Which one of the following is a material used as both a moderator and a coolant in nuclear reactors?

The answer is B) Heavy water. In a reactor, the moderator slows down fast neutrons so they can cause fission, and the coolant carries heat out of the core. Heavy water (D₂O) does both jobs in Indian PHWRs such as Rajasthan and Tarapur, and in Canada's CANDU reactors. Its key advantage is very low neutron absorption, so natural uranium (0.7% U-235) can sustain a chain reaction without enrichment.

Why the others fail:

• A) Ordinary water: It does moderate and cool in light water reactors, but it absorbs too many neutrons to run on natural uranium. It needs enriched fuel. This is the real source of confusion.
• C) Liquid ammonia: It is not used as a moderator in any commercial reactor.
• D) Liquid hydrogen: Not a reactor moderator or coolant. It is a rocket and cryogenic fuel.

Connect this to the three-stage nuclear programme: heavy water production and self-reliance in PHWRs underpin India's plan to use thorium later. For Mains GS-III, this links to energy security and the Indo-US civil nuclear deal.

Trap: Choosing ordinary water because it also acts as both moderator and coolant. The question expects the PHWR-based answer, so heavy water is the intended choice.