Identify the correct statement among the following in the context of a nuclear reactor.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Moderator is used to slow down the neutrons.
In a reactor, fission of uranium-235 is caused far more readily by slow (thermal) neutrons, while the neutrons released in fission are fast. Each part of the reactor has a separate job.
Option (1) Coolant is used to slow down the neutrons. — Incorrect. The coolant carries away the heat produced in the core; it is not used to slow the neutrons.
Option (2) Moderator is used to slow down the neutrons. — Correct. The moderator (water, heavy water or graphite) slows the fast neutrons down by collisions with light nuclei.
Option (3) Cadmium rods are used to slow down the neutrons. — Incorrect. Cadmium rods are control rods: they absorb neutrons to control the rate of fission, rather than slow them down.
Option (4) Energy released is of the order of keV. — Incorrect. One fission releases about 200 MeV, so the energy is of the order of MeV, not keV.
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 – “… Therefore, in reactors, light nuclei called moderators are provided along with the fissionable nuclei for slowing down fast neutrons. The moderators commonly used are water, heavy water (D2O) and graphite. … The reaction rate is controlled through control-rods made out of neutron-absorbing material such as cadmium.”