Assertion : Heavy water is used as moderator in nuclear reactor. Reason: Water cool down the fast neutron.

Step-by-step solution

When two bodies of same mass undergo an elastic collision, their velocities are interchanged after collision. Water and heavy water are hydrogenic materials containing protons having approximately the same mass as that of a neutron. When fast moving neutrons collide with protons, the neutrons come to rest and protons move with the velocity of that of neutrons.