During the nuclear fission process, the speed of neutrons is reduced to a critical value by using which of the following moderators?

Neutron moderators slow fast neutrons to thermal speeds, and graphite and heavy water are the classic choices in reactors.

Graphite is a solid moderator of low neutron absorption used in several reactor designs.

Heavy water slows neutrons efficiently while absorbing very few, allowing natural uranium fuel.

Deuterium as a bare gas is not itself used as a moderator, though heavy water carries it.

Monazite is a thorium and rare-earth ore, a nuclear fuel source rather than a moderator.

Tritium is a radioactive hydrogen isotope used in fusion, not for slowing neutrons.

A good moderator must be light enough to slow neutrons yet capture almost none.