A chain reaction in fission of uranium is possible because

The correct answer is Option (2) → more than one neutron is given out

A chain reaction needs each fission to trigger at least one more. The fission of uranium is started by a neutron, and each fission gives out two or three neutrons (2.5 on average), which can go on to split further uranium nuclei.

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

Option (1) small amount of energy is released — Incorrect. A fission releases a large amount of energy (about 200 MeV), not a small amount.

Option (2) more than one neutron is given out — Correct. The extra neutrons from each fission can cause further fissions, so the reaction can sustain itself.

Option (3) large amount of energy is released — Incorrect. The energy released is large, but energy does not cause the next fission; without the extra neutrons the reaction would stop.

Option (4) fragments in fission are radioactive. — Incorrect. The fragments are radioactive, but their decays do not split further uranium nuclei.

Note: A similar question was also asked in CUET (UG) 2026 on 18-05-2026 (Slot 1).

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.

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. This extra neutron is then available for initiating fission of another \( {}^{235}_{92}\mathrm{U} \) nucleus. 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.”