When lighter nuclei moving at a high speed are fused together to form a heavy nucleus, the process is called nuclear fusion.
In fusion reaction, the mass of heavier nucleus formed is less than the total mass of two lighter nuclei. Thus, just like a fission reaction, the source of energy in a fusion reaction is also the disappearance of mass, which gets converted into energy.
Nuclear fusion reaction takes place at very high temperature of about 108K. Therefore, this reaction is called thermonuclear reaction.
1H2 (Deuterium) + 1H3 (Tritium) → 2He4 (Helium) + 0n1 + Energy
The Mass loss is equal to 0.018 amu and the corresponding energy released is 1.79 × 109 Kj mol-1.