Fast neutrons can easily be slowed down by:

Moderator is the medium used to control the speed of neutrons in the reaction. In nuclear reactors, these moderators are used to control the speed of reaction or stop the reaction. In nuclear reactors, mainly water is used as moderator. Since, water contains Deuterium which slows down neutrons passing through the water. For faster moving neutrons, to slow them down, we have to pass them through the water.