Energy release in fission is due to what?

• A. The lost mass defect is converted to about 200 MeV of energy.
• B. The energy is all in the form of photons.
• C. No energy is released in fission.
• D. Energy release is negligible.

Answer: (A)

When a heavy nucleus fissions, the total rest mass of the fission products (and any emitted neutrons) is slightly less than the rest mass of the original nucleus. That missing mass, the mass defect, is converted into energy according to E = mc^2. For a typical fission event like uranium-235, this amounts to about 200 MeV per fission. Most of that energy appears as the kinetic energy of the two fission fragments, which quickly transfers to the surrounding material as heat, with smaller portions carried by prompt neutrons and gamma rays. So the energy release is fundamentally a mass-energy conversion, not all in photons and not negligible.