This section discusses reactor characteristics that change as a result of changing reactivity. A basic approach using a minimum of mathematics has been followed. Emphasis has been placed on distinguishing between prompt and delayed neutrons and showing relationships among reactor variables, keff, period, neutron density, and power level.

Core excess is the reactivity available which is above that necessary to achieve criticality. In other words, it is what the reactivity of the system would be if all of the control rods in the reactor were completely withdrawn. The core excess of a reactor is constantly changing due to many of the variables already discussed, such as fuel depletion, temperature, and fission product poisons. It is also an important parameter which is routinely assessed to provide assurance that there is always sufficient negative reactivity in the control rods to shut the reactor down; even perhaps with one rod, or one group of rods stuck out. As shown earlier, if the core excess is too low then it may not be possible to go critical during the xenon peak after shutdown.