What is nuclear fusion? Explain giving its nuclear equations and write down the definition of thermonuclear fusion.

(N/A) Nuclear fusion is a process in which two light nuclei combine to form a single heavier nucleus,resulting in the release of a large amount of energy.
Examples of nuclear fusion reactions:
$(1)$ ${ }_{1}^{1} H + { }_{1}^{1} H \rightarrow { }_{1}^{2} H + e^{+} + \nu + 0.42 \text{ MeV}$
(Proton + Proton $\rightarrow$ Deuteron + Positron + Neutrino + Energy)
$(2)$ ${ }_{1}^{2} H + { }_{1}^{2} H \rightarrow { }_{2}^{3} He + { }_{0}^{1} n + 3.27 \text{ MeV}$
(Deuteron + Deuteron $\rightarrow$ Helium-$3$ + Neutron + Energy)
$(3)$ ${ }_{1}^{2} H + { }_{1}^{2} H \rightarrow { }_{1}^{3} H + { }_{1}^{1} H + 4.03 \text{ MeV}$
(Deuteron + Deuteron $\rightarrow$ Triton + Proton + Energy)
For fusion to occur,nuclei must overcome the electrostatic Coulomb repulsion. This requires high kinetic energy,typically achieved at temperatures around $10^{7} \text{ K}$ to $10^{9} \text{ K}$.
Using the relation $K = \frac{3}{2} k_{B} T$,where $K \approx 400 \text{ keV}$,the required temperature is:
$T = \frac{2K}{3k_{B}} \approx 3 \times 10^{9} \text{ K}$.
Thermonuclear fusion is defined as the process of nuclear fusion achieved by raising the temperature of the system to such an extent that the particles possess sufficient kinetic energy to overcome the Coulomb repulsive barrier.