Which one among the following is used as a moderator in nuclear reactors?

In nuclear reactors, a moderator is used to slow down fast-moving neutrons to thermal speeds to sustain a controlled chain reaction [t1][t2]. An effective moderator must have a low atomic mass to ensure high energy loss per collision and a low neutron absorption cross-section to prevent neutron loss [t1]. Heavy water (D2O) is a primary material used for this purpose because it has excellent moderating properties and a very low neutron absorption rate compared to light water [t1][t5]. While light water and graphite are also common, heavy water is particularly valued in Pressurized Heavy Water Reactors (PHWR) as it allows the use of natural uranium fuel [t1][t5]. Other options like ozone and hydrogen peroxide are chemical oxidants related to atmospheric chemistry and do not possess the physical properties required for neutron moderation [c1][c3].

Sources

1. [1] https://en.wikipedia.org/wiki/Neutron_moderator
2. [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 19: Ozone Depletion > The chemical reaction > p. 269
3. [3] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 19: Ozone Depletion > Effects on air quatity > p. 272