moderator is dispensed with.
graphite is used as the moderator
heavy water is used as the moderator
moderator may or may not be used
no new neutron
at least one new neutron
one new neutron
more than one new neutrons
same
lower
higher
unity
graphite, C02
graphite, air
heavy water, C02
lead, H2
20
200
2000
20,000