CHAPTER 4 - Linear Reactor Process Dynamics with Feedback

Abstract

This chapter describes the three mechanisms leading to variations of the reactivity dependent on the state of the reactor, including the neutron population and power level. These mechanisms are inherent feedback, external plant feedback, and control effects. The chapter also describes the void effects. When a material is removed from a local region of a reactor to leave a void, there is a change of reactivity. The effect may be particularly prominent in liquid moderated reactors where the liquid coolant has an appreciable moderating effect and voidage is more likely to occur. In this case, fuel is not being removed, but the removal of moderator will affect absorption, moderation, and allow streaming of neutrons through the void. There may be local variations in reactivity effects so that the size and even the sign of the reactivity effect of a void vary along a coolant channel. The chapter also describes reactor thermal transients. Temperature effects in reactors can be conceptually divided between inherent and external feedback, the latter arising as changes in demand that affect the turbine and the way in which coolant is drawn off or its state on return to the reactor. It is significant that there is a certain amount of thermal inertia so that there are time delays in establishing new temperatures.