3.1. Introduction

The core assembly thermal hydraulics treatment in SAS4A/SASSYS‑1 includes the calculation of fuel, cladding, coolant, and structure temperatures, as well as coolant flow rates and pressure distributions. This treatment includes melting of the fuel and cladding. Boiling of the coolant is also handled, as described in Chapter 12. The relocation of fuel and cladding after pin disruption is described in Chapter 13, Chapter 14, and Chapter 16; and relocation of molten fuel before pin disruption is described in Chapter 15.

Prior to version 3.0, all of the core subassembly models in SAS4A/SASSYS‑1 were single pin models: a single fuel pin and its associated coolant were used to represent a subassembly; and pin-to-pin variations within a subassembly were ignored. A multiple pin option has been added to the code in version 3.0. A number of pins and their associated coolant can now be used to represent a subassembly, so variations within a subassembly can be accounted for. Currently the multiple pin option is only available for single-phase thermal hydraulics; it does not handle coolant boiling, in-pin fuel relocation, or pin disruption. Therefore, typical SAS4A/SASSYS‑1 cases that do not get into coolant boiling can be handled with the multiple-pin model, but typical core disruption cases can only be handled with single pin models.

Although SAS4A/SASSYS‑1 is mainly a transient code, both steady-state and transient temperatures and coolant pressures are calculated. The steady-state solutions are obtained from the transient equations after dropping all time derivatives. In general, the steady-state solutions in the single pin per subassembly model are not obtained by running a transient calculation at constant power and flow until the results approach a steady-state solution. Instead, the steady-state temperatures are obtained rapidly from a direct solution based on conservation of energy and the use of the same spatial finite differencing as used in the transient. On the other hand, a direct steady-state solution for the multiple pin option would be much more complicated, especially if subassembly-to-subassembly heat transfer is included. Therefore, a null transient with powers and flows held constant is used to obtain steady-state conditions for the multiple pin option.

The thermal hydraulics calculations are carried out in a number of separate modules, and each module is designed for a specific type of calculation. A steady-state thermal hydraulics module provides the initial conditions for the transient. The transient temperatures are calculated in a pre-voiding module (TSHTRN) until the onset of boiling. After the onset of boiling, the fuel-pin temperatures are calculated in a separate module (TSHTRV) that couples with the boiling module.

The core thermal-hydraulic routines interact with a number of other modules, as shown in Figure 3.1.1. Before the onset of voiding, TSHTRN calculates the coolant temperatures used in the hydraulic calculations, whereas the hydraulics routines calculate the coolant flow rates used in TSHTRN. After the onset of voiding, coolant temperatures are calculated in TSBOIL, and this module supplies the heat flux at the cladding outer surface or the fuel outer surface to TSHTRV. TSBOIL uses the cladding temperatures from TSHTRV in its coolant temperature calculations. The point kinetics module supplies the power level used in the heat-transfer routines, and the heat-transfer routines supply the Doppler feedback reactivity as well as other temperature-dependent reactivity feedback. TSBOIL supplies the voiding reactivity. The inlet plenum temperature computed by PRIMAR-4 is used in calculating the inlet temperature for TSHTRN or TSBOIL, and TSHTRN or TSBOIL provides the subassembly outlet temperatures used by PRIMAR-4 to compute the outlet plenum temperature. If flow reversal occurs in a subassembly, then the outlet plenum temperature computed by PRIMAR-4 is used in calculating the coolant temperature at the top of the subassembly, and the temperature computed by TSHTRN or TSBOIL for the coolant leaving the bottom of the subassembly is used by PRIMAR-4 to calculate the inlet plenum temperature. PRIMAR-4 supplies the inlet and outlet plenum pressures that drive the coolant hydraulics calculations, and the core channel flows are provided to PRIMAR-4 by TSBOIL and the pre-voiding hydraulics. The initial coolant flow rate and pressure distribution are supplied to TSBOIL by the pre-voiding hydraulics routines.

The transient calculations in the code used a multi-level time step approach, with separate time steps for each module. For the heat-transfer routines, all temperatures are known at the beginning of a heat-transfer step, and the routines calculate the new temperatures at the end of the step. The heat-transfer time step can be longer than the coolant time step or the PRIMAR time step, but the heat-transfer time step can be no longer than the main time step that is used for reactivity feedback and main printouts.

Figure 3.1.2 shows the flow through the pre-voiding core channel thermal hydraulics driver, TSCL0. This routine is entered once for each channel during each coolant time step. The coolant flow rates are calculated before the heat-transfer module (TSHTRN) is called. TSHTRN is only called if the current coolant time step completes a heat-transfer time step. The voiding model, TSHTRV, is described in Chapter 12.

In this chapter, Section 3.2 describes the mesh structure used for heat-transfer calculations. Then, Section 3.3 describes the pre-boiling transient heat-transfer calculations, followed by the steady-state thermal hydraulics calculations in Section 3.4. The pre-voiding transient heat transfer is discussed before the steady-state thermal hydraulics for two reasons. First, the code is primarily a transient code, so the transient calculations are more important. Second, the finite difference approximations were made with the transient calculations in mind, and the steady-state solution was formulated to be consistent with the approximations used in the transient. Section 3.5 describes TSHTRV, the fuel-pin heat-transfer calculations in the boiling module. Section 3.6 describes the treatment of the bond-gap conductance between the fuel and the cladding. Section 3.7 describes modifications to the fuel pin heat transfer calculations for PLUTO2 and LEVITATE. The heat transfer time step control is described in Section 3.8. Section 3.9 describes steady-state and pre-voiding transient hydraulics. Section 3.10 describes the multiple pin option. Subassembly-to-subassembly heat transfers described in Section 3.11. Section 3.12 describes interaction with other modules. It is followed by sections providing subroutine descriptions and flowcharts, subchannel model treatment details, thermal properties of fuel and cladding, and a description of the input to, and output from, the thermal hydraulic routines.

Figure 3.1.1 Interactions of Thermal-Hydraulic Routines with Other Modules

Figure 3.1.2 Flowchart for the Pre-Voiding Core Channel Thermal Hydraulics Driver (Subroutine TSCL0)