Publisher Summary This chapter continues the discussion on dynamic programming. Besides the fact that fission-product poisons play a central role in the xenon control problem, they also affect the long-term behavior of nuclear-reactor operation. As the reactor operates, fuel is consumed as it is being irradiated by the reactor neutron flux. Fuel cycle times are normally calculated in weeks to years. As long-term irradiation proceeds, the fissionable fuel available from the fertile fuel approaches an equilibrium concentration, because it is now also being consumed (burned) as well. As time passes, the fission-product concentration begins to increase rapidly, and as some of these are neutron absorbers, like xenon, the reactivity tends toward a negative value. The original fissionable fuel charge is being consumed as well, depleting the total amount of fuel, which further contributes to the negative reactivity trend. For the case of xenon, which is a consideration in short-term as well as long-term xenon kinetics, one must provide sufficient extra fuel to counteract the effect of the poison.