Fluid flows through rod bundles are observed in many nuclear applications, such as in the core of Gen IV liquid metal fast breeder nuclear reactors (LMFBR). One of the main features of this configuration is the appearance of flow fluctuations in the rod gaps due to the velocity difference in the sub-channels between the rods. On one side, these pulsations are beneficial as they enhance the heat exchange between the rods and the fluid. On the other side, the fluid pulsations might induce vibrations of the flexible fuel rods, a mechanism generally referred to as Flow Induced Vibrations (FIV). Over time, this might result in mechanical fatigue of the rods and rod fretting, which eventually can compromise their structural integrity. Within the SESAME framework, a joint work between Delft University of Technology (TU Delft), Ghent University (UGent), and NRG has been carried out with the aim of performing experimental measurements of FIV in a 7-rods bundle and validate numerical simulations against the obtained experimental data. The experiments performed by TU Delft consisted of a gravity-driven flow through a 7-rods, hexagonal bundle with a pitch-to-diameter ratio P/D=1.11. A section of 200 mm of the central rod was made out of silicone, of which 100 mm were flexible. Flow measurements have been carried out with Laser Doppler Anemometry (LDA) whereas a high-speed camera has measured the vibrations induced on the silicone rod. The numerical simulations made use of the Unsteady Reynolds-averaged Navier-Stokes equations (URANS) approach for the turbulence modelling, and of strongly coupled algorithms for the solution of the fluid-structure interaction (FSI) problems. The measured frequency of the flow pulsations, as well as the mean rod displacement and vibration frequency, have been used to carry out the benchmark.

Axial flow in tube bundles with small pitch-to-diameter ratio, a geometry encountered in nuclear reactor cores and heat exchangers, often displays periodic fluctuations. A significant velocity discrepancy between the inter-cylinder gap and subchannel center originates from the difference in through-flow area, feeding an instability. As it is associated with velocity-shear, it is similar to the Kelvin-Helmholtz type and the term 'gap instability' is adopted. A vortex street arises and structural vibration of the cylinders might develop due to the fluctuating pressure. Numerical simulations of this phenomenon were performed. The computational domain was constructed to match the most important geometrical features of an experimental setup. The bundle consists of 7 steel tubes in triangular array, placed in a hexagonal conduit. A flexible segment made of silicone is embedded in the central tube, with both extremes clamped to the steel parts of the cylinder. In the experiment, data of the fluctuating velocity was gathered using laser Doppler anemometry measurements. As first step, a completely rigid structure was considered. Unsteady Reynolds-averaged Navier-Stokes (URANS) simulations were used to test if this particular geometry also triggers the gap vortex street, which was the case. The phenomenon clearly appears as oscillations of the velocity components. Subsequently, fluid-structure interaction (FSI) simulations, taking into account the flexible part, allowed to assess the effect of the fluctuating flow field on the structure. A comparison between one-way and two-way coupled simulations was made.

Being able to quantify mechanical vibrations is of key importance for the safety of nuclear power plants, as they are able to induce damage. In this work, numerical simulations are used to compute water flow and vibration in a densely packed bundle of 7 rods, mimicking an experimental setup. This flow configuration is chosen to resemble the coolant flow through a nuclear reactor core. Because of the wall proximity, a considerable velocity difference between the narrow gaps and the subchannels exists, with an inflection point in the velocity profile. This yields an unstable situation, and large vortices are continuously created through a mechanism similar to the Kelvin–Helmholtz instability. The vortex streets in between the rods are associated with a fluctuating pressure field, causing vibrations of the rods. The experimental setup contains 7 steel cylinders, encased in a hexagonal duct. The central rod contains a section where the steel is replaced by a water-filled silicone tube, clamped at both extremes to the steel rod, and the vibrations of this section are examined. The numerical approach consists of coupled fluid–structure interaction (FSI) simulations, with the flow being modelled using computational fluid dynamics (CFD) and the structure using computational solid mechanics (CSM). The available experimental data consist of Laser Doppler Anemometry (LDA) measurements and high-speed camera footage of the wall movement of the silicone rod. Equivalent data is collected from the numerical simulations. The simulations are repeated for different flow rates. The frequency spectrum of the coherent structures, and the frequency and amplitude of the wall movement are compared for each operating point, as well as their trend as a function of the flow rate. The dominant frequencies found in the simulation results were similar to the experimental results, although slightly higher. They also showed a linear trend, just like the experiments. A larger mismatch was present for the structural response, the frequencies found using the FSI model being more than twice as high.

Gap vortex streets characterise many industrial applications involving rod bundle flows, such as heat exchangers and nuclear reactors. These structures, known as gap vortex streets, may excite the structural components of the bundle to resonance, leading to fretting and fatigue. This work aims to measure these coherent structures and the resulting displacement and oscillation frequency of the neighbouring rod, to provide unique data for fluid-structure interaction studies and to develop a general correlation for estimating the coherent structure's wavelength. A water loop was built to host a hexagonal rod bundle. Fluorinated Ethylene Prophylene (FEP), a refractive index matching (RIM) material, was used to have undisturbed optical access in the area around the central rod. The flow was measured with Laser Doppler Anemometry (LDA) to detect coherent structures, while the vibrations were measured with a high speed camera. A new correlation for estimating the wavelength of the coherent structures is derived with dimensional analysis based on experimental evidence. The correlation is tested on different geometries: rectangular channels with single or half-rods, and two rod bundles, within the pitch-to-diameter ratio (P/D) range 1.02–1.2. Moreover fluctuations in the flow, given by the detected coherent structures, govern the structural response of the rod. The rod is excited to resonance if these fluctuations match twice the natural frequency of the rod.

The core of a Liquid Metal Fast Reactor (LMFR) has wires wrapped helicoidally around the fuel pins. This solution prevents damage of the cladding by fretting, and pushes the coolant flow through the gaps between the pins, thus, enhancing the heat exchange. The wires make the flow through the core very complex to understand and model. The goal of this study is, thus, twofold: it aims to obtain insight into the physics of the flow field near the wire and to provide high-fidelity data for benchmarking numerical simulations. Water at room temperature flowing inside a 7 rods, wire-wrapped hexagonal bundle is studied within a Reynolds number range of 4000−14000 using the Particle Image Velocimetry (PIV) technique. The measured quantities are the axial and lateral velocity components, and their root mean square. The measurement area is close to the wire around the central rod. Fluorinated ethylene propylene (FEP), which is a refractive index-matching (RIM) material, provides optical access to the measured region without disturbance of the light beam. The flow direction below the wire follows the wrapping path, as expected. However, if the flow is measured at the front of the wire, the streamlines bend in the opposite direction. The Euler equations applied to the streamlines show that this effect is caused by the pressure gradient across the wire. These findings deepens the physical understanding of the rod bundle flow in the presence of wire spacers, improving the LMFR designs. Moreover, the generated data will support validation of numerical codes for Gen-IV nuclear reactors.

The core of a Liquid Metal Fast Breeder Reactor (LMFBR) consists of cylindrical fuel rods that are wrapped by a helicoidally-wound wire spacer to enhance mixing and to prevent damage by fretting. It is known that the liquid metal close to the rod is forced to follow the wires, and that liquid metal further away from the rod crosses the wires (called: migratory flow). This work aims at gaining more insight into the physics behind migratory flow and to provide a model for its bending angle. To this purpose, the flow field in a 7-rods, wire-wrapped, hexagonal bundle with water is studied within the Reynolds number range of 4990–16330 by using Particle Image Velocimetry (PIV). Refraction of the light is minimized by using Fluorinated Ethylene Propylene (FEP), which is a refractive index-matching (RIM) material. These measurements confirm that liquid near the rod follows the helicoid path and bends cross-wise with respect to the wire further away from the rod. A theoretical model for the bending angle of the flow is derived from the Euler equations and shows that the bending is primarily caused by the pressure gradient field induced by the wire. The model shows a very good correspondence with the experimentally obtained PIV data. These findings improve our understanding of the physics at play in rod bundle flows with wrapped wires and can be of assistance in developing practical correlations for frictional pressure losses and heat transfer in such bundles.

Liquid metal reactors typically employ wire wraps as spacers between the fuel pins. In the past, design and safety calculations were largely one-dimensional and based on experimental data. Nowadays, with modern state-of-the-art computer power and tools, three-dimensional Computational Fluid Dynamics (CFD) simulations allow designers and safety specialists to obtain much more detailed information on the flow and heat transport in liquid metal cooled fuel assemblies, obviously in close collaboration with experimental campaigns. This may lead to new insights possibly decreasing the safety margins. This paper intends to provide an overview on the activities in the frame of design and safety support for wire-wrapped fuel assemblies. It all starts with validation. Therefore, validation efforts will be shown for fuel assemblies as they are designed on the drawing board for ‘cold’ conditions. Such analyses will profit from the quantification of uncertainties and determination of most influencing parameters. Nevertheless, in reality a fuel assembly will not be employed as designed in ‘cold’ conditions. Therefore, they will probably deform. This requires an assessment of the effect of deformations. Another aspect possibly occurring during operational conditions is vibrations. State-of-the-art coupled CFD and finite element method fluid structure interaction techniques have been developed and applied to a wire wrapped fuel pins, providing insights in the vibration behavior of such assemblies. Apart from that, vibration experiments have been performed at complete fuel assembly scale providing important insights to safety analysts and designers. However, design and safety analysts will not only have to cope with operational conditions, but also have to show the heat transport behavior under accident conditions. Assessments of the effect and formation of blockages are necessary. In all above cases, it should be clear that experiments and numerical simulations go hand-in-hand. Numerical simulations are used to design the experiment, the experiment is used to validate the simulations, and the simulations are used to interpret the experimental results.

Liquid metal fast reactors (LMFRs) are foreseen to play an important role in the future of nuclear energy, thanks to their increased fuel utilization and safety features profiting from the optimal heat transfer performance of the metallic coolants. Accurate thermal-hydraulic analysis of their fuel assemblies, typically employed with wire-wraps as spacers, is recognized as a crucial scientific and engineering contribution to support the deployment of such technology. This challenges the modeling and simulation community. To this aspect, various reference databases (both experimental and numerical) for different wire-wrapped fuel assembly configurations have been created recently and are being used for validation of engineering simulation approaches based on Reynolds Averaged Navier Stokes (RANS) modelling. These databases include: • 7-pin rod bundle: A detailed experiment with Particle Image Velocimetry (PIV) is performed. In order to allow accurate measurements of the flow topology, a matched-index-of-refraction technique was used employing water as working fluid. • 19-pin bundle: A series of experiments is performed covering a wide range of Reynolds and Peclet numbers as well as thermal powers. The experiments use liquid lead-bismuth eutectic as working fluid. The measurements include pressure drop and local temperatures. • 61-pin rod bundle: This large eddy simulation including conjugate heat transfer from the pin cladding to the coolant allows to bridge the gap from small bundles (less than 37 pins) to large bundles (more than 37 pins). In literature, a fundamental different behavior has been observed for small bundles compared to large bundles. • 127-pin bundle: Isothermal experiments using lead-bismuth eutectic characterizing pressure drop are performed on a full scale fuel assembly representative for the MYRRHA reactor. • Infinite pin bundle: This reference quasi-direct numerical simulation profits from periodicity in all directions. It provides a detailed view into the flow field and in addition reveals details of the heat transfer from the rod bundle into the flow.Reference databases aim to serve the nuclear scientific community to validate engineering simulation approaches. The paper will introduce these reference databases, and how they have been used to validate RANS based turbulence modelling approaches within a mainly European context.

The enhancement of heat transfer from fuel rods to coolant of a Liquid Metal Fast Reactor (LMFR) decreases the fuel temperature and, thus, improves the safety margin of the reactor. One of the mechanisms that increases heat transfer consists of large coherent structures that can occur across the gap between adjacent rods. This work investigates the flow between two curved surfaces, representing the gap between two adjacent fuel rods. The aim is to investigate the presence of the aforementioned structures and to provide, as partners in the EU SESAME project, an experimental benchmark for numerical validation to reproduce the thermal hydraulics of Gen-IV LMFRs. The work investigates also the applicability of Fluorinated Ethylene Propylene (FEP) as Refractive Index Matching (RIM) material for optical measurements. The experiments are conducted on two half-rods of 15 mm diameter opposing each other inside a Perspex box with Laser Doppler Anemometry (LDA). Different channel Reynolds numbers between Re = 600 and Re = 30,000 are considered for each P/D (pitch-to-diameter ratio). For high Re, the stream wise velocity root mean square v_rms between the two half rods is higher near the walls, similar to common channel flow. As Re decreases, however, an additional central peak in v_rms appears at the gap centre, away from the walls. The peak becomes clearer at lower P/D ratios and it also occurs at higher flow rates. Periodical behaviour of the span wise velocity across the gap is revealed by the frequency spectrum and the frequency varies with P/D and decreases with Re. The study of the stream wise velocity component reveals that the structures become longer with decreasing Re. As Re increases, these structures are carried along the flow closer to the gap centre, whereas at low flow rates they are spread over a wider region. This becomes even clearer with smaller gaps.

The enhancement of heat transfer from fuel rods to coolant of a Liquid Metal Fast Reactor (LMFR) decreases the fuel temperature and, thus, improves the safety of the reactor. One of the mechanisms that enhance heat transfer consists of large coherent structures that can occur across the gap between two adjacent rods. This work is a preliminary investigation of the flow between two curved surfaces, representing the gap between two fuel rods in a fuel assembly. The aim is to provide a reliable benchmark on the flow in such geometry, to investigate the presence of the aforementioned coherent structures and to investigate the applicability of Fluorinated Ethylene Propylene (FEP) as Refractive Index Matching (RIM) material for optical measurements. The experiments are conducted on two half-rods of 15 mm diameter opposing each other inside a Perspex box with Laser Doppler Anemometry (LDA) to measure the velocity components. Different channel Reynolds numbers between Re = 600 and Re = 30,000 are considered for each P/D (rod pitch-to-rod diameter ratio). For high Re, the stream wise velocity root mean square v_RMS between the two half rods is higher near the walls, similar to common channel flow. As Re decreases, however, an additional central peak in v_RMS appears at the gap centre, away from the walls. The peak becomes clearer at lower P/D ratios and it also occurs at higher flow rates. Obviously this central peak in the v_RMS cannot be attributed to turbulence only. Periodical behaviour of the span wise velocity across the gap is revealed by the frequency spectrum and the frequency varies with P/D and decreases with Re. TU Delft, as partner in the EU SESAME project, will provide an experimental benchmark to support the development of new numerical approaches to reproduce the thermal hydraulics of Gen-IV LMFRs.