.. _section-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 :numref:`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 :numref:`Chapter %s<section-7>`, and with the
control system described in :numref:`Chapter %s<section-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:

..  figure:: media/Flowchart_5.1-1.svg
	:align: center
	:figclass: align-center

	Interaction between PRIMAR-4 and Other Modules.