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The characteristic relation between imposed reactor reactivity and exponential growth or decay rates in a point kinetics model. Several kinetic modes can coexist; the late-time behaviour is determined by the mode with greatest real growth rate that is actually excited.

The point kinetics model replaces the spatially and spectrally distributed neutron field by a single amplitude $n(t)$, effectively assuming a fixed or sufficiently slowly varying flux shape. The source amplitude is $s$. A single effective delayed neutron precursor population $c(t)$, delayed neutron fraction $\beta$ and decay rate $\lambda$ stand in for many precursor species with different lifetimes. $\Lambda$ is the prompt neutron generation time and $\rho$ the specified reactor reactivity.

The displayed equations omit explicit thermal-hydraulic, fuel-temperature and isotope-composition feedback. For the stated constant-coefficient transients, $\rho ,\beta ,\Lambda ,\lambda$ are prescribed rather than being solved as evolving fields. The model is useful for an elementary timescale calculation; it does not establish the spatial stability or operating limits of an actual core.

The effective time associated with successive prompt fission generations in a specified point kinetics model. It is distinct from a macroscopic population growth period, which also depends on reactor reactivity and delayed neutrons.