Advanced Thermal-Hydraulic Modelling for GEN IV Reactors: Multiscale Solutions Applied to Heavy Liquid Metal Experimental Facilities

In the context of GEN IV fission reactors, numerous advanced designs are under development, focusing on enhancing safety and improving waste management. The safety evaluation of these systems is intrinsically tied to the thermal-hydraulics analyses, ensuring that the cooling systems perform efficiently across various operational and accidental scenarios. These innovative reactor designs necessitate rigorous verification and validation processes, as well as reliable numerical modelling tools, especially during the design phase. To address these needs, numerous international projects have been launched in recent years, allowing collaboration and knowledge-sharing networks aimed at accelerating technological advancements. Specifically for liquid metal-cooled reactors, the EU project PATRICIA was launched as a comprehensive initiative to support the development of liquid metal reactor technology. This project provides a holistic framework that integrates the chemical behaviour of the coolant, the neutronic performance of the core, and the thermal-hydraulic behaviour of the system. This study focuses on advancing thermal-hydraulic modelling methodologies for liquid metal-cooled reactors, by addressing the challenges inherent in these systems. It aims to refine and validate numerical approaches to support the design, safety, and operational reliability of next-generation reactors. The research underscores the importance of multiscale analyses to capture the complex thermal-hydraulic phenomena in such reactors. At the ENEA Brasimone research centre, a new test section, CIRCE-THETIS, will be integrated into the CIRCE-pool facility, which operates with LBE. This upgraded configuration is designed to generate valuable experimental data for liquid metal-cooled pool-type reactors. A significant part of this work focuses on the design of CIRCE-THETIS and the identification of the most effective simulation approaches for capturing the key thermal-hydraulic phenomena involved. As a first step an initial assessment of STH codes RELAP5/Mod3.3 and RELAP5-3D revealed limitations in capturing the 3D behaviour of the pool environment. While RELAP5-3D was evaluated for its purported 3D capabilities, it was found to function more as a pseudo-3D model, not suitable for critical areas such as fluid mixing and computational efficiency. Consequently, RELAP5/Mod3.3 was used for preliminary transient analyses, yielding valuable experimental guidelines, such as optimizing the steam generator’s operation to prevent coolant freezing. A subsequent comparison of RELAP5/Mod3.3 with ANSYS Fluent highlighted the need for CFD-based modelling, showing the limitation of the 1D approach of RELAP5. However, CFD alone was limited in handling two-phase flow regimes, necessitating a coupled approach that integrated STH for the secondary side and CFD for the pool. This hybrid methodology proved effective in analysing Protected Loss of Flow Accidents (PLOFAs), confirming stable natural circulation as a reliable decay heat removal mechanism. A novel pressure coupling routine was developed within the myMUSCLE platform to integrate STH and CFD tools, addressing inconsistencies in pressure definitions between the SPECTRA and STAR-CCM+ codes. This strategy ensured consistent information exchange during simulations and was benchmarked against TALL-3D experimental data. The coupling improved simulation accuracy compared to standalone STH methods. The study extended to the NACIE-UP facility as part of an IAEA initiative aimed to investigate the thermal-hydraulic behaviour in liquid metal loop facility. A “smart” coupling approach integrated a simplified CFD representation of the Fuel Pin Simulator (FPS) region with an STH model for the rest of the system, balancing computational efficiency with accuracy. This hybrid model was tested under varying heating conditions, demonstrating strong agreement with experimental data for bulk temperature and flow rates. Minor deviations in wall temperature predictions during asymmetric heating scenarios were attributed to the simplified CFD geometry. Overall, this work underscores the important role of multiscale approaches as indispensable tools for the development and optimization of next-generation liquid metal reactor designs, bridging gaps in traditional modelling techniques and paving the way for more refined and efficient reactor technologies.