Reactor Dynamics

Reactor Dynamics

Reactor Dynamics vs Reactor Kinetics

In preceding chapters (Nuclear Chain Reaction), the classification of states of a reactor according to the effective multiplication factor – keff was introduced. The effective multiplication factor – keff is a measure of the change in the fission neutron population from one neutron generation to the subsequent generation. Also the reactivity as a measure of a reactor’s relative departure from criticality was defined.

In this section, amongst other things it will be briefly described how the neutron flux (i.e. the reactor power) changes if reactivity of a multiplying system is not equal to zero. We will study the time-dependent behaviour of nuclear reactors. An understanding of the time-dependent behavior of the neutron population in a nuclear reactor in response to either a planned change in the reactivity of the reactor or to unplanned and abnormal conditions is of the most importance in the nuclear reactor safety. This chapter is named the Reactor Dynamics, but also comprises the reactor kinetics. Nuclear reactor kinetics is dealing with transient neutron flux changes resulting from a departure from the critical state, from some reactivity insertion. Such situations arise during operational changes such as control rods motion, environmental changes such as a change in boron concentration, or due to accidental disturbances in the reactor steady-state operation.

In general:

  • Reactor KineticsReactor kinetics is the study of the time-dependence of the neutron flux for postulated changes in the macroscopic cross sections. It is also referred to as reactor kinetics without feedbacks.
  • Reactor Dynamics. Reactor dynamics is the study of the time-dependence of the neutron flux, when the macroscopic cross sections are allowed to depend in turn on the neutron flux level. It is also referred to as reactor kinetics with feedbacks and with spatial effects.

Time-dependent behaviour of nuclear reactors can be also classified by the time scale as:

  • Short-term kinetics describes phenomena that occur over times shorter than a few seconds. This comprises the response of a reactor to either a planned change in the reactivity or to unplanned and abnormal conditions. In this section, we will introduce especially point kinetics equations.
  • Medium-term kinetics describes phenomena that occur over the course of several hours to a few days. This comprises especially effects of neutron poisons on the reactivity (i.e. Xenon poisoning or spatial oscillations).
  • Long-term kinetics describes phenomena that occur over months or even years. This comprises all long-term changes in fuel composition as a result of fuel burnup.

This chapter is concerned with short-, medium- and long-term kinetics, despite the fact the fuel burnup and other changes in fuel composition are usually not a dynamic problem. At first, we have to start with an introduction to prompt and delayed neutrons because they play an important role in short-term reactor kinetics. Despite the fact the number of delayed neutrons per fission neutron is quite small (typically below 1%) and thus does not contribute significantly to the power generation, they play a crucial role in the reactor control and are essential from the point of view of reactor kinetics and reactor safety. Their presence completely changes the dynamic time response of a reactor to some reactivity change, making it controllable by control systems such as the control rods.