Heavy water and ________ is one of the substance used as moderator in reactors.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Graphite
In a fission reactor the neutrons released are fast, and slow neutrons are much more likely to cause the next fission. A moderator is a material of light nuclei that slows the neutrons down by elastic collisions. A neutron loses most of its energy when it collides with a nucleus of nearly its own mass, but it bounces off a heavy nucleus almost unchanged. So titanium and cobalt, which are heavy metals, cannot act as moderators, while carbon in the form of graphite can.
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.”