Nuclear fusion
The nuclei have to get very close in order to collide, which is approximately a million millionth of a millimetre. If the nuclei are moving very fast then they can overcome the electrostatic repulsion. The hotter a molecule is, the faster it will move and the more likely it is to collide.
The Sun and other stars use nuclear fusion to release energy. The sequence of nuclear fusion reactions in a star is complex, but overall hydrogen nuclei join to form helium nuclei.
One nuclear fusion reaction that takes place is hydrogen-1 nuclei fuse with hydrogen-2 (deuterium) nuclei to make helium-3 nuclei.
\(_{1}^{1}\textrm{H} +_{1}^{2}\textrm{H}\rightarrow_{2}^{3}\textrm{He}\)
For a nuclear fusion reactor to work, the temperature and pressure would each have to be very high. These extremely high temperatures and pressures are very difficult to reproduce and are very expensive. As a result, fusion as an energy source is a long way off.
In the The Tokamak is the most advanced fusion reactor design in the world., the deuterium (hydrogen-2) and tritium (hydrogen-3) nuclei are accelerated at very high speed around the reactor colliding with enough energy to cause nuclear fusion which will release energy.
\(_{1}^{2}\textrm{H} +_{1}^{3}\textrm{H}\rightarrow_{2}^{4}\textrm{He}+_{0}^{1}\textrm{n}\)
Both the neutron and the helium nucleus are moving at very high speed, but the neutrons can interact with the material that makes the Tokamak, which will then make it radioactive. The reactor therefore needs to be shielded with concrete to shield the radiation and inertial confinement by laser. Currently these technologies are not viable, as they cannot produce more energy than is required to initiate and sustain a fusion reaction.