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In a nuclear fission reactor, the neutrons must be slowed down to ensure an effective fission chain reaction process. This process of slowing down of neutrons down is known as moderation, and the chemical material that helps to slow down these neutrons is known as a neutron moderator. Heavy water is one of the two moderators that are used to allow a controlled process in a nuclear reactor. The other moderator is graphite.

I don’t think you can find any substitute for heavy water. Heavy water is highly enriched in the hydrogen isotope deuterium. Neutron interacts with billiard ball collision. Here neutron collides with nucleus of other atoms and loses its energy. If the colliding nucleus size is small, it will lose maximum energy. If nucleus is heavy then neutron hitting the nucleus changes its direction and slows down. Hence we use heavy water to slow down neutrons.

I totally agree with Parul. Water is used as moderator in reactors specially uranium reactors. The hydrogen present in water slows down the fast moving neutrons. The neutrons undergo collision with hydrogen molecule, loses its energy and slows down.

Lalit and Parul, you can use graphite as a moderator. Neutrons undergo large number of collisions before slowing down.While colliding with a lead, a neutron loses only a small fraction of its energy. While with water, the slowdown is faster requiring less number of collisions. Hence water is commonly used for a rapid slow down.