Development and Validation of Physics-based HVAC System Model for a Nuclear Fuel Cycle Facility Using Modelica-EnergyPlus Coupled Simulation

This study introduces a physics-based co-simulation framework designed to evaluate the differential-pressure control performance of HVAC systems in nuclear fuel cycle facilities. These facilities handle radioactive materials under stringent confinement requirements, making it crucial to maintain negative pressure gradients between different zones for safety. Since experimental testing under degraded or abnormal conditions is not feasible, a simulation-based verification approach was adopted. In this framework, the building envelope and ventilation zoning were modeled using EnergyPlus, while major HVAC components—such as fans, dampers, filters, and ducts—were represented with the Modelica Buildings Library, relying on physics-based formulations. The two domains were integrated through a functional mock-up interface (FMI), facilitating comprehensive analysis of thermal loads and dynamic airflow responses. The Modelica fan model demonstrated strong correlation with theoretical affinity laws, achieving an R² value above 0.99 for airflow, static pressure, and power consumption. The coupled Modelica-EnergyPlus simulation successfully reproduced the intended pressure cascade, with calculated pressure differences in the hot-cell zones (-334 to -149 Pa) satisfying the design criteria. These findings indicate that the proposed framework offers a reliable and safe method to assess HVAC performance in safety-critical nuclear facilities and lays the groundwork for future research on fault prediction, degradation assessment, and control optimization.