4.1. Introduction

The purpose of the SAS4A/SASSYS‑1 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 SAS4A/SASSYS‑1 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 SAS4A/SASSYS‑1 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 SAS4A/SASSYS‑1 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 Section 4.6, and subsequent sections provide details on code organization, data flow, input, and output.