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]