If α α is positive, then the core is supercritical and the rate of neutron production will grow exponentially until some other effect stops the growth. If α α is negative, then the core is "subcritical" and the number of free neutrons in the core will shrink exponentially until it reaches an equilibrium at zero (or the background level from spontaneous fission). If α α is exactly zero, then the reactor is critical and its output does not vary in time ( d N / d t = 0 , from above).