.. _section-7.1:

Introduction
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A balance-of-plant thermal/hydraulic model has been developed for use
with the SASSYS-1 liquid metal reactor systems analysis code. This model
expands the scope of SASSYS-1 so that the code can explicitly model the
waterside components of a nuclear power plant. Previously, only the
water side of the steam generators could be modelled, with the remainder
of the water side represented by boundary conditions on the steam
generator. This chapter is organized with a structure that reflects the
three major areas of the balance-of-plant thermal/hydraulics module: 1)
the network model, 2) the steam generator model, and 3) the component
models.

The balance-of-plant network thermal/hydraulics model represents the
water side as a network of components, similar to the representation
used by SASSYS for the sodium side of the plant. The model will handle
subcooled liquid water, superheated steam, and saturated two-phase
fluid. With the exception of heated flow paths in heat exchangers, the
model assumes adiabatic conditions along flow paths. This assumption
simplifies the solution procedure while introducing very little error
for a wide range of reactor plant problems.

The balance-of-plant steam generator thermal/hydraulics model is totally
new, and completely replaces the previous SASSYS-1 steam generator
model. A number of modeling improvements have been implemented in the
new steam generator model to achieve better accuracy, capability, and
computational performance. First, variable spatial nodalization has been
introduced to permit more accurate estimates of heat fluxes. Second, an
improved treatment of heat transfer regime transitions is employed to
promote numerical stability. Third, donor cell spatial differencing is
used with a semi-implicit time-differencing scheme to add further
numerical stability and to allow larger time stops. Fourth, the
steady-state initialization process has been improved to accommodate a
wider variety of heat transfer and flow regimes. Finally, the new steam
generator model contains a preliminary method for variable time step
selection.

The steam generator model simulates the two most likely configurations
for LMR plants with their high core outlet/inlet temperatures. The two
configurations are: 1) the once-through system where the superheater is
an integral part of the evaporator, and 2) the external recirculation
type where the balance-of-plant components are used to connect the
separate superheater to the evaporator through the steam drum.

The balance-of-plant components thermal/hydraulics models extend the
scope of the network model, to treat nonadiabatic and two-phase
conditions along flow paths, and to account for work done across the
boundaries of compressible volumes. Simple conservation balances and
extensive component data in the form of correlations constitute the
basis of models of heaters (deaerators, steam drums, condensers,
reheaters, flashed heaters, drain coolers, desuperheating heaters, and
desuperheater/drain coolers), turbines, and relief valves. Except for
the turbine nozzle, the mass and momentum equations for the component
models are the same as the network equations. The component energy
equations contain a heat source due to energy transfer across a flow
boundary or to work done through a shaft. To handle two-phase
conditions, the equation of state is expressed differently for each
phase in terms of the quality and separate intensive properties.

The next three sections describe the thermal-hydraulic network, steam generator, and
component models.