.. _section-1.1:

SAS4A/SASSYS‑1 Background
-------------------------

In the late 1960s, the then U.S. Atomic Energy Commission gave
development of a liquid-metal-cooled fast reactor (LMR) a high priority,
and the development of the Fast Flux Test Facility (FFTF) became a
cornerstone of that program. To provide adequate support for the FFTF
and for the expected LMRs to follow, a major base technology program was
established which provided a continuous stream of experimental
information and design correlations. This experimental data would either
confirm design choices or prove the need for design modifications. At
the time, the "tremendous amount of data and experience pertaining to
thermal design" of LMRs was recognized as providing the technical
foundation for the future commercial development of LMRs.[1‑1]

Along with the generation of experimental data came the development of
safety analysis methods that used that data in correlations for
mechanistic, probabilistic, or phenomenological models. These models
were developed for a variety of needs ranging from individual
components, such as heat exchangers, pumps, or containment barriers, to
whole core or even whole-plant dynamics. A major portion of the overall
technical effort since that time has been allocated to safety
considerations, and the SAS4A/SASSYS‑1 safety analysis code is the
result of that dedication.

Perhaps the strongest factor that influenced early fast reactor safety
analysis was the concern over the possibility of core compaction
followed by an energetic core disassembly --- the so-called Bethe-Tait
accident.[1‑2] In the late 1960s, the Hanford Engineering Development
Laboratory (HEDL) began developing the MELT code[1‑3,1‑4] to evaluate
the initiating phase of hypothetical core disruption accidents (HCDA) as
part of the FFTF project. The MELT series of codes has the capability to
model the transient behavior of several representative fuel pins
(channels) within a reactor core to allow for incoherency in the
accident sequence. By 1978 MELT had evolved into the MELT-IIIB code.[
1‑4]

Around the same time that development on MELT began, Argonne National
Laboratory began developing the SAS series of codes.[1‑5-1‑9] Like MELT,
SAS has the capability to model the transient behavior of several
representative channels to evaluate the initiating phase of HCDAs. SAS1A
originated from a sodium boiling model and includes single- and
two-phase coolant flow dynamics, fuel and cladding thermal expansion and
deformation, molten fuel dynamics, and a point kinetics model with
reactivity feedback. By 1974, SAS evolved to the SAS2A computer
code[1‑6] which included a detailed multiple slug and bubble coolant
boiling model which greatly enhanced the ability to simulate the
initiating phases of loss-of-flow (LOF) and transient overpower (TOP)
accidents up to the point of cladding failure and fuel and cladding
melting.

The SAS3A code [1‑7] added mechanistic models of fuel and cladding
melting and relocation. This version of the code was used extensively
for analysis of accidents in the licensing of FFTF. In anticipation of
LOF and TOP analysis requirements for licensing of the Clinch River
Breeder Reactor Plant (CRBRP), new fuel element deformation, disruption,
and material relocation models were written for the SAS4A version of the
code,[1‑8] which saw extensive validation against TREAT M-Series test
data. In addition, a variant of SAS4A, named SASSYS-1, was developed
with the capability to model ex-reactor coolant systems to permit the
analysis of accident sequences involving or initiated by loss of heat
removal or other coolant system events. This allows the simulation of
whole-plant dynamics feedback for both shutdown and off-normal
conditions, which have been validated against EBR-II Shutdown Heat
Removal Test (SHRT) data and data from the FFTF LOF tests.

Although SAS4A and SASSYS‑1 are generally portrayed as two computer
codes, they have always shared a common code architecture, the same data
management strategy, and the same core channel representation.
Subsequently, the two code branches were merged into a single code
referred to as SAS4A/SASSYS‑1. Version 2.1 of the SAS4A/SASSYS‑1 code
[1‑10,1‑11] was distributed to Germany, France, and Japan in the late
1980s, and it serves as a common tool for international oxide fuel model
developments.

Beyond the release of SAS4A/SASSYS‑1 v 2.1, revisions to SAS4A/SASSYS‑1
continued throughout the Integral Fast Reactor (IFR) program between
1984 and 1994,[1‑12] culminating with the completion of SAS4A/SASSYS‑1 v
3.0 in 1994.[1‑13] During this time, the modeling emphasis shifted
towards metallic fuel and accident prevention by means of inherent
safety mechanisms. This resulted in 1) addition of new models and
modification of existing models to treat metallic fuel, its properties,
behavior, and accident phenomena, and 2) addition and validation of new
capabilities for calculating whole-plant design basis transients, with
emphasis on the EBR-II reactor and plant [1‑14], the IFR prototype. The
whole-plant dynamics capability of the SASSYS-1 component plays a vital
role in predicting passive safety feedback. Without it, meaningful
boundary conditions for the core channel models are not available, and
accident progression is not reliably predicted.

By the mid 1990s, SAS4A/SASSYS‑1 v 3.1 had been completed as a
significant maintenance update, but it was not released until
2012.[1‑15]