.. _section-4.1:

Introduction
------------

The purpose of the |SAS| reactor point kinetics, decay heat,
and reactivity feedback models is to provide an estimate of the reactor
power level to be used in the prediction of energy deposition in the
fuel. Reactor material temperature changes and relocations determine the
reactivity, which in turn determines the reactor power level and the
rate of heating of the reactor materials.

The |SAS| reactor point kinetics, and reactivity feedback
models are based on concepts used in the SAS3A [4-1] computer code. A
time-independent reactor power spatial shape is assumed, along with a
space-independent (point) reactor kinetics model. However, the decay
heat model in |SAS| has been rewritten for version 5.0.
First-order perturbation theory is used to predict reactivity feedback
effects associated with material density changes. Fuel temperature
(Doppler) effects are calculated assuming a logarithmic dependence on
the local absolute temperature ratio, with a linearly dependent
variation of the local Doppler coefficient on the coolant void fraction.

The fundamental basis for the assumptions of a time-independent power
distribution, point kinetics, and first-order perturbation theory is the
underlying supposition that the reactor neutron flux distribution is
invariant in time. This means that in the transient simulation, the
effects of changes in the reactor environment (geometry, dimensions,
temperature and density distributions) on the neutron flux shape are
neglected. This significantly reduces the complexity and computational
expense of the overall neutronics model, with some loss of accuracy. In
general, this inaccuracy can be expected to be significant mainly in the
estimate of the reactivity feedback accompanying large-scale fuel
material relocations, and large-scale fuel relocation is usually not
included in a |SAS| case.

The sections that follow describe the mathematical formulations for the
total reactor power, delayed-neutron precursors, decay heat, and net
reactivity. The numerical solution methods are described in :numref:`section-4.6`,
and subsequent sections provide details on code organization, data flow,
input, and output.