Nuclear Physics 1 - Reactivity

Reactivity is the term given to the quantity that tells you how much the
neutron economy is out of balance.

If a reactor is exactly critical - that is, the neutron production is exactly
equal to neutron destruction - then the reactivity is zero.

If the reactivity is positive - then the reactor is supercritical. If the
reactivity is negative - then the reactor is sub-critical.

The Reactivity is defined as the ratio of an adjoint weighted average
of the excess neutron production divided by an adjoint weighted average
of the fission production.

The quantity "reactivity" is one of the terms in the "Point Kinetics"
equations, which is a zero-dimensional set of equations that describe the
transient behavior of a reactor.

Look up "point kinetics equations" in a reactor physics text.