In the sun, four hydrogen nuclei fuse to form one helium nucleus with the release of 26.7 MeV. The fission of one \({}^{235}_{92}\mathrm{U}\) nucleus releases about 200 MeV. The energy released per nucleon in this fusion is about how many times the energy released per nucleon in the fission?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 8
To compare the two processes for the same amount of fuel, divide the energy of each reaction by the number of nucleons that take part. Four protons take part in the fusion. The uranium nucleus has 235 nucleons.
\( \begin{aligned} E_{\mathrm{fusion}} &= \frac{26.7\ \mathrm{MeV}}{4} \\ &\approx 6.7\ \mathrm{MeV\ per\ nucleon} \end{aligned} \)
\( \begin{aligned} E_{\mathrm{fission}} &= \frac{200\ \mathrm{MeV}}{235} \\ &\approx 0.85\ \mathrm{MeV\ per\ nucleon} \end{aligned} \)
\( \begin{aligned} \frac{E_{\mathrm{fusion}}}{E_{\mathrm{fission}}} &= \frac{6.7}{0.85} \\ &\approx 8 \end{aligned} \)
Therefore, per nucleon, the fusion of hydrogen into helium gives about 8 times more energy than the fission of uranium.
NCERT (2026-27):
Nuclear fusion – energy generation in stars – “Thus, four hydrogen atoms combine to form an 42He atom with a release of 26.7 MeV of energy.”
Fission – “The energy released (the Q value ) in the fission reaction of nuclei like uranium is of the order of 200 MeV per fissioning nucleus. …”