How do ordinary water and heavy water act as neutron moderators?

In nuclear fission, neutrons released during the process are highly energetic, typically possessing an average energy of about two million electron volts. To sustain a chain reaction in a thermal reactor, these fast neutrons must be slowed down to thermal energies through a process called down-scattering.12

Both ordinary water ($H_{2}O$) and heavy water ($D_{2}O$) act as moderators by facilitating this energy loss through collisions with light nuclei. Neutrons lose kinetic energy most efficiently when they collide with atoms of similar mass, such as the hydrogen (mass 1) found in ordinary water or the deuterium (mass 2) found in heavy water.1

While both substances effectively slow down neutrons, heavy water is particularly advantageous because it absorbs neutrons only slightly, whereas ordinary water has a higher probability of capturing them. In many reactor designs, these materials serve a dual purpose by also functioning as coolants to remove the heat generated during the fission process.312