4.7. Code Organization and Data Flow

The subroutines in SAS4A/SASSYS‑1 that are mainly related to the solution of the reactor kinetics, decay-heat, and reactivity feedback models are listed in Table 4.7.1. Most of these routines are called directly from the steady-state and transient driver routines, SSTHRM and TSTHRM. Flow diagrams for these routines are included in Section 2.2.

Table 4.7.1 Listing of Reactor Kinetics Subroutines

Subroutine	Description
SSPK	Initializes all steady-state point kinetics data including power, reactivity, delayed-neutron precursor concentrations, and decay heat sources.
POWING	Extrapolates power at the beginning of a main time step for external power vs. time option via call to PREA. Updates parabolic power history coefficients.
PREA	User-supplied external power or reactivity history.
PAR	Fits a parabola to three points.
POINEX	Extrapolates reactivity at the beginning of a main time step for reactivity vs. time option by parabolic extrapolation of calculated internal reactivity and direct evaluation of external reactivity via call to PREA.
TSPK	Given an extrapolated or calculated reactivity, drives the calculation of the reactor power. Resets beginning time-step values and calls PKSTEP.
PKSTEP	Performs the integration of the point kinetics equations across a main time step.
RHOEND	Updates end-of-time step values.
FEEDBK	Performs calculation of Doppler and coolant void reactivity feedback for a channel.

At steady state, the driver routine SSTHRM calls subroutine SSPK, which performs all required data initialization. In the transient calculation, the point kinetics equations are solved on each main time step. From driver routine TSTHRM, subroutine POWING is called at the beginning of each main time step to provide an extrapolation in time of the reactor power for the power versus time option (IPOWER = 1). For the reactivity versus time option (IPOWER = 0), subroutine POINEX is called to provide an extrapolation of the reactivity, and subroutine TSPK is then called to compute the reactor power given the reactivity. Subroutine TSPK performs data initialization and calls subroutine PKSTEP, which integrates the point kinetics and decay-heat equations over the main time step. At the end of the main time step, subroutine FEEDBK is called from driver routine TSTHRM to compute the current reactivity feedback, and subroutine TSPK is called once more to compute the final estimate of the reactor power. Subroutine RHOEND is then called to update the coefficients used in the integration of the point kinetics equations.