Special Issue on Advances in Thermal Hydraulics of Nuclear Power Plants

It is our great pleasure to present this Special Issue on Advances in Thermal Hydraulics of Nuclear Power Plants. Since the advent of nuclear power, thermal hydraulics has served as a core discipline governing reactor safety, operational stability, and system innovation. Its importance grows as contemporary reactor concepts introduce increasingly challenging conditions, including high-heat-flux environments, complex geometric scales, advanced coolant formulations, and tightly integrated multiphysics phenomena. Today, thermal hydraulics stands at the intersection of next-generation reactor development, advanced diagnostics, and high-fidelity computation, making it more important than ever to understand, predict, and manage the behavior of fluids and heat in nuclear systems.

The 10 papers presented in this Special Issue highlight both the maturity and forward trajectory of the field. Contributors explore thermal hydraulic phenomena in the conventional nuclear fleet as well as in advanced fission concepts and emerging fusion technologies. This Special Issue presents a broad rigorously peer-reviewed collection of studies advancing the field of nuclear thermal hydraulics, spanning two-phase flow behavior, critical heat flux, dryout, steam separation, passive safety, natural circulation, fluid–structure interactions, data assimilation, and enhanced boiling surfaces. A central theme across this Issue is how modern experimental, computational, and data-driven methods are being used to strengthen the scientific foundations needed for safer, more efficient, and more resilient nuclear energy systems. The work within this Special Issue collectively reflects the increasing complexity of reactor environments and the growing demand for high-fidelity understanding of thermal hydraulic mechanisms.

Several contributions provide new insight into complex two-phase flow phenomena and next-generation reactor technologies. Objective neural-network classifiers for improved accuracy of two-phase flow structure identification and advanced modeling are reported by Contribution 1 and applied to diverse geometries including vertical narrow rectangular channels and round pipes. Advanced interface-capturing simulations clarifying steam separation behavior in Boiling Water Reactors are presented by Contribution 2, who analyzed two-phase flow in steam separators with the aim to achieve more accurate predictions of separation mechanisms. Experimental and numerical research of droplet entrainment phenomena in the steam supply systems of water-cooled Small Modular Reactors (SMRs) was conducted by Contribution 3 with the aim of optimizing moisture separation and enhancing plant thermal hydraulic efficiency.

Other studies advance thermal hydraulics for emerging reactor systems including fluoride-salt-lubricated bearings and thorium-based fuels in heavy water reactors. Contribution 4 analyzed the operational performance and scalability potential of hydrodynamic bearings lubricated by fluoride salts within the specific environments of fluoride-salt-cooled high-temperature reactors. Contribution 5 present a multiphysics safety analysis of a heavy water reactor using thorium-based fuel, integrating thermal hydraulic and solid mechanics models to evaluate reactor core behavior under both normal operating conditions and transient scenarios.

Contribution 6 investigated the fluid–structure interaction (FSI) of foreign objects within steam generator tube bundles, utilizing numerical simulations to assess the impact of flow-induced vibrations on the mechanical integrity and wear of the tubes. Contribution 7 analyzed the dynamics of bubble motion in narrow channels on basis of experimental investigations and numerical simulations. Specific focus was placed on the effects of spatial confinement on bubble deformation, rise velocity, and flow regimes.

This Special Issue also highlights progress in passive safety analysis, which is essential for the safety of advanced Small Modular Reactors (SMRs) and Generation III+/IV reactors, uncertainty reduction, and thermal hydraulic surface engineering. Natural circulation behavior is examined by Contribution 8 through RELAP5-3D validation with water and high-Prandtl-number fluids, identifying pathways to improve molten salt system studies. Deterministic data assimilation methods used by Contribution 9 demonstrate how real-time measurements can refine system predictions and safety margins in thermal hydraulic analysis of natural circulation loops.

Finally, the universal, mechanism-based classification system for boiling surfaces introduced by Contribution 10 offers a unifying framework that strengthens the evaluation and deployment of advanced surface technologies while enabling clear, consistent comparisons with prior studies.

Together, these contributions showcase the evolving capabilities of thermal hydraulics research and its essential role in enabling the next generation of nuclear energy systems.

We extend our sincere gratitude to all authors and reviewers for their commitment, expertise and scientific rigor. Their contributions not only enrich the literature, but also help drive the continual advancement of nuclear thermal hydraulics as a discipline.