Thermal-Hydraulic analysis and characterization of the HEXACOM facility

The design and safety assessment of innovative nuclear systems such as MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) require dedicated experimental facilities able to reproduce and investigate the complex thermal–hydraulic behaviour of key components. Among them, the Primary Heat eXchanger (PHX) plays a central role in the reactor’s primary cooling system, ensuring effective heat removal under both nominal and transient conditions. To this end, the HEXACOM facility was conceived as a scaled experimental loop reproducing the thermal–hydraulic behaviour of the PHX under representative operating conditions. This paper presents the results of a comprehensive experimental campaign carried out on HEXACOM operated in Stand-Alone Open Loop mode, aimed at characterizing the system’s steady-state and transient performance. The experimental activities were designed to provide high-quality data for code validation and to enhance the understanding of the coupled dynamics between primary, intermediate, and secondary cooling circuits. In parallel, a detailed system thermal–hydraulic (STH) model of the HEXACOM loop was developed using the RELAP5 code. To ensure reliability, stand-alone models of the main components—such as valves, the pump, and the PHX—were first implemented and validated against nominal data. These efforts ensured that the most influential components were represented with sufficient accuracy before their integration into the complete facility model. A post-test analysis was then performed by comparing RELAP5 simulation results with experimental data obtained from the HEXACOM test campaign. The comparison demonstrated a satisfactory agreement in terms of pressure, flow distribution, and heat transfer behaviour, confirming the model’s predictive capability. On this basis, the RELAP5 model can be considered validated and reliable for further applications. Finally, a pre-test analysis was conducted for a specific operating configuration whose experimental results are not yet available. This predictive study provides useful insights for the planning and interpretation of future experiments and supports the broader objective of validating the PHX thermal–hydraulic behaviour in MYRRHA.