System and safety studies of accelerator driven transmutation ... - SKB

Appendix B:
Short Summary of the SAS4A Code
The SAS4A/SASSYS-1 [69] code is an integrated safety analysis computer code for the
analysis of reactor plant transients in liquid-metal cooled reactors. The development of the
SAS family of computer codes began at Argonne National Laboratory in the middle of the
1960’s. The acronym SAS is an abbreviation for the Safety Analysis Section of the Reactor
Analysis Division at Argonne. Over the years, SASSYS-1 has been employed extensively in
the U.S. liquid metal reactor development programs. It has been the principal tool for the
analysis of accidents in the licensing of the Fast Flux Test Facility (FFTF) and the Clinch
River Breeder Reactor Plant (CRBRP) and in the passive safety design evaluation of the
Integral Fast Reactor (IFR). Traditionally, the SAS code is used to track the initial phase of a
core disruptive accident, through coolant heat-up and boiling, fuel element failure, and fuel
melting and relocation. The information obtained can then be used to determine whether a
noncritical and permanently cooled configuration could be established or whether there is a
remote possibility for recriticality, and in that case provide the initial conditions for the
disassembly phase.
The SAS4A code is built on a multiple-channel thermal-hydraulics core treatment coupled
with a point kinetics neutronics model with reactivity feedbacks. Reactivity feedbacks are
calculated from each channel. A channel contains a fuel pin, its associated coolant, and a
fraction of the structure of the subassembly casing. Usually a channel represents an average
pin within a subassembly or a group of similar subassemblies. The SASSYS-1 core treatment
is built on the same models as SAS4A. In addition, SASSYS-1 is combined with detailed
thermal-hydraulic models of the primary system and secondary coolant circuits and balanceof-plant
steam/water circuit, including pumps, plena, pipes, valves, heat exchangers and steam
generators. Models for two-phase coolant thermal-hydraulics, fuel and clad melting and
relocation events were developed for sodium-cooled reactors with oxide fuel and stainless
steel clad or specialized metallic fuel. Many of these models were validated with experimental
test data from the EBR-II, FFTF, and TREAT reactors [88-90]. More recently, the code has
been adapted to enable the analysis of heavy liquid-metal cooled reactor designs and
accelerator-driven systems. The SAS4A has been coupled to the DIF3D-K [91] and
VARIANT-K [92, 93] nodal spatial kinetics codes to provide accurate analysis of coupled
spatial kinetics and thermal-hydraulics problems [94].
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