## 4 Conclusions  
In the present paper, the preliminary results of a coupled STH/CFD application involving the CIRCE-HERO facility were reported.  
The facility represents a complicated thermal-hydraulic system which involves two-phase flow, non-condensable gas transport and flow conditions in an intrinsically three-dimensional environment. These characteristics make a stand-alone approach of the available modelling techniques hardly reliable presenting aspects that may impair the predicting capabilities of both STH and CFD codes.  
As a consequence, a coupled STH/CFD application represents the best option for obtaining a suitable representation of the selected operating conditions. The computational domain was subdivided into several parts and each region was assigned to the code that could better represent its phenomena. The regions were consequently connected in order to allow mass and heat transfer between the various domains.  
The obtained results suggested that coupled applications have promising capabilities in predicting the observed phenomena. A good matching between the measured and calculated trends was obtained for thetementperature distributions; for the mass flow rate instead, several hypotheses concerning the observed mismatching were formulated. Nevertheless, given all the uncertainties of the experimental setup, the obtained results may be considered as a good prediction of the experimental data. The coupled approach also allowed to observe and highlight the interesting phenomena occurring inside the pool. In particular, changes in the direction along which heat is transferred between the pool and the FPS were here clearly spotted and the pathlines distribution suggested possible explanations for the observed phenomena.  
The performed work represents a valuable benchmark for the next applications to be developed at the University of Pisa as it highlighted both capabilities and weaknesses of the selected approach. In particular, the improved capabilities and the quality of the results provided by the addressed coupled STH/CFD code with respect to the STH stand-alone application seem worth the required  
Figure 10: Calculated temperature distribution at the bottom of the pool for t = 2060 s.  
additional modelling and computational cost. The lessons drawn during the present application will be considered in order to refine and improve the presently adopted coupled STH/CFD approach in view of applications in future works. The modelling and discretizing approach adopted in the present work obviously has common characteristics with some of the referred papers thus suggesting that the considered technique may be approaching a sufficiently mature status. Since most of the presently referred applications were performed in the frame of the nuclear field, it is a common interest that this approach is considered suitable by licensing agencies. The development of Best Practice Guidelines to be adopted in coupled STH/CFD applications could be a sound strategy for paving the way for this achievement; the present paper thus calls the scientific community to pursue this common effort.  
Investigation and Formal Analysis; P.A.; writing original draft, writing, reviewing and editing; P.A., G.F., M.M., F.N., M.D.; project administration and supervision; F.N. All authors have read and agreed to the published version of the manuscript.  
This work was performed in the framework of H2020 MYRTE project. This project has received funding from Euratom research and training program 2014-2018, under grant agreement No 662186.  
The authors declare no conflict of interest.