A light-element material, most often ordinary water, heavy water or graphite, that slows neutrons through repeated collisions until they reach thermal energies where uranium-235 fission is far more likely. In light water reactors the coolant is also the moderator, so heating or boiling the water reduces moderation and adds negative reactivity. The moderator temperature coefficient is a key parameter in core design and startup physics testing.
Moderator temperature coefficient is still slightly positive at beginning of cycle, so watch the boron.
Also heard as
- moderating material
Related terms
Void coefficient
The change in reactivity caused by steam bubbles or voids forming in the coolant.
Reactivity
A measure of how far a reactor is from criticality, positive when power will rise and negative when it will fall.
Criticality
The state in which a chain reaction sustains itself at a steady rate, with each fission causing on average exactly one more.
Poison
Any material that absorbs neutrons strongly, whether added deliberately for control or produced by fission.