5.1. Introduction

The PRIMAR-4 module computes coolant pressures, flow rates, and temperatures in the primary and intermediate heat transport loops. This module is designed for analysis of a wide range of transients, from fast unprotected LOF or TOP cases to slow operational transients or natural circulation shut-down heat-removal cases. Also, an arbitrary arrangement of components in either a loop-type or a pool-type system can be treated. Semi-implicit and fully implicit numerical schemes have been developed to handle the full range of transients efficiently. This chapter describes the physical models and the numerical algorithms used in PRIMAR-4. As indicated in Figure 5.1.1, PRIMAR-4 couples with the driver and blanket subassembly coolant dynamics calculations in the pre-voiding thermal hydraulics module TSCL0, and the boiling module TSBOIL. Also, PRIMAR-4 couples with the detailed steam generator model described in Chapter 7, and with the control system described in Chapter 6.

The PRIMAR-4 module contains both a simple PRIMAR-1 type option and the more detailed PRIMAR-4 treatment. The PRIMAR-1 option supplies only the minimum quantities of information required to drive the subassembly coolant dynamics calculations. It is often used when the more detailed treatment is not required. This minimum information consists of the inlet and outlet plenum pressures, the subassembly inlet temperatures, and the outlet plenum temperature in the event of flow reversal. All of these quantities are computed as functions of time by PRIMAR-1 from user-supplied information independently of what is happening in the core or in the rest of the primary loop.

The detailed multi-loop PRIMAR-4 treatment models heat transfer and coolant flow in the inlet and outlet coolant plenums, the pipes, pumps, and valves in the primary and intermediate loops, the intermediate heat exchangers, the steam generators, and the pool in a pool system. It utilizes a modular approach. The user specifies the properties of various components and arranges them in an arbitrary manner. Each type of component is treated in a separate section of the code. Components can be added or existing component treatments can be modified without impacting the rest of the code.

The detailed PRIMAR-4 treatment includes both a steady-state initialization sub-module and a transient sub-module. The main parts of the transient sub-module are a hydraulic section in which liquid coolant flow rates and pressures are calculated, a liquid temperature section, and a gas section that calculates gas flow between cover gasses. For a given time step, the liquid flow rates and pressures are calculated first, taking into account pressure and flow rate changes during the time step, but ignoring temperature changes and gas flow changes during the time step. This not only simplifies the calculations, but it makes explicit use of the fact that the incompressible single-phase liquid flow rates are more sensitive to pressure changes than they are to temperature and gas flow changes. Then, with the flow rates known, the liquid temperatures are calculated for the time step. Finally, with the liquid pressures, flow rates, and temperatures known, the gas flow rates and pressures are calculated, and the liquid pressures are adjusted to account for changes in the cover-gas pressures.

Figure 5.1.1 Interaction between PRIMAR-4 and Other Modules.