.. _section-1.3:

Documentation Overview
----------------------

The rest of this manual contains details of the modeling capabilities of
SAS4A/SASSYS‑1. The chapter organization shown in :numref:`table-1.3-1` reflects
the major model delineations. Each chapter provides in-depth
descriptions of the models, including model formulations, solution
techniques, and input descriptions. It is critical that users understand
the relationships between their model input and the model formulations
given in this manual. Failure to understand these relationships can
result in broken models and misleading results.

SAS4A/SASSYS‑1 provides a detailed, multiple-channel thermal/hydraulic
treatment of the reactor core. Each channel represents a fuel pin, its
cladding, the associated coolant, and a fraction of the subassembly duct
wall. Other positioning hardware, such as wire wraps or grid spacers, is
usually lumped into the structure field with the duct wall. Within a
channel, the flow is assumed to be one-dimensional in the axial
direction, and the temperature field in the fuel, cladding, coolant, and
structure is assumed to be two-dimensional in the radial and axial
directions. Usually, a channel represents an average fuel element in a
subassembly or a group of subassemblies. A channel may also represent
pins in blanket or control subassemblies. Alternately, a single channel
may also be used to represent the hottest pin in an assembly, or any
other subset of a subassembly. The axial extent of a channel covers the
entire length of a subassembly, including the core, the axial blankets,
the fission gas plenum and the spaces above and below the pin/cladding
geometry. Different channels may be used to account for radial and
azimuthal design geometry, power, coolant flow, and burnup variations
within the reactor core.

.. _table-1.3-1:

.. list-table:: Organization of the SAS4A/SASSYS‑1 Manual
    :header-rows: 1
    :align: center
    :widths: auto

    * - Chapter
      - Subject
    * - 1
      - Introduction (this chapter)
    * - 2
      - SAS4A/SASSYS‑1 User's Guide
    * - 3
      - Pin Heat Transfer and Single‑Phase Coolant Thermal/Hydraulics Model
    * - 4
      - Reactor Point Kinetics and Reactivity Feedback Models
    * - 5
      - PRIMAR‑4: Primary and Intermediate Loop Thermal/Hydraulics Model
    * - 6
      - Plant Control and Protection Systems Model
    * - 7
      - Balance‑of‑Plant Thermal Hydraulics Model
    * - 8
      - DEFORM‑4 Oxide Fuel and Cladding Mechanics Model
    * - 9
      - DEFORM‑5 Metal Fuel Cladding Mechanics Model
    * - 10
      - SSCOMP Metal Fuel Characterization Model
    * - 11
      - FPIN2 Metal Fuel and Cladding Mechanics Model
    * - 12
      - TSBOIL Two‑Phase Coolant Thermal/Hydraulics Model
    * - 13
      - CLAP Molten Cladding Dynamics Model
    * - 14
      - PLUTO2 Fuel‑Coolant Interaction Model
    * - 15
      - PINACLE In‑Pin Fuel Relocation Model
    * - 16
      - LEVITATE Fuel Relocation Model

:numref:`Chapter %s<section-2>` contains a general user's guide for SAS4A/SASSYS‑1, including
a complete description of the standard input file. Although :numref:`Chapter %s<section-2>`
includes a summary description of every input parameter, it is essential
that users consult the relevant chapters to understand the relationship
between the input and the model formulations.

:numref:`Chapter %s<section-3>` contains the description of the formulation for the
SAS4A/SASSYS‑1 pin heat transfer and single-phase coolant
thermal/hydraulics model. The subassembly-to-subassembly heat transfer
model has been improved, and axial conduction in the coolant has been
added. A sub-channel model has been introduced to provide accurate
predictions of intra-assembly temperature and flow distributions.[1‑16]
This modeling addition is being validated with results from the EBR-II
Shutdown Heat Removal Tests [1‑17] as part of an International Atomic
Energy Agency Coordinated Research Project.[1‑18]

:numref:`Chapter %s<section-4>` contains the description of the formulation for the
SAS4A/SASSYS‑1 reactor point kinetics, decay heat, and reactivity
feedback models. A new addition to this module is the ability to
represent more detailed decay heat characteristics in multiple regions
of the core. This module provides the reactor power level to the core
thermal/hydraulics models for determination of the heating rate in the
fuel, and receives core materials temperature and geometry information
to calculate the reactivity feedbacks employed in the solution of the
point kinetics equations.

:numref:`Chapter %s<section-5>` presents a full description of the formulation for the
PRIMAR‑4 sodium loops thermal/hydraulic model. This model provides
boundary coolant pressure and flow conditions for the core channel
models, including transient heat losses through normal and emergency
heat removal systems and the transient performance of pumps. PRIMAR‑4
includes the option for multiple core inlet and outlet coolant plena,
permitting exact representation of the actual EBR-II coolant systems
geometry. Compressible volumes in PRIMAR‑4 may also be coupled with
external computational fluid dynamics simulations to better represent
flow and temperature distributions during transients.

The plant control and protection system model described in :numref:`Chapter %s<section-6>` is
mostly unchanged from prior versions of SASSYS‑1, except for the
addition a sinusoidal function to represent oscillations in
control-system signals.

The balance-of-plant (BOP) model described in :numref:`Chapter %s<section-7>` was implemented
to permit 1) improved simulation of EBR-II design basis transients, 2)
whole-plant analysis of IFR designs for optimization of advanced reactor
control system strategies, and 3) core temperature margin assessments in
unprotected accident sequences (i.e. beyond design basis accidents
(BDBA) and anticipated transients without scram (ATWS)). In these latter
sequences, core response depends strongly upon the performance of the
balance-of-plant, because the core neutronic and thermal/ hydraulic
behavior is determined by the availability of heat sinks outside the
core. The BOP model couples to PRIMAR‑4 at the steam generator.

:numref:`Chapter %s<section-8>` provides a description of the DEFORM‑4 fuel element behavior
model for stainless steel-clad oxide fuel, which is unchanged from prior
versions of SAS4A/SASSYS‑1.

:numref:`Chapter %s<section-9>` contains the description of the DEFORM‑5 model, which treats
the transient behavior of stainless steel and advanced (HT‑9) cladding
for metal fuel elements. This model is aimed at predicting margin to
cladding failure, and timing and location of failure in limiting
transients. It includes physical phenomena unique to metallic fuel, such
as fuel/cladding chemical interactions.

The SSCOMP model described in :numref:`Chapter %s<section-10>` reflects available metal fuel
material properties evaluations recorded in the IFR Material Properties
Handbook [1‑19]. An efficient correlation technique has been implemented
in all SAS4A/SASSYS‑1 material properties routines that accurately
generates the data from the IFR Handbook for use in all the modules of
the code. It is planned to revise the material migration capability in
SSCOMP for ternary fuel, to add models for fission gas generation and
release, swelling, and all other phenomena needed to describe the
transition from cold, clean, unirradiated conditions to hot irradiated
conditions.

:numref:`Chapter %s<section-11>` contains the description of the FPIN2 metal fuel pin
mechanics model [1‑20]. FPIN2 is a validated model for metal fuel pin
transient behavior. Unlike DEFORM‑5, which treats only the cladding
response, FPIN2 provides a finite-element solution of the fuel and
cladding mechanics equations for the elastic/plastic response, including
fission gas pressurization and migration, molten cavity formation and
growth, and fuel/cladding chemical interaction and cladding thinning.
The interface between SAS4A/SASSYS‑1 and FPIN2 has been designed to
permit stand-alone execution of FPIN2 for direct verification or to
replace the FPIN2 thermal/hydraulics calculation with the SAS4A/SASSYS‑1
counterparts for coupled calculations. The application for this model is
design basis analysis of driver and experimental fuel elements in EBR-II
for the purpose of margin-to-failure assessments.

The TSBOIL module for liquid metal coolant boiling and two-phase
thermal/hydraulics calculations has been retained intact from previous
versions of SAS4A/SASSYS‑1. The current model includes a set of
modifications to describe the sudden release of non-condensable fission
gas from a cladding rupture in the upper fission gas plenum of metal
fuel elements and the subsequent plenum blow-down and liquid coolant
expulsion. This option has been used to assess the safety implications
of long-term fuel element irradiations in EBR-II [1‑21].

The CLAP and PLUTO2 models described in :numref:`Chapter %s<section-13>` and :numref:`Chapter %s<section-14>` are relevant
only to oxide fuel, and have remained unchanged since the previous
documentation.

The PINACLE model described in :numref:`Chapter %s<section-15>` and the LEVITATE model
described in :numref:`Chapter %s<section-16>` have been upgraded for applications to metallic
fuel [1‑22]. The model enhancements added to PINACLE and LEVITATE for
metal fuel include fuel/cladding and fuel/structure chemical
interactions and fission gas generation and migration with fuel
swelling. Preliminary analyses of TREAT M-Series in-pile metal fuel
tests have been completed [1‑23], and applications to severe accident
sequences in metal-fueled IFR cores have been completed and documented
[1‑24].