Multi-scale modelling of the CIRCE-HERO facility

This work describes the activities performed at the University of Pisa (UniPi) and at the Nuclear Research and Consultancy Group (NRG) concerning the application of different in-house developed coupling methodologies for thermal-hydraulic analysis of innovative nuclear reactor. The application of a tool for System Thermal-Hydraulic (STH) and Computational Fluid Dynamics (CFD) coupled calculations aims at exploiting the advantages from both the codes to perform thermal-hydraulic analysis of complex systems restricting a higher-degree of detail in a smaller part of the domain. Indeed, STH codes are commonly used for thermal-hydraulic analysis of entire complex nuclear systems, though these codes become generally inadequate to investigate phenomena with relevant 3D characteristics, such as mixing and thermal stratification in large pool systems. On the other hand, the exclusive use of CFD codes to simulate large systems is still too demanding in term of computational efforts. In such frame, the coupling between two or more scales represent a promising compromise, especially for those applications in which the small-scale phenomena take place in a limited part of the domain. Two different models of the CIRCE facility are created. At NRG, the CFD code ANSYS CFX is coupled with the STH code SPECTRA, while at UniPi, ANSYS Fluent is used for the CFD calculations and a modified version of RELAP5 is adopted for the STH calculations. Modelling and coupling strategies for both models are discussed in the present paper. Furthermore, simulation results of both models are here discussed, both for steady-state cases representing experiments at full power, as well as Protected Loss Of Heat sink and Loss Of Flow (PLOH + LOF) accident scenarios. Special attention is paid to pool phenomena such as the 3D characteristic of the heavy-liquid metal (Lead-Bismuth Eutectic, LBE) fluid flow, the thermal stratification and the temperature evolution during the postulated transients.