Where reactor operation is designed with fast neutrons such as in reactors using highly enriched fuel then no moderator is needed.

• A fast-neutron reactor is a category of a nuclear reactor in which the fission chain reaction is sustained by fast neutrons.
• Nuclear reactors can be either thermal or fast.
• Thermal nuclear require a moderator to slow down fast neutrons to the thermal level so that nuclear fission can continue.
• However, in fast reactors, a moderator is not needed, and the neutrons within it move much more quickly.

Fast reactors are beneficial as they enhance the sustainability of nuclear power. This is because they have the ability to get more neutrons out of their fuel, can transform nuclear waste into products that decay more quickly, and they respond better to potentially catastrophic equipment failures.