2. Point Reactor Kinetics

To describe the reactor kinetics, the number of neutrons and the number of delayed neutron precursors that change with time are considered. The following ordinary differential equations can be gotten if the space dependence of these variables is ignored and the neutron energy is handled in one group. This is called the point reactor kinetics model. Actually, the reactor is not treated as a single point but the assumption is made that the space distribution of parameters does not change with time. When a slow disturbance is treated in spatial asymmetry, the point reactor kinetics model can be used by weighting the reactivity feedback amount determined with the importance function. Generally, the point reactor approximation can be used to approximate a slow change of the space distribution of parameters. It can be applied to many transient events that contain the disturbance to be handled by reactor control. In contrast, when handling a local and fast (that is, approximately 0.1 s or less) reactivity disturbance, the space-dependent kinetics model must be used. An example is the accident that may occur if asymmetric control rods are quickly withdrawn.