The moderator helps slow down the neutrons produced by fission to sustain the chain reaction. A moderator in a Nuclear reactor does not absorb neutrons but slows down the neutrons as early as possible.
Desirable properties of moderator
It should have high thermal conductivity as heat transfer requires between neutron and moderator at a very high rate.
It should be chemically stable and should not decompose due to nuclear radiation.
It should not absorb neutrons but slow down the neutrons as early as possible.
It should have high resistance to corrosion as it has to work under high pressure and temperature.