Analysis of loss of flow without scram test in the FFTF reactor: Coupled 3D neutronics and thermal hydraulics analysis with DYN3D/ATHLET code system

• ULOF transient simulation with coupled 3D neutronics and system thermal hydraulics. • Comparative analysis: 3D Spatial Kinetics vs. Point Kinetics vs. Experiment. • Validation case for DYN3D/ATHLET coupled code system for SFR applications. • Transient decay heat estimation without applying decay heat standards. • Key conclusions from the FFTF LOFWOS Test #13 benchmark study. The coupled code system DYN3D/ATHLET is continuously developed and validated for transient analyses of sodium-cooled fast reactors (SFRs). In this study, the system was applied to model the loss of flow without scram (LOFWOS) test conducted at the Fast Flux Test Facility (FFTF), with the aim of further assessing the coupled 3D neutronics and system-level thermal hydraulics analysis capabilities. Two simulation approaches were employed. The first approach utilized the point kinetics module of ATHLET for neutron physics. The second relied on the 3D spatial kinetics solver of DYN3D. In both cases, the thermal hydraulics of the system was handled by ATHLET, while neutronics data was generated by the Serpent Monte Carlo code. The performance of both approaches was evaluated by comparing their results with experimental data. Additionally, the feasibility of using the adaptive decay heat model, a recent enhancement in DYN3D, was demonstrated. This paper concludes the FFTF LOFWOS study, extending the validation base for the DYN3D/ATHLET system in comprehensive SFR modeling.