Theory:

When a heavy nucleus is split into two smaller nuclei, energy is produced. Similarly, energy is produced when two lighter nuclei join to form a heavier nucleus. This type of phenomenon is known as nuclear fusion. The word 'fusion' means 'joining' or 'combining' together, hence,
The process in which two lighter nuclei combine to form a heavier nucleus is termed nuclear fusion.
Nuclear fusion

$$_{1}H^{2}$$ $$+$$ $$_{1}H^{2}$$ $$\rightarrow$$ $$_{2}He^{4}$$ $$+$$ $$Q$$ ($$Energy$$)

Here, $$_{1}H^{2}$$ represents an isotope of hydrogen known as 'deuterium'. The average energy released in each fusion reaction is about $${3.84} \times {10^{-12}\ J}$$.

Nuclear fusion is the combination of two lighter nuclei of positive charge. According to electrostatic theory, the charges repel each other as they get closer. At higher temperatures of the order of $$10^7$$ to $$10^9\ K$$, the kinetic energy of the nuclei will overcome the repulsive force. Alpha rays, positrons, and neutrinos are generally emitted as a product along with light and heat energy.

The main advantage of a fusion reaction is that it does not release any harmful radiation. Unlike fission, nuclear fusion reactions are not chain reactions. The energy released in a nuclear fusion reaction is much higher than the fission reaction.
Conditions necessary for nuclear fusion:
Some traces of hydrogen are generally found in the Earth's atmosphere. If nuclear fusion occurs spontaneously at normal temperatures and pressures, then a number of fusion reactions could occur in the atmosphere, potentially resulting in explosions. But, such type of explosions does not occur. It is because nuclear fusion is only possible under certain circumstances.

Nuclear fusion occurs only at a very high temperature of the order of $$10^7$$ to $$10^9\ K$$, along with a very high pressure to fuse the hydrogen nuclei. This type of reaction is known as the Thermonuclear reaction.