Neutron moderator

In a nuclear reactor, a reactor moderator controls the flow of neutrons. Moderators are often in the form of control rods, which increase nuclear fission when withdrawn and decrease fission when inserted; a SCRAM is an emergency shutdown in which all rods are inserted to their maximums.

Moderators and reactor coolants have comparable properties, and, if the reactor design permits, making them of the same material increases safety.

The key figure of merit for a moderator material is the moderator ratio (MR).

MR = (ζ ∑_s) ⁄ ∑_a

where

ζ = energy loss per collision

∑_s = scattering cross-section

∑_a = absorption cross-section

Other desirable properties include high density, chemical stability, and resistance to radiation damage.

Light (ordinary) water

Light water is both the most common moderator and reactor coolant in Boiling Water Reactors and Pressurized Water Reactors used for electricity generation and naval nuclear propulsion.