Generation IV prismatic high-temperature gas-cooled reactors (PHTGRs) require coupled thermal-hydraulic and neutronic analysis to characterize safety margins, yet explicit resolution of TRistructural ISOtropic (TRISO) particles in computational fluid dynamics (CFD) remains computationally prohibitive. Standard homogenization approaches are efficient, but they do not directly provide the intraparticle temperature information needed for accurate Doppler feedback evaluation. To address this, this study develops a multiscale coupling framework that combines Serpent Monte Carlo neutronics, OpenFOAM CFD, and a dedicated analytical heat conduction solver. In the proposed approach, the fuel compact is treated as homogeneous in the CFD solution, while TRISO coating layer and kernel temperature profiles are analytically reconstructed for each CFD fuel cell from the homogenized compact temperature field and supplied to the neutronics for cross-section evaluation. Compared with recently reported Serpent-OpenFOAM applications to research reactors, small modular reactors, molten salt reactors, liquid metal-cooled reactors, and light water reactors, the present work focuses on a PHTGR with TRISO fuel and distinguishes itself through full integration with Serpent's unstructured mesh coupling interface and a per cell analytical reconstruction of TRISO temperatures within a block-scale Monte Carlo/CFD framework. The coupling approach is applied to a high-temperature engineering test reactor-based fuel block model, where the coupled simulation resolves intrablock power peaking, absorber proximity effects, and subpin radial power and temperature variations, predicting a maximum fuel rod temperature of 1249 K and a coolant outlet temperature of 950 K. The results show that the framework provides high-resolution thermal feedback information for PHTGR analysis without the prohibitive cost of explicit TRISO meshing and can serve as a high-fidelity reference tool for validation of lower-order system models.

The dynamics of Taylor bubbles is investigated in a counter-current water flow, where the downward liquid flow in a vertical pipe counterbalances buoyancy, allowing extended steady-state observations. Using a combination of Particle Tracking Velocimetry (PTV), Laser-Induced Fluorescence (LIF), shadowgraphy, and dynamic masking, we unveil interfacial flow features, including tiny capillary waves. Velocity fields are measured for Taylor bubbles of varying lengths (1.2 D-14.9D) in D=12.4mm diameter pipe. Upstream of the bubbles, the liquid flow is laminar with Re = 1270 whereas the bubble behavior is characterized by Eo = 21 and Nf = 4830. In the liquid film region, the flow accelerates rapidly reaching up to about eight times the bulk velocity for the longest bubbles, yet no transition to turbulence is observed. The liquid film stabilizes at its final thickness and terminal velocity is reached for bubble lengths at about 5 D. Our measurements show that the bubble wake region hosts a strong primary toroidal vortex accompanied by a weaker, oppositely rotating secondary vortex. A particularly interesting is the emergence of stationary capillary waves at the bottom of the annular liquid film, likely associated to a localized widening of the liquid film region, reminiscent of a hydraulic jump where rapid flow abruptly decelerates. Velocity profiles, captured with a 20 mu m spatial resolution, expose significant interfacial shear, observed exclusively downstream of the stationary capillary waves while remaining negligible elsewhere. This increased shear not only explains the observed phenomena but also suggests local flow motion within the gas bubble itself.

Wind power is intermittent at any single location—but it is always blowing somewhere across a continent. Here we test this premise using a decade of actual generation data from 29 European countries (2015–2024), the longest operational record analyzed to date. Unlike reanalysis-based studies, operational data capture curtailment, maintenance outages, and evolving fleet composition—effects invisible in simulated output. Aggregated European wind production never fell below 5.5 GW (1.8% of 2024 installed capacity), and geographic diversification reduced wind production variability by a stable ~48%. Yet Europe is failing to harness this potential. Although installed capacity doubled, the guaranteed baseload floor barely grew—a 2.1% conversion rate—because new capacity concentrated in already-correlated Northwestern Europe rather than in weakly correlated regions like Iberia and Scandinavia. Adding solar raises the floor by 61% and, more importantly, cuts battery storage costs by 81–93%. These findings identify geographic diversification as a highly effective but largely untapped reliability mechanism for wind energy. Yet even under optimal siting, the wind baseload floor remains at just 3–5% of installed capacity, covering only a small fraction of demand; a reliable zero-carbon grid requires wind alongside battery storage and firm clean generation.

The TRIGA Mark II research reactor at the Jo & zcaron;ef Stefan Institute is a key facility in the field of nuclear research, characterized by its versatility and applicability in a wide range of scientific disciplines. This document highlights its operational history, contributions to nuclear safety, education and various scientific applications, including advances in reactor and radiation physics, neutron activation analysis, environmental science and even contributions to the fight against the COVID-19 pandemic. It highlights the reactor's significant role in fostering international collaborations, improving computer modeling techniques for nuclear research, and providing invaluable educational experiences. The great versatility and applicability of the JSI TRIGA reactor is emphasized by its adaptability to various research needs and its ability to enable groundbreaking studies in both fundamental and applied sciences.

This study reports the identification of a previously uncharacterized type of capillary wave on the moving interface of a Taylor bubble in vertical countercurrent flow. In our experiments, the buoyancy of the Taylor bubble is precisely balanced by a downward laminar water flow (Re = 1400) within a 12.4 mm diameter glass pipe. These previously uncharacterized waves appear at a height one-fourth to one-half of the pipe diameter above the bubble's tail and remain stationary in the laboratory frame, implying that their upward propagation speed exactly matches the downward motion of the interface. The waves persist for 5–10 wavelengths, spanning one-fifth to one-third of the pipe diameter. We measured their wavelength for bubbles of different lengths (1.5–11 pipe diameters) and determined the liquid film thickness at the wave location. From the measured mass flow rate, we calculated the laminar velocity profile in the liquid film and the interface velocity. We have shown that the measured wave propagation speeds align with the interface velocities computed from the film thickness, confirming their capillary nature within experimental uncertainty. Furthermore, our analysis reveals the underlying physics: these capillary waves originate from a newly formed toroidal vortex near the bubble tail that rotates opposite to the primary air vortex inside the bubble, consistent with previous computational fluid dynamics (CFD) simulations and our new observations.

Within the SARENA programme, students from different parts of the world have the opportunity to obtain a Master's degree in nuclear engineering by participating in a two-year curriculum combining academic programs in France, Spain, Finland and Slovenia (depending on the selected study track). The concept and organization of the SARENA programme are described. The experience from the first four cohorts is presented and assessed both quantitatively and qualitatively. So far, the programme has mostly fulfilled expectations. As the students went through a very selective admission process, their mostly satisfactory academic performance was expected. An additional aspect was the cultural interaction between SARENA students and their new educational environments.

The TRIGA Mark II research reactor at the Jožef Stefan Institute in Slovenia achieved first criticality in 1966. Since then, the reactor has been playing an important role in developing nuclear technology. The reactor has been mainly used for research, education of university students, training of operators of the Krško nuclear power plant (start of operation in 1983) and other nuclear specialists, isotope production and beam applications. The reactor is experiencing a high level of activity today, engaging in a diverse range of experiments and studies across reactor physics, environmental research, radiation hardness testing as well training and education. The future of nuclear technology in Slovenia is focused on new NPPs, while the research community is looking forward to a possible new nuclear reactor. The basic initiatives are at a very preliminary stage: the primary choice is dual-core pool-type reactor, with a zero-power core and a separate MW-size core, cooled and moderated with light water. Such a dual-core configuration is designed to meet the varied requirements of the European Union member states. Another option would be hosting one or more micro-reactors with electrical and/or heating power producing capability that could offer stronger support toward demonstration of prototype small modular reactors in prototype future electrical grids.

The TRIGA Mark II research reactor at the Jožef Stefan Institute in Slovenia, achieved first criticality in 1966. Since then the reactor has been playing an important role in developing nuclear technology. The reactor has been mainly used for research, education of university students, training of operators of the Krško nuclear power plant (start of operation in 1983) and other nuclear specialists, isotope production, and beam applications. Despite the age of the reactor, there is a wide range of research activities going on in the three main areas. 1) Reactor Physics activities are related to verification and validation of computer codes and nuclear data, testing and development of experimental equipment used for core physics tests at the Krško NPP, neutron radiography, neutron activation studies, development of bio-dosimeters, radiation hardness studies, safeguards activities. 2) Some of the environmental studies are using reactor for neutron activation analyses and for production of radioactive tracers. 3) Reactor is being used by particle physics department for radiation hardness studies of ATLAS detector in CERN. The future of nuclear technology in Slovenia is focused on new NPPs, while the research community is looking forward to a possible new nuclear reactor. The basic initiatives are at a very preliminary stage: the primary choice is dual-core pool-type reactor, with a zero-power core and a separate MW-size core, cooled and moderated with light water. Such a reactor will be capable of supporting the European fleet of existing and future nuclear power plants, including small modular reactors based on pressurized water reactor technology. Another option would be hosting one or more micro reactors with electrical and/or heating power producing capability. In this way the knowledge and infrastructure available for research and development could offer stronger support towards demonstration of prototype small modular reactors in prototype future electrical grids.

This paper investigates dynamics of Taylor bubble in counter-current flows, leveraging large eddy simulations combined with the volume of fluid method. Utilizing the OpenFOAM framework, we have implemented a high-order Runge–Kutta time-integration scheme, along with a piecewise linear interface calculation method for precise geometric reconstruction of the bubble interface. We examine the performance of algebraic vs geometric capturing techniques in the context of Taylor bubble breakup, focusing specifically on the transitional flow regime with a liquid Reynolds number of 1400. Our results reveal that the geometric capturing technique offers superior accuracy, improving our understanding of the breakup process and providing valuable insight for multiphase flow simulations in various engineering fields. Our study also reveals the emergence of a secondary vortex in the turbulent wake region behind the Taylor bubble, a phenomenon most prominent at finer mesh resolutions. This vortex represents a novel discovery in counter-current Taylor bubble flows.

Pipe bends disrupt the flow, resulting in an asymmetric velocity field across the pipe diameter (D). We examined the recovery length required for the flow to return to a symmetric velocity profile downstream of a sharp elbow. The wall-resolved Large Eddy Simulation (LES) approach was applied to reproduce turbulent fluid flow at Reynolds numbers (Re) of 5600 and 10,000. An additional case in the transitional laminar-turbulent-laminar regime was analyzed at Re=1400. This analysis explored the behavior of the Dean vortices downstream of the elbow and revealed that, in turbulent cases, these vortices reverse their vorticity direction in the region between 8 D and 10 D. However, they eventually decay in structure as far as 25 D from the elbow. Flow asymmetry was analyzed in a 100 D long pipe section downstream of the elbow using four different criteria: wall shear stress (WSS), streamwise velocity, its fluctuations, and vorticity fields. This study found that in turbulent flows, the distance required for flow recovery is a few tens of D and decreases with increasing Re. However, in the transitional case, the flow separation within the elbow induces instabilities that gradually diminish downstream, and flow asymmetry persists even longer than the 100 D length of our outlet pipe section. WSS proved sensitive for detecting asymmetry near walls, whereas flow profiles better revealed bulk asymmetry. It was also shown that asymmetry indicators derived from velocity fluctuations and vorticity were less sensitive than those obtained from streamwise velocity.

Fluid dynamics is a fundamental pillar of modern engineering, playing a pivotal role in enhancing the efficiency and safety of various industrial and energy systems. In nuclear power systems, understanding two-phase flows becomes particularly critical during events such as boiling heat transfer and accident management scenarios, including the Loss of Coolant Accident (LOCA). During these scenarios, the slug flow regime-characterized notably by elongated, bullet-shaped bubbles known as Taylor bubbles-often occurs, particularly within steam generators. This can pose significant challenges to the thermodynamic stability of the system. This study focuses on the behavior of Taylor bubbles in counter-current air-water flows through a dual methodology approach, incorporating both high-fidelity numerical simulations and advanced experimental techniques. We present the dynamics in the flow regime with a transitional Reynolds number (Re) of 1400. The investigation focused on bubbles of varying lengths under steady-state conditions, where buoyancy is balanced by the downward flow's inertial drag. Experimentally, we concentrated on three pivotal aspects: the disintegration of bubbles, in-depth analysis of bubble interface dynamics, and velocity field measurements in the liquid phase with Particle Image Velocimetry technique. Numerically, we used the open-source Computational Fluid Dynamics (CFD) framework OpenFOAM to study bubble behavior and disintegration mechanisms. We have developed a new solver, which uses high-order Runge-Kutta time integration schemes alongside a Volume-Of-Fluid (VOF) approach for precise bubble interface reconstruction. Our analysis primarily focused on the efficacy of algebraic versus geometric interface capturing techniques. The comparison of the high-resolution experimental and numerical velocity fields in the bubble wake region show good qualitative agreement with minor quantitative differences.

The stagnant Taylor bubble in vertical isothermal turbulent counter-current flow was analyzed using 2D shadowgraphy experiments and two distinct high-fidelity numerical simulations. One simulation employed the geometrical VOF interface tracking method within the OpenFOAM code, while the other utilized the explicit front tracking method of the TrioCFD code. Interface recognition algorithms were applied to the photographs and compared with the results of 3D simulations performed with LES and pseudo-DNS accuracy in OpenFOAM and TrioCFD, respectively. The measured Taylor bubbles exhibited an asymmetric bullet-train shape and a specific speed, which were compared with the predictions of both numerical approaches. Reproducing the experiment proved challenging for both otherwise well-established methods frequently used in interface tracking simulations of two-phase flows. Grid resolution and subgrid turbulent models, known for their success in single-phase turbulence, were less accurate near the water–air interface. Additional experimental parameters compared with simulations were related to the dynamics of tiny disturbance waves with amplitudes ranging from 10 to 100 µm along the interface of the Taylor bubbles. The speed and spectra of the surface disturbance waves were reproduced numerically with moderate success despite detailed grid refinement in the relevant region of the computational domain.

Turbulent heat transfer in a square duct, where a finite section of a single duct boundary is heated with an infinitely thin foil, was analyzed with LES and DNS approaches. Both computations were performed with OpenFOAM code with passive scalar approximation, using second order accurate schemes. Results are obtained for the key velocity and temperature statistics in the duct, with an emphasis on the temperature profiles and temperature fluctuations at the foil boundary, which appear due to the imposed constant heat flux boundary condition at the foil. Comparison of DNS and standard LES-WALE model show 20% higher foil temperatures and temperature fluctuations in LES results. These differences are explained with detailed analysis of the LES-WALE model near-wall behavior. It has been demonstrated that LES-WALE yields highly accurate results only when employed with a finely resolved mesh. However, the computational costs associated with such refinement may not be necessarily justified especially with respect to DNS.

Large gas bubbles separated by the liquid slugs are the main characteristic of the slug flow regime. We have analyzed the stagnant Taylor bubble in the vertical isothermal turbulent counter-current flow with high speed videos at 100 to 800 frames per second. A single Taylor bubble was captured in each experiment through dynamical balance of the bubble drag in the downward liquid flow. Bubbles of around two to six diameters length were observed in the pipe of 26mmdiameter with liquid Reynolds numbers in front of the bubble around 6000. Video frequencies at around 400 Hz were found sufficient to capture all temporal fluctuations of the bubble interface. Algorithms for two-phase interface recognition have been developed and applied on the images of the cap and the body of the Taylor bubbles. We have observed asymmetric Taylor bubbles of bullet-train shape with the thinnest liquid film on the belly of the bullet-train shape bubble. Even the long time averaging of up to 10 min did not produced axisymmetric time-averaged shape of the bubble in turbulent liquid flow. Azimuthal position of the bubble's belly is randomly determined during the injection of the bubble into the test section. In addition, dynamics of the tiny disturbance waves with tenth of mm amplitudes has been tracked along the interface of the Taylor bubbles. Cross-correlations of time-dependent interface fluctuations were measured at different spatial positions and used to determine propagation speeds of the traveling interface waves.

The stagnant Taylor bubble in vertical isothermal turbulent counter-current flow was analyzed using 2D shadowgraphy experiments and two distinct high-fidelity numerical simulations. One simulation employed the geometrical VOF interface tracking method within the OpenFOAM code, while the other utilized the explicit front tracking method of the TrioCFD code. Interface recognition algorithms were applied to the photographs and compared with the results of 3D simulations performed with LES and pseudo-DNS accuracy in OpenFOAM and TrioCFD, respectively. The measured Taylor bubbles exhibited an asymmetric bullet-train shape and a specific speed, which were compared with the predictions of both numerical approaches. Reproducing the experiment proved challenging for both otherwise well-established methods frequently used in interface tracking simulations of two-phase flows. Grid resolution and subgrid turbulent models, known for their success in single-phase turbulence, were less accurate near the water–air interface. Additional experimental parameters compared with simulations were related to the dynamics of tiny disturbance waves with amplitudes ranging from 10 to 100 µm along the interface of the Taylor bubbles. The speed and spectra of the surface disturbance waves were reproduced numerically with moderate success despite detailed grid refinement in the relevant region of the computational domain.

EDITORIAL article Front. Energy Res., 09 June 2023Sec. Nuclear Energy Volume 11 - 2023 | https://doi.org/10.3389/fenrg.2023.1233257

The relationship between the industry and academia is widely considered the key driver or obstacle in producing and using new knowledge. This is, at least in part, a consequence of different forms of knowledge being of different utility for the industry and academia (conceptual, e.g., why? and procedural, e.g., how?). In so-called “high-tech” industries, where the attraction and development of new talents are in significant part carried out through academic research-based higher education system, successful cooperation in the development of new knowledge automatically results in successful attraction and development of new talents. In well-established, strongly regulated, and very procedural industries, such as nuclear power, the pragmatic focus on procedural over conceptual knowledge may also result in the pathways of talent development outside of the research-based higher education system. Some definitions, features, and risks of different approaches to conceptual and procedural knowledge are outlined and discussed in the paper and an attempt is made to connect them with the notions of basic and applied research. It is suggested that suitable sequences and balance of know-why? and know-how? may lead to the best results in the attraction and development of new nuclear talents, while minimizing the risks of reduced ability to manage unexpected, reduced need for innovation and weakening the nuclear knowledge centers outside of the industry and regulators. This is supported by an analysis of 51 Ph.D. theses in nuclear engineering developed in Slovenia since 1993 to discern their contributions towards basic or applied research. The Pasteur’s quadrant, developed by (Stokes, 1997), was used as the underlying framework. Ph.D. graduates and supervisors were independently asked to evaluate the basic and applied contributions through three variables, describing the stages of the creative research process: input, processing, and output, respectively. The predominantly mixed (basic and applied) contributions of the analyzed Ph.D. theses indicate that academic nuclear engineering education is an enabler of successful careers in academia and industry.

Dynamics of the Taylor bubble interface in the vertical counter-current flow was analyzed with video recordings at 100 - 800 frames per second. Taylor bubbles in air-water mixture were studied on time intervals of up to several minutes in stagnant conditions, where buoyancy is dynamically balanced by the inertial bubble drag in the downward turbulent flow. Taylor bubbles of length from 4 to 10 cm were observed in a pipe of 26 mm diameter at Reynolds numbers based on liquid superficial velocity around 6000. Algorithms, developed for analysis of the interface from the video frames were dedicated to the analysis of the cap and the body of the Taylor bubble. The long time averaging of up to 10 min samples do not end up with axisymmetric time-averaged shape of the bubble, but with an asymmetric bullet-train shape, with the thinnest liquid film observed on the belly of the bullet-train shape bubble. The main result of this study is based on high relative sensitivity of our measurements, which was sufficient to track the dynamics of the tiny disturbance waves with a tenth of mm amplitudes traveling along the interface of the Taylor bubble. Cross-correlations of time-dependent interface fluctuations measured at different spatial positions allowed us to measure propagation speeds of the interface waves. When averaged over sufficiently long time intervals of around a minute, the time averaged propagation velocities are shown to be equal to the convective velocity of the interface. Moreover, waves propagating on both sides of the two-dimensional photographs show clear correlation; crest of the wave on one side of the bubble photograph corresponds to the trough of the wave on the other side.