A moderator, apart from its high neutron slowing power, and low non-productive neutron, should be

Question

RRB Paper · Accepted Answer

The moderator helps slow down the neutrons produced by fission to sustain the chain reaction.

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.

Stable under nuclear radiation