Piyush Sabharwall

Heat pipes are among the most efficient thermal transport technologies, widely utilized in aerospace systems, electronics cooling, and advanced nuclear reactors. Alkali metal heat pipes enable kilowatt-level heat transfer through evaporation and condensation with minimal temperature gradients, making them uniquely capable in high-performance thermal management applications. Achieving reliable and high-quality heat pipes requires meticulous design, fabrication, and testing. This article presents consolidated best practices for alkali metal heat pipe development process, documentation, quality assurance practices, and validation protocols. Long-term reliability of heat pipes is dependent on addressing geometric tolerances, operational limits, transient stresses and impurities from the earliest design stages. Implementing these practices reduces risk and supports the production of high-quality heat pipes.

Micro-scaled high-temperature gas-cooled reactors (micro-HTGRs) offer a promising option for reliable power in remote or off-grid locations. While the safety characteristics of modular HTGRs have been widely studied, a micro-HTGR configuration alters several key thermal-fluid phenomena that govern both normal operation and passive decay-heat removal. In many proposed concepts, the reactor vessel is oriented horizontally and integrated into an ISO shipping container to enhance transportability and modular deployment. This report documents a Phenomena Identification and Ranking Table (PIRT) exercise focused on the thermal hydraulic safety phenomena relevant to all micro-HTGRs. The objective is to systematically identify, describe, and rank the importance, uncertainty, and modeling complexity of the key phenomena that control core and vessel temperatures during normal operation, pressurized conduction cooldown (PCC), and depressurized conduction cooldown (with air ingress) conditions.

Gas–solid granular systems exhibit complex transient dynamics that are of fundamental interest in fluid physics and various industrial applications, ranging from chemical reactors to geophysical phenomena. In this study, a graphics processing unit accelerated smooth particle hydrodynamics–discrete element method (DEM) framework is developed to resolve coupled hydrodynamic and thermal behaviors from dense packed beds to dynamic spouting regimes. The fluid phase is governed by the volume-averaged Navier–Stokes equations, where a key feature is the direct evaluation of local porosity and its temporal derivative (dε/dt) from instantaneous particle configurations. By applying the chain rule to interpolated quantities, the porosity change rate is expressed as an inner product between kernel gradients and DEM particle velocities. This approach enables the explicit integration of transient porosity effects into the momentum balance without introducing additional empirical closures, ensuring consistent pressure–velocity coupling under highly transient conditions. The framework is validated against benchmark experiments, demonstrating accurate predictions of pressure drop and thermal responses in granular beds. Notably, the model reveals that accounting for transient porosity evolution systematically amplifies the velocity and elevation overshoot during spouting onset, a phenomenon found to be negligible in the subsequent quasi-steady state. These findings emphasize the critical necessity of considering temporal porosity variations to capture the full physics of early stage gas–solid interactions and provide a robust computational basis for extending toward more complex multiphase systems.

It is our great pleasure to present this Special Issue on Advances in Thermal Hydraulics of Nuclear Power Plants

I. This special issue of Nuclear Technology contains selected papers from the 2024 International Congress on Advances in Nuclear Power Plants (ICAPP 2024). The contributions assembled here represen...

Steam generators are integral components of both conventional and advanced reactors utilizing steam power cycles, as they influence the power conversion performance. Steam generator designs are highly influenced by the type of reactor coolant used, which mainly governs the different control, monitoring, and safety system actuation requirements. Therefore, the present work reviews the thermal hydraulics instrumentation within steam generators designed for pressurized water reactors (PWRs), high-temperature gas-cooled reactors (HTGRs), sodium fast reactors (SFRs), and molten salt cooled reactors (MSRs). Emphasis is given to measurement techniques that are either unique or particularly important to specific reactor types. Lastly, a discussion is provided on the instrumentation and measured parameters. The work identifies standard parameters of interest and is expected to aid the development of test beds and novel measurements techniques.

Droplet entrainment in steam-flow is a prominent phenomenon that needs adequate safety and risk analysis of postulated transient and accident scenarios—including experimental investigation and representative modeling and simulation (M&S)—for small modular reactor (SMR) system design and demonstration. This study identifies knowledge gaps by evaluating experimental and computational fluid dynamics modeling approaches to support early-stage reactor system design, testing, and model evaluation. Previous studies reported in the literature for steam-flow entrainment primarily focused on gigawatt capacity pressurized water reactor (PWR) systems. However, entrainment phenomena are even more prominent for PWR-type SMRs due to their more compact integrated designs, which need further research and development. To fill the research gaps, this study provides insight by specifying the phenomena of interest by leveraging the lessons learned from past research, adopting advanced M&S techniques and advanced instrumentation and control. The findings and recommendations are applicable for evaluating steam-flow entrainment models and for designing integral effect test and separate effect test facilities for gaining reactor design approvals.