What term best describes a nuclear reaction where the number of neutrons produced balances the number used?

Question

What term best describes a nuclear reaction where the number of neutrons produced balances the number used?

Answer 1

The word for such state is "criticality".

Answer 2

It's a term called criticality.

When dealing with nuclear reactors, you're playing a numbers game.  You can't control the individual neutrons so much as you can influence them.

For example, it takes a certain number of neutrons starting out to create an equivalent number of neutrons from fission.  If say, you move the control rods in, they will suppress some of the reaction (control rods absorb neutrons, preventing them from reaching the fuel).  So now you have less neutrons at the end than when you started.  This is known as "sub-criticality" and will reduce your reactor's power.

But if you pull the control rods out a little, this exposes more fuel and less rods, therefore encouraging the reaction.  You'll end up with more neutrons than when you started.  This is known as "super-criticality" and will raise your reactor's power.