Why does nuclear fusion reaction yield more energy than Nuclear fission reaction?

Fission only produces more energy than it consumes in larger nuclei (eg: Uranium & Plutonium) which have around 240 nucleons.

Fusion only produces more energy than it consumes in small nuclei (in stars, Hydrogen & its isotopes fusing into Helium).

The energy released when four Hydrogen nuclei fuse in to a Helium nucleus is around 27MeV or about 7 MeV per nucleon.

For fission of U or P, energies released are around 200 MeV or so. The energy per event is greater in fission, but the energy per nucleon (fusion = about 7MeV/nucleon; fission = about 1 MeV/nucleon) is much greater in fusion.

Now lets look at fission. An example of fission is when a U-235 atom is split by a neutron into a Ba-144 and Krypton-89 atoms and 3 neutrons. The binding energy per nucleon for Uranium is about 7.6 MeV and for Barium around 8.3 MeV giving an increase in binding energy during fission of about 0.7MeV per nucleon or a total of 164.5MeV in total.

In a fusion reaction firstly two Hydrogens form a Deuteron, a positron and an electron neutrino. Then the Deuterium fuses with  another Hydrogen to form  He-3 and a photon. Finally two He-3's fuse forming a Helium nucleus and two hydrogen nuclei.

Considering the mass of four  protons/hydrogen nuclei and the mass of helium produced we get a mass difference of 24.69 MeV.

Conclusion: We can conclude that fusion reactions give out more energy per reaction. This is 0.7 MeV for fission and 6.2 MeV for fusion.  

I think you have cleared your doubt. Have a nice time ............bye.

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