A chain reaction in fission of uranium is possible because

The correct answer is Option (3) → more than one neutron is released

A fission of \( {}^{235}\mathrm{U} \) is started by one neutron and releases, on average, 2 to 3 new neutrons. Each of these can cause a further fission, so the number of fissions can keep growing. This self-sustaining multiplication is the chain reaction.

    \( \displaystyle {}^{1}_{0}\mathrm{n} + {}^{235}_{92}\mathrm{U} \rightarrow {}^{144}_{56}\mathrm{Ba} + {}^{89}_{36}\mathrm{Kr} + 3\,{}^{1}_{0}\mathrm{n} \)

Why the other options are incorrect

• Option (1) large amount of energy is released — Incorrect. A large energy release is true of fission, but energy does not trigger further fissions; neutrons do.

• Option (2) small amount of energy is released — Incorrect. Fission releases a large amount of energy (about 200 MeV per fission), and energy is not what sustains the chain anyway.

• Option (4) fragments in fission are radioactive. — Incorrect. The radioactivity of the fragments does not produce the neutrons that keep the chain going.

Note: A similar question was also asked in CUET (UG) 2025 on 23-05-2025 (Slot 2).

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), but it is still in the CUET 2026 syllabus, so do not skip it.

NCERT (Pre-rationalised edition, 2022-23):
Nuclear Reactor – “… When \( {}^{235}_{92}\mathrm{U} \) undergoes a fission after bombarded by a neutron, it also releases an extra neutron. … In fact, on an average, 2½ neutrons per fission of uranium nucleus are released. The fact that more neutrons are produced in fission than are consumed raises the possibility of a chain reaction with each neutron that is produced triggering another fission.”