Nuclear fusion

The helium formed has mass 4 u, so energy released per u $= \left(\frac{0.02866}{4}\right)(931\ MeV) = 6.675$ MeV

For fusion, two light nuclei must come within the range of the nuclear force (about $10^{-15}$ m).
Both nuclei are positively charged, so they repel each other; this coulomb barrier must be overcome before they can fuse.
At very high temperatures (of the order of $10^7$ to $10^9$ K), the nuclei have enough thermal kinetic energy to overcome this repulsion.
Hence fusion needs high temperature because the kinetic energy is then high enough to overcome the coulomb repulsion between nuclei.

Reaction: $^2_1H + ^2_1H \rightarrow ^4_2He$ + energy
Binding energy of one deuterium nucleus (2 nucleons) = $2\times1.1 = 2.2$ MeV
Total binding energy of the two deuterium nuclei = $2\times2.2 = 4.4$ MeV
Binding energy of the helium nucleus (4 nucleons) = $4\times7.0 = 28.0$ MeV
Energy released = binding energy of product − binding energy of reactants = 28.0 − 4.4
= 23.6 MeV