High-Fidelity Simulation Of Forced-To-Natural Convection Transition In Tall-3D Facility Using Spectral Element Method

The pool-type design of advanced metal-cooled reactors offers several significant benefits, including reduced risk of coolant leaks, simplified structural design, and enhanced passive safety. In particular, the primary coolant is contained within a large pool, which minimizes the potential for leaks and provides an extra barrier against sodium leakage. This configuration allows for a more compact primary system, reducing the overall size of the reactor while also enhancing safety through substantial thermal inertia; the large volume of coolant acts as an immense heat sink, enabling passive decay heat removal via natural circulation even in the event of system failures. The TALL-3D liquid-metal facility was introduced to support these advancements as part of the SESAME project (Simulations and Experiments for the Safety Assessment of MEtal-cooled reactors). In this study, Large Eddy Simulation (LES) is performed using the GPU-accelerated spectral element code NekRS to investigate the forced, transient, and natural convection operational conditions of the TALL-3D facility in experiment TG03.S301.04 and T01.09, using four single- and one overlapping-domain cases. For the transient case, time-dependent inlet mass flow and temperature histories are prescribed from the experiment using a compact piecewise Chebyshev series representation. The overlapping-mesh methodology (NekNek) in NekRS is employed to couple multiple subdomains with distinct mesh configurations, enabling interaction via boundary conditions. The NekRS single-domain temperature data were compared with experimental measurements, demonstrating good agreement across all operational conditions of interest.