The rapid evolution of Small Modular Reactor (SMR) technology has triggered renewed interest in exploring innovative applications within the framework of nuclear hybrid energy systems. SMR, given their greater flexibility in terms of siting and power rates, are one of the most suitable candidates to be integrated in energy systems aimed at providing multiple energetic assets. In this context, feasibility studies as well as techno-economics analysis should be supported by proper modelling tools able to facilitate the integration of SMRs with other energy system components, such as renewable plants, energy storage systems, hydrogen production, etc. This paper presents the TANDEM Modelica library developed within the TANDEM Project – a Horizon Europe project aimed at developing methodologies and tools to facilitate the safe and efficient integration of SMRs into smart low-carbon hybrid energy systems. The TANDEM library is based on several staple libraries (e.g., ThermoPower, ThermoSysPro, Modelica Standard Library) from which the main components of a hybrid energy system are derived, e.g., NSSS, BoP and power conversion systems, electrical and thermal energy storage, electrical grid, conventional power plants, hydrogen production with low and high temperature electrolyzers, district heating…. The library is meant to be a versatile platform, offering a unified framework for the dynamic simulation and analysis of complex interactions within nuclear hybrid energy systems. Key features of the library include modularity, extensibility, and compatibility with existing library and simulation tools, including safety codes. Possible applications enabled by the library include the analysis of different operational strategies and the optimization of the hybrid energy system configuration and components’ design in terms of efficiency, reliability and/or CO2 emissions.