atom bomb which creates the high temperature and pressure required for fusion when it explodes. Then, fusion takes place in the hydrogen core and leads to the release of a very large amount of energy in an uncontrolled manner. The energy released in a hydrogen bomb (or fusion bomb) is much higher than that released in an atom bomb (or fission bomb).

Features of Nuclear fission and nuclear fusion

NUCLEAR FISSION

1. The process of breaking up (splitting) of a heavy nucleus into two smaller nuclei is called 'nuclear fission'.

2. Can be performed at room temperature.

3. Alpha, beta and gamma radiations are emitted.

4. Fission leads to emission of gamma radiation. This triggers the mutation in the human gene and causes genetic transform diseases.

NUCLEAR FUSION

1. Nuclear fusion is the combination of two lighter nuclei to form a heavier nucleus.

2. Extremely high temperature and pressure is needed.

3. Alpha rays, positrons, and neutrinos are emitted.

4. Only light and heat energy is emitted.