Motivated initially by the specific needs of Small Modular Reactors (SMR) transient analysis, including Molten Salt Reactors (MSR) draining or leak scenarios, or Sodium Natural Circulation Reactor (SNCR) scenarios, this paper presents the development of a generic two-substance liquid-gas medium and an approach used for a liquid Sodium medium. Such media have a low isothermal compressibility, which is a key issue for the numerical robustness of the simulation. These media are designed for compatibility with standard libraries like Modelica.Fluid. We detail their formulation, capabilities, and limitations. A significant portion of the paper is then dedicated to investigating a crucial aspect arising in such models: the representation of the liquid phase’s compressibility. We compare different approaches, analyzing their impact on the resulting equation system structure and numerical robustness using the OpenModelica debugger. This analysis demonstrates the trade-offs involved and provides insights into selecting appropriate liquid models for dynamic simulations involving non-miscible flows.