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general heat and mass transfer package with built-in models for steam/water/non-
condensable gases, including natural and forced convection, condensation, and boil-
ing. The fluid properties and heat transfer correlations can be defined separately by the
user. This makes it possible to analyze various types of coolants, including liquid
metals and various types of molten salts, without any code modifications. A point
reactor kinetics model is available, with an isotope transformation model to compute
concentrations of important isotopes (e.g., Xe-135). The radioactive particle transport
package deals with radioactive fission product chains, release of fission products,
aerosol transport, deposition, and resuspension. A large number of verification and
validation tests have been performed for the various reactor types, using International
Standard Problems and international code-to-code benchmarks. For liquid-metal
applications, important benchmarks are related to the Ph enix, ASTRID, and EBR-
II sodium-cooled reactors and the benchmark for the ELFR lead-cooled reactor and
the CIRCE lead-bismuth-cooled facility. The code was used as a background
thermal-hydraulic code for interactive educational simulators of generic PWR,
BWR, and the low flux research reactor of NRG.

4.5.4 SAS4A/SASSYS-1 code
This STH code was developed at Argonne National Laboratory for thermal, hydraulic,
and safety analysis of power and flow transients in liquid-metal-cooled nuclear reac-
tors (Rui Hu, 2017; Fanning et al., 2017). With its origin as SAS1A in the late 1960s,
the SAS series of codes has been under continuous use and development for nearly
five decades and represents a critical investment in advanced fast reactor safety anal-
ysis capabilities for the US Department of Energy (DOE).
SAS4A/SASSYS-1 is capable of performing a comprehensive analysis of whole-
plant dynamics including the response of primary and intermediate heat transport sys-
tems. Modeling flexibility is maintained by allowing the user to choose from a variety
of built-in component models. SASSYS-1 assesses the safety margins in design-basis
accidents and consequences of beyond-design-basis accidents. SAS4A analyzes the
consequences of severe accidents with coolant boiling and/or fuel failures that are ini-
tiated by a very-low-probability coincidence of an accident precursor and failure of
one or more safety systems. Although SASSYS-1 and SAS4A were originally devel-
oped as separate computer codes, they have always shared a common code architec-
ture, a similar data management strategy, and the same core channel representation.
Subsequently, in the late 1980s, Argonne merged both codes into a single code
referred to as SAS4A/SASSYS-1.
SAS4A/SASSYS-1 has been coupled to a variety of analysis and optimization
tools, such as STAR-CCM+, Dakota, RAVEN, SAM, and PDC (the Argonne Plant
Dynamics Code that models supercritical CO2 Brayton cycle for energy conversion).
Several benchmark models have been developed for the validation of whole-plant pas-
sive safety response based on EBR-II tests conducted in the 1980s. Two of these tests,
shutdown heat removal tests 17 and 45R, are the basis of an IAEA Coordinated
Research Project led by Argonne. DOE and Argonne are currently preparing