## 1 Introduction  
Liquid Metal Fast Breeder Reactors (LMFBRs) represent one of the proposed technologies and concepts for the upcoming Generation IV of Nuclear Power Plants. The University of Pisa, in the frame of several programs promoted by the European Union, has joined the common effort for the development of such technologies in the near future. In particular, the development of coupled STH/CFD applications for the analysis of complex thermal-hydraulics phenomena is one of the main commitments of UniPi inside the MYRTE EU H2020 project. Unlike the LWRs, in fact, LMFBRs consist of several plant components that cannot be reliably modelled adopting STH approaches such as the large reactor pool (see e.g., Ref. [1, 2]) or components that may require a three-dimensional approach for a better comprehension of the involved phenomena, as in the case of fuel assemblies adopting wrapped wire rod bundles. As a consequence, stand-alone STH applications are no more suitable for performing thermal-hydraulics analyses addressing the various plant operating and accidental conditions. Computational Fluid Dynamics (CFD), yet not being considered a reliable tool for licensing matters, usually provides instead valuable estimations for flow conditions in complex three-dimensional environments. In this sense, CFD may be adopted in order to provide better estimations for parameters to be used in STH calculations (see, e.g., Ref. [3]); nevertheless, a more comprehensive approach is represented by coupled STH/CFD methods that try to obtain the advantages of both approaches without facing the corresponding drawbacks. The main aim is to combine the capabilities of STH codes in predicting flow conditions in complicated pipe systems at limited computational costs, with the capabilities of CFD codes in providing highly detailed predictions of selected regions where complex multi-dimensional phenomena may occur. In the available literature, examples of the application of coupled STH/CFD codes to liquid metal thermal hydraulics exists, proving the capabilities of the adopted method in predicting the considered experimental conditions. Particularly, several coupled simulations were performed in works by [PERSON] [4; 5; 6], aiming at reproducing the behavior of the MYRRHA reactor in postulated conditions, highlighting the advantages with respect to the stand-alone STH and CFD calculations. In their works, [PERSON] and co-authors also analysed the complex behaviours observed in the E-SCAPE facility (see e.g., Ref. [7]) simulating the lower plenum and the Above Core Structure (ACS) by means of CFD (ANSYS Fluent) calculations while simulating the pipes system with the help of the RELAP5-3D STH code. Improvements with respect to the STH stand-alone approach were observed; in particular, CFD allowed the analysis of the thermal stratification in the ACS which also granted the prediction of an improved pressure distribution in the reactor core. Further works concerning applications of coupled STH/CFD codes to liquid metals were performed at the KTH [8; 9]; here, the TALL-3D facility was investigated by means of the CFD code STAR-CCM+ and the STH code RELAP5/Mod3.3. CFD was again considered for one of the large 3D environments components by which the facility is composed: the use of the coupled STH/CFD provided improved results in comparison to the STH stand-alone application, also highlighting the presence of thermal-hydraulic phenomena that could not be spotted without the CFD analysis. At the University of Pisa, coupling techniques have been developed for the analysis of experimental facilities in order to improve the understanding of phenomena relevant for LMFBRs during both operating and accidental conditions. The coupling methodology, developed in the frame of recent studies [10; 11; 12; 13; 14; 15], combines the STH code RELAP5 Mod3.3 with the CFD code ANSYS Fluent; the coupling interface of the simulation is provided by MATLAB scripts. Coupled calculations were performed for the analyses of the NACIE-UP (see e.g., Ref.