A nuclear reactor with gaseous coolant, usually helium or CO₂. Reactors relying on thermal neutrons typically use graphite as a moderator (Magnox, pebble bed type, HTGR), while fast neutron reactors have no moderator and use helium as a coolant. The design allows the utilization of natural uranium, without the need for enrichment. The use of gas as a coolant increases safety (the gas does not boil) and also allows a high outlet temperature, which improves energy conversion.