A2 Daily A Level Physics question

An R&D team compares two fuels for a compact power source: (i) deuterium–tritium fusion, 2H + 3H → 4He + n, and (ii) fission of 235U to typical mid‑mass fragments. Take average binding energy per nucleon for the fission fragments ≈ 8.5 MeV and for 235U ≈ 7.6 MeV. For fusion, the total binding energies of 2H and 3H are about 2.2 MeV and 8.5 MeV respectively, and the binding energy per nucleon of 4He is about 7.1 MeV. Assuming the energy released equals the increase in binding energy and that mass per nucleon is effectively constant across fuels, which statement about energy released per kilogram must be true?

1. A Fission yields more energy per kilogram by roughly a factor of 2, because each heavy nucleus contains many more nucleons.
2. B Fusion yields more energy per kilogram but only slightly (under 10%), since the reactants are already fairly well bound.
3. C Fusion yields more energy per kilogram by about a factor of 4, because the average increase in binding energy per nucleon is around 3.5 MeV for D–T but about 0.9 MeV for 235U fission. (correct)
4. D They are similar per kilogram, since fission releases about 200 MeV per nucleus whereas D–T releases only about 18 MeV, and the difference in nucleus size cancels.