Normally, the equations for neutron kinetics are obtained from the general space and time dependent multi-group diffusion equations for a finite reactor of non-uniform properties. The 3-D neutron kinetic calculations are often made with fast and thermal energy groups and six groups of delayed neutrons. To reduce the sophistication, a modified one group diffusion equation can be considered for fast calculations. In this, only the dynamics of the thermal energy group is simulated in detail.

For simplicity of calculations, most of the large system codes consider the point kinetics model assuming the overall buckling to be almost constant during the transient. The parameters used in these equations such as delayed neutron fraction, prompt neutron life time, delayed neutron precursor concentration, etc. are defined taking into account some phenomena formally neglected during the derivation of the point kinetics equation. An improvement to the point kinetics model is reflected in the one-dimensional kinetics equation that is also adopted in BWR calculations. In this, the radial buckling is assumed not to change during the transients. In point kinetics, reactivity coefficients for the void and Doppler effect are provided as inputs for the whole core. However, 1D and 3D neutron kinetics models facilitate direct evaluation of these feed back effects.