Performance evaluation of a coupled CFX-RELAP5 tool adopting experimental data from the TALL-3D facility

The interest for multiscale simulations for the thermal–hydraulic analysis of nuclear systems has increased with the grow of computational power and the need of complex three-dimensional analysis for the generation IV nuclear reactors. To perform analyses at different scales, coupling codes are expected to be a valuable solution. Particularly, the coupling between System Thermal Hydraulic (STH) and Computational Fluid Dynamics (CFD) codes promises to achieve the goal of a detailed local solution at CFD level, while keeping the speed of a STH approach elsewhere in the system, where 1D phenomena are predominant. The validation of new developed coupled tools requires specific experimental facilities in which three-dimensional phenomena affect the behaviour of the entire system. TALL-3D is a liquid Lead Bismuth Eutectic loop designed in such a way to produce experimental data for coupled STH and CFD codes validation. In this work, a Ansys CFX − RELAP5/Mod3.3 coupled tool is adopted to replicate a forced to natural circulation transition on the TALL-3D facility, comparing different coupling strategies regarding the spatial domain discretization (decomposition and overlapping) and time advancing scheme (explicit and semi-implicit). Results obtained with the coupled models show a clear advantage with respect to the RELAP5 standalone one. The analysis of the phenomena involved show the inability of the STH code in reproducing them, while the coupled tool proved to be a reliable solution thanks to its capability to take advantage of both the component and system scales analysis. Moreover, the impact of the selected coupling strategy on the stability of the tool is assessed, and the required computational time for the analysed transient is evaluated.