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The understanding of flow regimes in helical coils is essential in the design and operation of modern heat exchangers and has been studied therefore intensively over the past decades. However, due to the subjective nature in defining these flow regimes, previous studies showed conflicting results. The scope of the present study is to perform automatic flow regime recognition for helical two-phase flows by a novel two-step machine learning approach without any prior knowledge. The machine learning algorithms are trained on X-ray radiography based high-resolution high-speed void fraction measurements at adiabatic conditions. The trained machine learning models successfully predict the number of flow regimes as well as the flow regimes themselves. The classifier models show excellent classification accuracy >98%. Comparison of the experimental results with available flow regime models revealed significant discrepancies >2 orders of magnitude in the superficial liquid velocity. The novel machine learning based flow regime recognition methodology presented herein shows not only a significant improvement in the classification accuracy compared to previous studies but offers also a novel data-driven way to define and investigate the very nature of the flow regimes themselves in a more consistent and objective way., International Journal of Multiphase Flow, 161, ISSN:0301-9322, ISSN:1879-3533

A transport model was developed and numerically solved for trace substances dissolved in the coolant flowing through a complete BWR fuel element. The integration of the trace substance transport equations is carried out on the same computational grid used by a two-phase three-field subchannel code, the output of which provides the necessary coolant mass flow data, including field exchange terms. The results consist of complete, field specific trace substance distributions within the domain of the simulation and with the same spatial resolution as used by the subchannel code. Downstream the onset of annular flow, it was observed that the trace substance concentrations in liquid films on heated rods are significantly higher than the concentrations in droplets and in films on unheated surfaces. A mechanistic model for disturbance waves was further introduced to assess the additional trace substance concentration increase in the time period of base film evaporation between two successive disturbance waves. The highest trace substance concentrations were found next to part-length rods in the corner of the considered fuel assembly just upstream of the two uppermost spacer grids, which approximately corresponds to the location of trace substance deposition observed during fuel inspections at a Swiss nuclear power plant., Nuclear Engineering and Design, 407, ISSN:0029-5493, ISSN:1872-759X

With the development of the next generation of nuclear reactor safety system codes fast underway, increased importance has been placed on enhancing physical closure correlations and amassing representative benchmark-quality experimental data for validation purposes. Wire-mesh sensors, a reputable experimental measurement technique with sufficient spatial and temporal resolution to serve such goals, and related data reconstruction algorithms have been the subject of renewed interest as researchers attempt to characterize their measurement uncertainty. To assist in such investigations, the present work establishes a comprehensive numerical framework with which to quantify the electric potential field around wire-mesh sensors. Using the finite-volume foundations of OpenFOAM, a numerical solution algorithm is developed to predict the transmitted electric current between transmitter and receiver electrodes for both homogeneous and heterogeneous electrical conductivity fields. A detailed verification against seminal numerical calculations and robust validation procedure is included to ensure the accuracy of the proposed methodology. Parametric studies of spherical bubble diameter, lateral crossing position, and spheroidal shape influence are conducted to provide preliminary insights into wire-mesh sensor operation and the suitability of various calibration approaches. Observed trends in the transmitted currents reveal overshoots relative to calibration conditions, which are fundamentally linked to the maldistributed electric potential field in heterogeneous bubbly flows. The present investigation offers a vital first step towards a comprehensive multi-physics model of multiphase flow around a wire-mesh sensor.

A Filtered Containment Venting System (FCVS) is a safety system incorporated in Nuclear Power Plants (NPPs) to filter the steam and noncondensable gases released from the containment during a severe accident scenario to avoid overpressure in the containment structures while isolating most radioactive products inside. This paper presents the experimental and numerical simulation results from confirmatory tests performed in the VEFITA (Venting Filter Assessment) test facility at the Paul Scherrer Institut (PSI) in Switzerland. This facility is designed to evaluate the performance of an FCVS by assessing key phenomena and parameters such as internal recirculation patterns, changes in inventory, level swell, and the Decontamination Factor (DF) of aerosol and iodine species in simulated venting processes. The current study focuses on the level swell (gas holdup) phenomenon. The level swell during gas injection is particularly of interest since it is associated with the heights of gases and liquids in the venting vessel and thus affects the design and operation of an FCVS. The experiment is performed by injecting the noncondensable gas and steam at various mass flow rates through a sparger nozzle to the vessel, which is partially filled with washing fluid as a bubble column. The level swell and other physical variables are recorded and analyzed. System codes (RELAP5 and TRACE) input decks are developed for this facility, and experimental transients are simulated. The code simulation results, as well as previous physical gas holdup models based on homogeneous-heterogeneous regime transition, are compared with current experimental data to study the level swell in this FCVS.

A new technique development for high spatial and temporal resolution film-thickness measurement in oil-water flow is presented in this work. A capacitance measurement system is proposed to measure thin water films or local water fractions near to the pipe wall. The measurement system is based in a planar sensor which is flexible and can be placed close to and following the curvature of the inner wall of the tube. Experimental results of the capacitive technique showed that the system is able to measure film thickness between 400 μm and 2200 μm. Dispersed and core-annular flows were studied in a vertical 12-m-height glass pipe with 50.8 mm of internal diameter, using mineral oil (860 kg/m3 of density and 220 mPa s of viscosity) and tap water. Local time-domain signals and axial and circumferential distributions of water film thickness near to the pipe wall and 3-D images of water film are presented. The measured film thickness, obtained by the proposed technique, was compared with that obtained through a camera-based imaging technique and the agreement was good, with a deviation less than 20%. The new technique allows the observation and characterization of average interfacial topologies in separated flows, and high frequency phenomena related to dispersed flows that occur in liquid-liquid flow near the pipe wall.

Compact Deuterium-Deuterium fast neutron generators are generally limited to a low neutron yield due to outgassing of implanted deuterium in case of ion beam target overheating. This is even more pronounced for a generator with a small emitting spot tailored to transmission-based imaging applications, resulting in long exposure times. In this work a novel water-cooled rotating copper target rod with a 5 µm thin titanium coating as a deuteron beam target was developed. Using extensive computational fluid dynamics and heat transfer analysis, an optimized target configuration was chosen. The details of the target rod design, construction and loading with deuterium are outlined. The first results after loading showed a peak total neutron output of 4.8 × 107 n/s for −107 kV target high voltage and 0.9 mA average target current at 220 rpm target rotational velocity. The comparison between actual and theoretical ideal neutron yield suggested that no loss of deuterium density in the target due to outgassing occurred.

This article presents a feasibility study of using a compact deuterium-deuterium (D-D) fast neutron generator for element-sensitive fast neutron tomography. The energy range of such neutron generators is quasi-monoenergetic from about 2.2–2.8 MeV, depending on the angle of emission. Unlike X-rays and gamma photons, fast neutron cross-sections have unique energy sensitivity in this range depending on the nuclide, making differentiation of materials in principle possible by performing tomographic attenuation measurements at multiple energies. In this context, detailed neutron spectrum models were created and attenuation versus emission angle data were simulated for several uniform test samples. An automated mechanical apparatus and plastic scintillator fast neutron detectors were used to interrogate these samples over the full range of emission angles around a custom neutron generator located at the Paul Scherrer Institute. The experimental attenuation data were compared with simulations. Simulations and experiments were found to be generally in good agreement, demonstrating the fundamental feasibility of the approach.

Helical coil heat exchangers are known for their superior heat transfer, compactness and robustness against thermal expansion compared to systems with straight pipes. Due to all these benefits, they became quite popular in the last few decades in various industries such as pharmaceutical, chemical or nuclear. Generally, they are used in a single-phase environment or with evaporation taking place on the shell side. However, in most of the steam generator designs adopted in advanced nuclear reactors, the two-phase mixture flows upward within the vertically oriented helical coils. In addition, physical two-phase phenomena taking place inside helical tubes differ significantly from those in straight pipes due to inherent centrifugal and torsion effects. To improve the computational models used to predict the thermal hydraulic performance of such steam generators, experimental characterization and identification of different two-phase flow patterns in helical coils are essential. In the present paper, high-resolution measurements of the void fraction distribution for an adiabatic upward air–water two-phase flow were conducted in a transparent helically coiled acrylic pipe by means of high-speed X-ray radiography at ambient conditions. Various void fraction based features such as probability density function estimates, profiles over the height of the pipe cross-section and temporal evolutions were used to identify and characterize flow regimes. A total of 13 measurements are presented in this paper, classified into six flow regimes, namely: bubbly, plug, wavy, slug, slug-annular and annular flow. The obtained flow regimes are in accordance with flow regime characterizations and transition criteria already reported in literature., Nuclear Engineering and Design, 373, ISSN:0029-5493, ISSN:1872-759X

Neutron cross-sections have an energy dependence which is uniquely varying according to isotopic composition. By measuring the local neutron beam attenuation of an object as a function of energy, this can be related to its elemental composition. Therefore material decomposition measurements using a deuterium-deuterium (D-D) compact fast neutron generator in an energy-selective fashion have been carried out. Previous work described determining setup-specific effective reference cross-section data for homogeneous samples. This work presents first tests of this non-destructive analysis principle in a 2D computed tomography (CT) setup using heterogeneous samples, based on the aforementioned reference cross-sections. Measurement data were collected at five angles around the neutron generator corresponding to five effective energies. At each angle a CT reconstruction was performed to obtain energy-specific CT images. These CT images were use to calculate an elemental decomposition of the samples of interest. Good agreement between theory and experiment was found for most of the samples, while the more challenging cases highlighted limitations of the technique. These first tomographic measurements are discussed along with an outlook for further developments of the technique. © 2020 The Author(s), Radiation Physics and Chemistry, 181, ISSN:0969-806X

The droplet size distributions in an industrial self-priming Venturi scrubber with liquid film injection have been investigated for varying gas flow rates and different submergence levels. For this purpose, imaging method analysis, commonly applied in spray measurements, was adopted and validated by means of synthetic data. The droplet size distributions characterized by probability density functions show a unimodal shape and can be satisfactorily represented by Nukiyama-Tanasawa distribution functions. For an increase in the gas flow rate, the cumulative distribution function shifts consistently to smaller equivalent droplet diameters. On the other hand, no statistically significant difference in the droplet size distribution was found for a change in the submergence level. The present results are compared with two droplet size correlations based on spray injection and commonly applied to Venturi scrubbers. Both correlations significantly overpredict the experimental results by a wide margin. In contrast, droplet size correlations developed for annular flows in straight tubes show a superior consensus in magnitude with the experimental data. The pronounced decreasing trend in droplet size with increasing gas superficial velocity, however, is weaker in our results, possibly due to the geometry and the self-priming characteristics of the Venturi scrubber., International Journal of Multiphase Flow, 142, ISSN:0301-9322, ISSN:1879-3533

The international EU-SGTR and ARTIST projects investigated the transport of fission products in the form of aerosols during SGTR severe accidents. The major finding of the two projects was that there was significant retention of aerosols in the steam generator secondary side, and that the retention was increased by more than an order of magnitude if the secondary side of the steam generator was flooded with water. Furthermore, the experiments with the flooded secondary side showed that the aerosol particle retention was significantly increased due to the presence of submerged structures, i.e., the tube bundle, as compared to an empty pool. The increased aerosol retention was attributed to the interactions of the high velocity gas jet discharged from the tube break with the dense bundle of the steam generator tubes. Under these conditions, the two-phase flow is very complex due to the high gas velocities and complicated geometry of the steam generator secondary side. To determine the effect of the tube bundle on aerosol retention, hydrodynamic characteristics of an empty pool and a flooded steam generator secondary side were measured in a facility equipped with wire-mesh sensors. The facility was equipped with either a tube bundle consisting of 221 steam generator tubes, or with a single tube in the center of the facility. The flow development could be followed by making the measurements at different distances between the gas injection and the measurement point, and using different flow rates. The facility was operated at close to ambient conditions. Void fraction, bubble size distributions, gas phase velocity as well as interfacial area concentration were determined based on the wire-mesh sensor data. In addition, the penetration depth of the initial large gas bubble into the channel was studied for the closest break-sensor distances. The investigations show distinct differences between the flow characteristics in the single tube geometry and in the tube bundle. As compared to the single tube, the flow was more confined in the tube bundle due to the interactions of the flow with the tubes. The interfacial area between the liquid and gas phase is larger with the tube bundle than with the single tube and also the bubble size distributions show distinct differences between the two geometries., Nuclear Engineering and Design, 366, ISSN:0029-5493, ISSN:1872-759X

Void fraction has always been an important parameter in the study of multiphase flows and its measurement has proven difficult over the years. This paper is a state of the art review of the application of conductivity based wire-mesh sensors (WMS) for the measurement of void fraction, bubble size, and gas fraction velocity in multiphase flows and their associated uncertainties. At this point in time there is no golden standard for void fraction measurement, so a large bulk of this work is on the uncertainty of the WMSs relative to other void fraction measurement methods, namely radiative methods. It is shown using the available data that the WMS have a void fraction measurement uncertainty of ±10.5% over a variety of flow regimes relative to other measurement methods. However, the accuracy of the instrument is largely based on its applicability to a particular flow. For example, the WMS is an excellent choice when entrapment in the sensor due to surface tension is minimized resulting in best results at higher flow rates compared to radiative methods. An assessment into the uncertainty of velocity and bubble size measurements is also performed: analyzing the current algorithms available and studies on these measurements in comparison with high speed cameras and ultrafast X-ray tomography. The current functioning form of the wire-mesh sensors were developed by Prasser in 1998 as a tomographic technique for the measurement of void fraction using a conductivity approach, as performed by earlier researchers. Later developments with the senors resulted in various techniques that allow for the measurement of velocity and interfacial area concentration.

The installation of passive autocatalytic recombiners (PARs) in the containment of operating nuclear power plants (NPPs) is increasingly based on three-dimensional studies of severe accidents that accurately predict the hydrogen pathways and local accumulation regions in containment and examine the mitigation effects of the PARs on the hydrogen risk. The GOTHIC (Generation Of Thermal-Hydraulic Information for Containments) code is applied in this paper to study the effectiveness of the PARs installed in the Gosgen NPP in Switzerland. A fast release of a mixture of hydrogen and steam from the hot leg during a total station blackout is chosen as the limiting scenario. The PAR modeling approach is qualified simulating two experiments performed in the frame of the OECD/NEA (Organisation for Economic Co-operation and Development/Nuclear Energy Agency) THAI (Thermal-hydraulics, Hydrogen, Aerosols and Iodine) project.The results of the plant analyses show that the recombiners cannot prevent the formation...

Mid-wave infrared thermography was investigated to measure the surface temperature of water films flowing in condensing and evaporating environments. The measurements were performed through a sapphire window and a confined environment with the presence of water vapor. An in situ real time calibration was developed to account for the light attenuation and emitted radiation caused by the absorbance of steam and sapphire on the optical path of detection. Thermal mapping reveals spatiotemporal fluctuations measured with a sensitivity of 47 mK for evaporation and 82 mK for condensation. The thermal images were synchronized with the film thickness mapping obtained from the imaging absorption of near infrared light in the film. The images from both techniques were dewarped and cropped to obtain identical measurement fields. Superposed temperature and thickness mappings were recorded with an image size of 246 × 180 pixels, a projected pixel size of 0.63 mm and frame rates of 125 and 250 fps, respectively. Measurements were performed in the LINX facility at the Paul Scherrer Institute (PSI). Liquid films were created on a temperature controlled wall placed vertically inside a pressure vessel with controls on thermal-hydraulic conditions. Films were produced either by condensation or via a film injector. The results obtained for selected tests are presented which involve condensation and evaporation on a wavy films with steady state conditions. Measurements with the surface partially wetted by an evaporating rivulet as well as drop wise condensation are reported. Confirmation could be brought that the perturbations caused by the presence of waves on the film impact locally the surface temperature of the film. The convolution and synchronization of mid-wave and near infrared imaging represent a strong analytical tool for the study of two phase flows involving liquid films subject to heat and mass transfers.

A novel technique developed to measure liquid film thickness up to 2 mm was tested under thermodynamic conditions encountered in containment during severe accident scenario. The tests were performed in the thermal-hydraulic facility LINX located at PSI (Villigen, Switzerland), a 10 m 3 pressure vessel with precise control of the boundary conditions. In the experiments, liquid films were created on a temperature controlled wall placed in the center of the vessel. Selected tests involving condensation and evaporation on film with steady state conditions are presented. In each test, sequences of thickness mappings were recorded with an image size of 246 × 180 pixels, a projected pixel size of 0.63 mm and a frame rate of 250 fps. Uncertainty on the film thickness was estimated to 35 μm absolute over the entire measurement range based upon current calibration. Large and small structure can be observed spatially but also temporally through extracted local signal. Average and fluctuating part of the film thickness are presented and discussed for four tests where the controlled wall temperature and bulk composition were varied. Strong time-correlation in the signal was observed for rivulets whose formation is enhanced by an increase of the wall temperature.

High-quality boiling crisis, i.e. dry out of the liquid film is an important operational safety issue for boiling water reactors. The phenomena involved in dry out are complicated and difficult to capture in full details in experiments. We follow a step-wise approach to the problem and utilize innovative, high-resolution measurement techniques. First extensive testing have been carried out on partial nuclear fuel bundle models containing typically a couple of neighboring subchannels. These first series featured adiabatic, air-water annular flows with and without the presence of prototypical functional spacers. Cold neutron imaging has been used to map the distribution of the liquid film thickness on the virtual fuel rods and on the spacer structures. As a next step a dedicated loop using a special working fluid has been designed enabling examining convective boiling two-phase flow in similar geometries as the adiabatic tests. Cold neutron and X-ray imaging can be used for the heated set-up to try to capture actual dry out phenomena. An overview of the result of the adiabatic test series and the first result with the heated experiments are given below.

The bubble pairing scheme was devised to quantify three-dimensional velocity of each bubble. We used two sets of Wire-Mesh Sensors to identify locations of each bubble according to bubble identification algorithm, which was developed by HZDR. The devised scheme was applied to the vertical upward air-water flow at 0.64 m/s for both air and water superficial velocities in a large diameter pipe (i.d. 224 mm). The bubble pairing scheme visualized the developing process of two-phase flow: large bubbles coalesced with each other to move toward the center, while the rest of bubbles broke up into smaller bubbles and decelerated.

State-of-the-art turbulence models are expected to accurately predict flow behavior in a wide range of geometries and flow conditions for a wide range of fluid properties. These models rely on accurate time resolved measurements of scalar and vector flow variables. One such measurement technique for obtaining a high density of flow field variables with high time resolution is in the form of electrode-mesh sensors (WMS) which measure the local instantaneous electrical conductance of a flow at a large number of positions, typically in the cross-section of a conduit at frequencies up to 10 kHz. Extensive studies on single and multiphase flows have been carried out with mesh sensor technology in the past 15 years, often focused on flows directly related to nuclear power generation. However, essentially all of these experiments have taken place at low temperatures ( A new mesh sensor construction has been designed at the ETH Zurich for operation in high temperature and pressure pipelines by implementing novel materials, assemblies and a new sealing methodology. The mesh sensor package design is scalable so as to be compatible with standard flanged pipelines of size estimated to be DN10. The new design solves the problems discussed related to prior state of the art while at the same time allowing the sensor to operate at higher temperatures and to be manufactured and assembled more easily and more cheaply. The pressure barrier is no longer guaranteed by an epoxy, as in prior state of the art developed by other researchers, but rather by a standard graphite gasket between the sensor housing flanges, the same type that would normally be sealing a bolted flange joint, and commercially available threaded sealing glands. While still maintaining space for the standard number of bolts for a given flange size, such sealing glands could conceivably allow for a very high number of electrode signals to be extracted. The ability to carry a large number of conductors through the pressure barrier, in addition to the housing flange design, enables the installation of multiple mesh sensors, three layer sensors, or other instrumentation such as thermocouples, enabling the reconstruction of additional flow properties from measured data such as velocity and interfacial area concentration. A 16-transmitter, 16-receiver prototype sensor has been constructed for a DN80 pipeline; its individual components and some assemblies thereof have been tested in an autoclave at temperatures up to 285 °C in a liquid water-vapor environment with boiling at saturation pressure. Furthermore, a test section construction has enabled the sensitivity of the sensor to temperature changes via the variable electrical conductivity of deionized water as a function of temperature to be investigated.

Wire-mesh sensors are widely used to characterize gas-liquid two-phase flows and single-phase mixing processes. The geometry of the electrode grids and the way of the signal readout generates a three-dimensional electrical field in the vicinity of the electrode wires. Resulting electrical currents at the receiver electrodes, representing the primary measuring information, are calculated by a three-dimensional potential field simulation within the sensitive volume formed by the electrode wires, whereas bubbles are taken into account as simplified, spherical or elliptical objects placed at different locations in the calculation domain. The response of the sensor to the passage of such synthetic bubbles is studied. A significant deviation from the linear dependency between the received current and the local instantaneous gas fraction is found. Overshoots of the current above the reference value obtained by calibration in plain liquid occur. Furthermore, the response of the sensor depends on the axial distance between the transmitter and the receiver electrode grids. Swarms of bubbles of small size passing through the grids of the wire-mesh sensor lead to an average decrease of the current which can be described by the average conductivity of an emulsion according to Maxwell.

This work reports on the development and application of a new approach, corrective interface tracking, to simulate finite size bubbles. Finite-size bubbles, which are by definition bigger than the grid cell size but not well resolved, are not capable of being modeled with any standard two-phase flow approaches, such as interface tracking (IT), Euler-Euler (EE), or Lagrangian particle tracking (LPT). This poses a problem when simulating bubbly flows with different bubble sizes on the same computational grid. The Finite-size Lagrangian particle tracking (FSL) approach (Badreddine et al., 2015), aimed at simulating finite-size bubbly flows by inheriting features of IT and LPT approaches, simulated a single bubble with good accuracy. However, deficiencies with the FSL approach led to a newly developed approach based on interface tracking with the addition of a correcting force. The correcting force, derived from modeling the hydrodynamic forces on a bubble, attempts to correct for errors introduced when a coarse grid is used and the flow and pressure fields around the bubble are under-resolved. Therefore, as a finer grid is used the correcting force decreases. The corrective interface tracking approach is validated against a single bubble rising in stagnant and linear flow, and then results are compared to FSL and to finely resolved IT simulations.

In order to develop an iodine scrubbing model for nuclear severe accidents, experimental data on filtration efficiency of a wet scrubber for gaseous molecular iodine are collected to improve understanding of the basic mechanisms involved in iodine chemical scrubbing. A bubble column reactor 1.5 m high and 0.2 m in diameter equipped with an injection nozzle and a bubble breaker is used to investigate retention under relevant flow regimes. Scrubber is loaded with a scrubbing solution containing sodium hydroxide and sodium thiosulphate, no irradiation is provided. Retention efficiency is found to be strongly dependent on flow regime and on residence time as models predict. It was found that jet-like injection has a strong effect on filtration efficiency introducing a significant dependence on iodine initial concentration in gas phase believed to be due to nozzle hydrodynamics.

High cycle thermal fatigue (HCTF) damage in the vicinity of T-junction piping, where mixing of coolant streams at significant temperature differences occur, is considered to be an important problem in the context of operational safety in nuclear power plants (NPPs). The present study deals with the experimental and numerical investigations of T-junction flow mixing at realistic temperature differences encountered in NPPs. Experiments were carried out at the fluid-structure interaction (FSI) T-junction test facility at the University of Stuttgart. Near-wall thermocouples were used in the mixing zone to record temperature information inside the flow and the structure. A conductivity based wire mesh sensor (WMS) instrumentation designed at the Laboratory of Nuclear Energy Systems at the ETH Zurich for high-pressure high-temperature applications was also used during the measurements to characterize the cross-sectional thermal field in the mixing zone. Aside from experiments, numerical studies were carried out using the large-eddy simulation (LES) turbulence model to gain further insights into the flow mixing behavior and its predictions were validated with measurement data. With a constant mass flow rate ratio (main/branch) of 4:1, thermal mixing tests were carried out at temperature differences (Δ T ) between the mixing fluids of 65 °C (Case 1) and 143 °C (Case 2), respectively. Two distinct flow trends were identified in the mixing region: (i) an unstable stratified flow being subjected to extreme oscillations in case 1 and (ii) a stable stratified flow being subjected to significant buoyancy effects in case 2. Near-wall thermal fluctuations (a factor in thermal fatigue analysis) with highest amplitudes were recorded by the thermocouples located in the vicinity of the stratification layer in both the cases. Frequency analyses of thermal fluctuations using the power spectral density (PSD) method indicates no dominant frequency (spectral peak) in the frequency range identified to be of relevance for HCTF (0.1–10 Hz). Instead, the energy of thermal fluctuations were mainly contained in the frequency range of 0.1–2 Hz. LES predictions of parameters, viz., mean temperature, thermal fluctuations and its frequency distribution showed good agreement with measurement data.

We assess the quantitative capabilities of three condensation models. These models are: (1) Numerical iteration technique; (2) heat flux balance equation; (3) phase field. The numerical iteration technique introduces a mass and energy transfer at the interface, if the temperature of the corresponding cell differs from the saturation value. The second approach solves the heat flux balance equation at the interface, hence, the resolution of the thermal boundary layer around the liquid-vapor interface is necessary to obtain an accurate value for the condensation rate. The third technique is based on a recently derived phase field model for boiling and condensation phenomena. The models were implemented in FLUENT and the interface was tracked explicitly with the volume of fluid (VOF) method. The models were tested on the LAOKOON facility, which measured direct contact condensation in a horizontal duct. The results showed that the phase field model fit best the experimental results.

In the pebble-bed high-temperature reactor under construction in China, called the HTR-PM, the spherical fuel elements continuously flow downward in the cylindrical core. The burnup of each pebble is checked at the core outlet and, according to the achieved burnup level, the pebble might be disposed or reinserted into the upper section of the core. Upon reinsertion, each pebble is radially distributed in a random manner and, according to its downward path, faces different burnup conditions. Hence, the number of passes necessary to achieve the average discharge burnup of 90 MWd/kgU may vary. Discrete element method (DEM) simulations have been carried out to achieve a clear understanding of the movement of the 420000 fuel pebbles in the HTR-PM core. At the same time, neutronics properties have been investigated for a single pebble and for the full core with the Serpent 2 Monte Carlo code. As a result, one-group microscopic cross sections (XS) have been parametrized at the core level. The pebble movement has been loosely coupled with the depletion of a single pebble in a dedicated burnup script called moving pebble burnup (MPB), developed in matlab. 3000 single pebble burnup histories were simulated to obtain sufficient statistics and an insight into the HTR-PM burnup process. The decrease of the average burnup gained per single pass implies that a miss-handling of recirculated fuel elements is unlikely to lead to an excess of the maximum allowed burnup of 100 MWd/kgU. The core demonstrates a self-compensation effect of burnup, meaning that it always compensates burnup under- or over-runs in the successive passes. In addition, gamma detection of 137Cs has been studied as a practical method for monitoring the burnup of the discharged pebbles, turning out to be an applicable measurement technique. Finally, it is possible to conclude that the fuel cycle of the HTR-PM, as it has been laid out, is well designed and feasible.

The electrical liquid film thickness (LFT) sensor developed at ETH Zurich is applied to study wavy annular flows. The LFT sensor consists of a matrix of electrodes put flush to the surface of the wall at which the liquid film of interest is present. The conductance between transmitter and receiver electrodes is sampled and converted into a film thickness using a calibration function obtained for a flat liquid film without waves. Due to limited lateral resolution and non-linear sensor response, waves are characterized with measuring errors, which depend on the wave height, length and angle of attack. The paper presents the result of potential field simulations of waves passing over the sensor surface. The wave parameters obtained from the simulated sensor signals are compared to the input. The results are used to quantify the uncertainty of dynamic film thickness measurements. The obtained detailed information on the sensor response allows a better interpretation of experimental results., Nuclear Engineering and Design, 380, ISSN:0029-5493, ISSN:1872-759X

For the validation of advanced two-fluid CFD models at boiling water reactor (BWR) conditions, it is necessary to have good experimental data challenging for the code by reflecting relevant flow phenomena. The present experimental study aims to provide detailed film thickness distributions in a BWR-like subchannel geometry, including spacer grids and heat transfer. This is achieved using chloroform as a working fluid, indirectly heated by hot water channels simulating BWR fuel rods. X-ray equipment was used to produce tomographic 3D-data of time-averaged attenuation coefficients which essentially represent the liquid holdup. The conversion to film thickness was done by integrating the attenuation coefficient along lines perpendicular to the rod wall and using an effective chloroform attenuation coefficient determined by numerical simulation of the experimental setup, including X-ray sensor behavior. The simulation method was validated by using two different sensors types. Measurements were performed over the whole cross-section of the channel and an axial length of 11 cm. The experiments cover a range of liquid flow rates in the pre- and post-dryout regime., Nuclear Engineering and Design, 380, ISSN:0029-5493, ISSN:1872-759X

A compact D-D fast neutron generator system was developed with an emphasis on imaging applications. It is equipped with an electron cyclotron resonance microwave ion source, an electric field method electron suppression electrode, and a rotating beam target. The design, mechanical assembling, and initial performance of the system is presented in this work. Electrostatics and charged particle tracing simulations were employed to dimension components in the vacuum system and estimate the size of the ion beam spot on the target surface. Stable operation of the neutron generator was possible with accelerating potentials of up to −120 kV and an estimated averaged ion beam current of around 0.5 mA. At these conditions, the neutron yield was estimated to 6 ⋅ 107 s−1 based on a combination of detailed Monte Carlo MCNP6 models and the ambient dose rate reading of an LB6411 neutron probe. The neutron emitting spot size was compared to the ion beam spot size and experimentally determined with an attenuating edge technique to be between 1.7 and 2.6 mm. With these neutron yield and emitting spot size values, the system compares favorably with commercially available neutron generators, and this context is also presented.

As a means of further elucidating turbulence- and stratification-driven thermal fatigue in the vicinity of T-junctions in nuclear power plants, a series of experiments have been conducted at the high temperature high pressure fluid–structure interaction T-junction facility of the University of Stuttgart with novel fluid measurement instrumentation. T-junction mixing with large fluid temperature gradients results in complex flow behavior, the result of density driven effects. Deionized water mixing at temperature differences of up to 232 K at 7 MPa pressure have been investigated in a T-junction with main pipe diameter 71.8 mm and branch line diameter 38.9 mm. The experiments have been performed with fixed flow rates of 0.4 kg/s in the main pipe and 0.1 kg/s in the branch line. A novel electrode-mesh sensor compatible with the DN80 PN100 pipeline upstream and downstream of the T-junction has been utilized as a temperature sensor providing a high density information in the pipe cross-section in both space and time. Additionally, in-flow and in-wall thermocouples quantify the damping of thermal fluctuations by the wall material. The results indicate that large inflow temperature differences lead to strong turbulence damping, and ultimately stable stratification extending both downstream and upstream of the T-junction resulting in large local thermal gradients.

High conversion light water reactors (HCLWR) having triangular, tight-lattice fuels bundles could enable improved fuel utilization compared to present day LWRs. However, the efficient cooling of a tight lattice bundle has to be still proven. Major concern is the avoidance of high-quality boiling crisis (film dry-out) by the use of efficient functional spacers. For this reason, we have carried out experiments on adiabatic, air-water annular two-phase flows in a tight-lattice, triangular fuel bundle model using generic spacers. A high-spatial-resolution, non-intrusive measurement technology, cold neutron tomography, has been utilized to resolve the distribution of the liquid film thickness on the virtual fuel pin surfaces. Unsteady CFD simulations have also been performed to replicate and compare with the experiments using the commercial code STAR-CCM+. Large eddies have been resolved on the grid level to capture the dominant unsteady flow features expected to drive the liquid film thickness distribution downstream of a spacer while the subgrid scales have been modeled using the Wall Adapting Local Eddy (WALE) subgrid model. A Volume of Fluid (VOF) method, which directly tracks the interface and does away with closure relationship models for interfacial exchange terms, has also been employed. The present paper shows first comparison of the measurement with the simulation results.

In order to allow development and validation of CFD models for hydrogen mixing and transport in the containment, a comprehensive experimental test campaign was performed at the small scale MiniPanda facility at ETH Zurich. The considered test series aimed at studying the turbulent erosion of a stratified light gas by means of a vertical air jet with different momenta. Due to its new and innovative measurements, e.g., with temperature wire mesh sensors, the global mixing and the local interaction of jet and stratification are characterized in a high resolution in space and time. Both are essential for a detailed model assessment, to identify possible error sources and rate their effect on the global scenario evolution. Consequently, the tests are well suited for CFD model development and validation and complement the data basis gained before, e.g., in the frame of the joint OECD/NEA-SETH-2 Project (2007–2010) ( OECD/NEA, 2012 ). Based on a description of the MiniPanda facility and the ‘layer erosion’ test series, the application of a U-RANS CFD approach, capable to be applied also for large scale application, is discussed. Numerical model uncertainties are minimized according to the best practice guidelines before a systematic comparison against the experimental data is performed and the capability of the model to predict the turbulent mixing at the interface and the inter-compartment mass transfer is successfully validated.

Thermal hydraulic experiments at system pressures close to the reference reactor are difficult and costly. The pressure chamber technology described in the present paper was developed to improve the accessibility of the test fluid for an application of advanced instrumentation. The main idea consists in accommodating the actual test rig inside a pressure tank and operate it in pressure equilibrium with a filling gas in the chamber. In this way, the walls of the test equipment do not have to be designed as a pressure-bearing boundary. An important feature is a passive approach to maintain the needed very good pressure equilibrium between the fluid in the test setup and the filling gas of the chamber. It is in particular needed in experiments requiring high mass flows of steam supply. Auxiliary systems for pressurizing and cooling the chamber are described. Furthermore, several useful solutions of particular problems are presented, such as the accommodation of delicate devices, like high-speed cameras and infrared cameras, in the pressure chamber, a blurring-free imaging through large observation windows, the sealing of the latter, auxiliary equipment for an efficient handling of the test equipment and the design of penetrations for multiple electric signals through the pressure boundary. Finally, an overview of the experimental programs conducted by now is given, whereby the emphasis is put on examples of measuring results illustrating the potentials of the pressure chamber technology rather than on the scientific output of the given experimental programs.

Despite the beneficial effects of part-length rods (PLRs) in BWR fuel, they also introduce inhomogeneities to the coolant flow. These effects are hard to simulate and might affect cooling efficiency, dryout margin and CRUD deposition. So far, no fundamental experimental research was conducted to explore the details of their influence to the liquid film behavior in the dominating annular flow regime. To investigate the fluid dynamical effects of PLRs, we analyzed three different types of subchannel geometries. We varied the positions of PLRs in a pair of adjacent subchannels as well as the liquid and gas flow rates, staying within the annular flow regime. The adiabatic air-water test section was equipped with two types of sensors measuring the liquid film thickness distribution on half a fuel rod and on a fuel element wall. The measurement frequency was 10 kHz and the lateral resolution in the two-dimensional measuring domain was 2 mm. This corresponds to 64 × 16 and 16 × 32 measuring points, for rod and wall sensor respectively. Closely downstream of a PLR, we found a recirculation zone with less shear driving the liquid film. This is reflected by an increase of the average film thickness and a decrease of the wave velocity. A cross-flow of gas and liquid towards the open space downstream of the PLR was detected. The side of neighboring fuel rods facing this open space sees an increase of liquid flow. On the PLR-averted side, the coolant flow decreases, reducing the dryout margin. Further downstream, more gas tends to traverse into the open space left behind the PLR, which again increases the driving shear stress on the liquid film. The film becomes thinner and faster. The precise channel geometry featuring PLRs have a strong and complex influence on the local average liquid film thickness and the wave velocities. Their qualitative distribution at different flow conditions however, is similar.

This work aims to describe experimental efforts to further development of an energy-selective based interrogation technique using a deuterium-deuterium (D-D) fast neutron generator. In this approach, elemental decomposition of a sample can be performed. Limitations of the technique include uncertainties related to the neutron spectrum and the actual elemental cross-section data. To circumvent these challenges, a calibration method using well-characterized homogeneous reference samples has been implemented. For this procedure, twenty-two reference samples were used to produce setup-specific reference data which can afterwards be used for elemental decomposition measurements using fast neutrons. Data for individual elements were obtained by measuring pure elemental samples or by subtracting elemental sample data from compound samples. Ultimately full cross-section characterization was obtained for nineteen individual elements. Although limitations to this approach were found, the general agreement between simulation and experiment was good. This technique, the measured results, and how it can be used for fast neutron elemental decomposition are described.

Temporal and spatial-resolved data on the interfacial structure in upward vertical and inclined two-phase annular flows were accumulated using a novel non-intrusive multilayer conductance sensor. The sensor provides simultaneous measurement of the film thickness across the entire pipe circumference, enabling a three dimensional reconstruction of the wavy interface. Measurements were performed for two liquid (water) flow rates and a single high gas (air) flow rate. Three types of interfacial waves were identified, including ripples, disturbance and rogue waves. Rogue waves can be described as an infrequent solitary disturbance wave propagating over a ripple-dominant interface. Detailed statistical properties of the interfacial shape, such as the mean film thickness, wave height distribution, wave frequency spectra, wave propagation velocities and more, were obtained as a function the pipe inclination and azimuthal angle. The statistical analysis of the wavy interface presented in this study sheds light on a complex flow pattern of annular flow in inclined pipes, which has remained relatively unstudied experimentally. For inclined pipes, gravity imposes an asymmetric film distribution resulting in the thickest film at the bottom of the pipe. At this location, waves attain larger amplitudes while maintaining slower propagation velocities as compared to smaller amplitude waves at the top of the pipe. Generally, the wave frequency throughout the pipe circumference increases with inclination angle. For a larger liquid flow rate, the interface was found to be primarily dominant by disturbance waves. For a lower liquid velocity, the interfacial structure was found to be highly dependent on both the azimuthal and the inclination angles. An interface wave type map is presented as a function of those angles.

In heat exchangers employing flow boiling, e.g., boiling water reactors and industrial boilers, the heat transfer mechanism will transition from nucleate boiling to forced convective evaporation as the flow pattern transition from bubbly to annular flow at relatively high steam quality. Such thermal transition is called suppression of nucleate boiling (SNB). The occurrence of SNB affects the local heat transfer coefficient, the stability of liquid film, as well as the characteristics of entrained liquid droplets in the gas core. Despite its importance, there has been hitherto few direct measurements of the SNB conditions. Furthermore, the existing prediction approaches of SNB are only approximate, since they are based on extrapolation of empirical heat transfer correlations valid for nucleate boiling and forced convective evaporation regimes, rather than SNB mechanism. The objective of this study is to, experimentally and theoretically, fill the gap in understanding SNB phenomenon, using a modern set of diagnostics and a semi-empirical modeling approach. We leveraged synchronized infrared thermometry and an electrical conductance-based liquid film thickness sensor to investigate the details of the SNB phenomenon with high spatial and temporal resolutions. Such advanced diagnostics measure two crucial boundary conditions for SNB, i.e., the distribution of the temperature and heat flux on the heated wall, as well as the thickness of the liquid film. Such direct measurement revealed a clear dependency of SNB heat flux and wall superheat on both steam quality and mass flux. The experimental database has informed a more accurate semi-empirical model for predicting the SNB condition.

A relatively uniform coolant temperature profile at the outlet of reactor core, resulted from good mixing behavior inside fuel assemblies, is desirable for better safety and economic performance. In order to enhance mixing between adjacent sub-channels, various kinds of mixing vanes have been designed and attached to the edge of spacer grids. Both experimental and computational investigations have been ongoing on this topic for decades. However due to the complexity of the geometrical configuration of the spacer grids and mixing vanes, the flow field in the rod bundle involves complicated mechanism, and is not yet fully understood by researchers. A sub-channel mixing test facility SUBFLOW was designed by PSI jointly with ETH Zurich. The test section is an adiabatic 4*4 rod bundle at atmospheric pressure. Both single-phase and two-phase mixing experiments were carried out and valuable data has been obtained. This paper presents the numerical prediction of the single-phase mixing at a Reynolds number of 28000. Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations were performed on a combination of hexahedral and trimmed meshes. A passive scalar source was introduced as an indicator of the mixing behavior. Various turbulence models have been studied. Lateral mixing of passive scalar, velocity profiles and swirl behavior along the channel have been investigated. Fluctuations induced by the mixing vanes were analyzed. The results showed that among the tested methods only low-Reynolds number model v2f with unsteady solver was capable of obtaining quantitatively correct lateral mixing rate and swirl decay rate along the channel, and capturing the fluctuations of the flow field with qualitative agreement with experimental data.

Flow behavior of thin liquid films in an annular flow regime is an important element of the thermal hydraulics of a BWR – it controls the heat transfer from the fuel rods to the coolant. Despite its relevance, the flow behavior of dynamic liquid films is not well understood even in adiabatic conditions. To investigate it, unsteady numerical simulations with an interface tracking model were performed in a double subchannel geometry with a P/D ratio of 1.325. Different turbulence models, namely the large eddy simulation (LES) and linear/nonlinear eddy viscosity unsteady-RANS (URANS) models, were tested. A novel approach for generating turbulent inlet conditions in a periodic flow domain was developed to reduce computational efforts. Validation against experimental data revealed shortcomings of the linear eddy-viscosity RANS model in predicting key flow parameters. By capturing the effects of the secondary flow structures in a subchannel geometry, improved predictions were obtained with a non-linear SST k–ω (QCR) turbulence model. Time-averaged liquid film thickness (LFT) and tracer distribution obtained with LES were found to have the best agreement with experimental data. Instantaneous and time-averaged velocity profiles were analyzed to understand the influence of the gas-liquid interface. Secondary flow structures in the subchannel gap region were found to enhance the turbulent mixing of the passive scalar in the liquid film. This finding is relevant towards the prediction of thermal-hydraulic parameters in a multichannel flow assembly by accounting for inter-channel mixing phenomena.

The enhancement of safety, sustainability, and social acceptance is the main requirements for a nuclear renaissance. Two Gen IV concepts, the molten salt reactor and the high-temperature reactor, have a potential for the paradigm shift in nuclear safety, and Switzerland is involved in the international research efforts dedicated to both of these reactors. Due to the liquid fuel state, especially molten salt reactor (MSR) can act as a safe, sustainable, and acceptable reactor for the future. Main aim of the MSR research program in Switzerland is the safety. Nonetheless, possible MSR designs and so the fuel cycles are also evaluated. At the same time, tools are being developed for MSR transient analysis and thermochemical simulations.

The existing evidence clearly shows that the physical phenomena governing gas/liquid two-phase flows are quite complicated even in the case of smooth conduits and simple geometries. Needless to say, the development of experimental, analytical and computational methods for predicting such flows in complex geometries is an even more complicated and challenging task. A configuration of interest to a broad range of industrial applications, including nuclear reactors, deals with the flow of two-phase mixture along the channels formed between narrow arrays of multiple parallel cylindrical elements (tubes or rods). The alignment of such elements is normally accomplished by installing spacer grids placed at regular distances along the flow. The presence of spacers actually affect flow conditions, including the velocity field, pressure drop, heat transfer and, in the case of two-phase flows, phase distribution. The objective of this paper is to present a comparative analysis of the results of a combined experimental, theoretical, and computational study of phase distribution around and downstream from complex-geometry spacer grids with split-vane type mixing devices. The main emphasis has been given to the analysis of the effect of proper interpretation of the experimental data on the modeling consistency. The importance of the understanding of uncertainties and limitations associated with the results of multidimensional computer simulations performed using mechanistic modeling principles based on an average bubble size is also discussed.

In this paper we investigated reflux condensation in a single pressurized water reactor steam generator tube in the presence of non-condensable gases. We observed the gas plug formation, behavior and impact on heat removal under thermo-hydraulic conditions typical for the LOCA stage during which the reflux condenser regime is characteristic. We used nitrogen and helium to observe differences of the plug behavior for heavy and light gases. Measurements reveal that the non-condensable gas type plays a crucial role in the plug formation. Light gas (He) forms sharp and narrow interface and limits condensation efficiently while heavy gas (N2) develops wide, diffuse interface with gradual temperature drop.

Fast neutrons are highly penetrating and exhibit macroscopic attenuation coefficients which do not strongly vary from one material to another. This makes them an attractive alternative to X-rays when imaging bulky samples, particularly when a low-Z material of interest, like water or oil, is shielded within a thick high-Z (e.g. steel) structure. A compact fast neutron generator based on deuterium-deuterium (D-D) fusion is under development at the Paul Scherrer Institute to work with techniques taking advantage of those properties. This generator was designed with fan-beam transmission imaging as its primary use and has a small emitting spot size (~2 mm) to reduce emitting spot blur. The source emits neutrons quasi mono-energetically depending on emission angle from about 2.2 to 2.8 MeV. Like other compact D-D neutron generators, the source has an intrinsic low neutron output which brings challenges of achieving reasonable exposure times for imaging applications. The neutron generation rate is mainly limited according to the maximum heat deposition rate on the ion beam target. If the temperature of the host material reaches a critical level, deuterium outgasses and neutron yield decreases. To overcome this temperature limitation, a rotating target rod was designed, implemented and tested. This upgrade increased the total neutron output by a factor of about 4.4 to ~3∙107 neutrons/s over 4π at an acceleration of roughly -100 kV and a beam current of ~1 mA. In addition to the details of the design and recent performance improvements of the rotating beam target, the latest work in characterizing the neutron generator thoroughly using COMSOL Multiphysics and MCNP6 simulations as well as experiments will be described.

In the pebble-bed high temperature reactor under construction in China, called HTR-PM, the spherical fuel elements continuously flow downward in the cylindrical core. After the discharge, the burnup of each pebble is checked at the core outlet and, according to the achieved burnup level, the pebble might be disposed or reinserted into the upper section of the core, distributing randomly in the radial direction and defining a variable number of passes necessary to achieve the average maximum burnup of 90 MWd/kgU. Discrete Element Method (DEM) simulations have been carried out to achieve a clear understanding of the movement of 420,000 fuel pebbles in the HTR-PM core. At the same time, neutronic properties have been investigated for a single pebble and for the full core with Serpent 2 Monte Carlo code in order to perform a parametrization of the one-group microscopic cross sections at the core-level. The pebble movement has been coupled with the neutronic behavior of a single pebble in a dedicated burnup script called Moving Pebble Burnup (MPB), developed in Matlab. 3,000 single pebble burnup histories were simulated to obtain sufficient statistics and insight on the burnup process in the HTR-PM. The decrease of the average burnup gained per single pass implies that a miss-handling of recirculated fuel elements is unlikely to lead to exceedance of the maximum allowed burnup of 100 MWd/kgU. Furthermore, the core demonstrates a self-compensation effect of burnup, meaning that it always compensates burnup under- or over-runs in the successive passes. Finally, it is possible to conclude that the fuel cycle of the HTR-PM, as it has been laid out, is well-designed and feasible.

After the accident in the Fukushima Daiichi nuclear power plant, the interest of adding Filtered Containment Venting Systems (FCVS) on existing nuclear power plants to prevent radioactive releases to the environment during a severe accident has increased. Wet scrubbers are one possible design element which can be part of an FCVS system. The efficiency of this scrubber type is thereby depending, among others, on the thermal-hydraulic characteristics inside the scrubber. The flow structure is mainly established by the design of the gas inlet nozzle. The venturi geometry is one of the nozzle types that can be found in nowadays FCVS. It acts in two different steps on the removal process of the contaminants in the gas stream. Downstream the suction opening in the throat of the venturi, droplets are formed by atomization of the liquid film. The droplets are contributing to the capture of aerosols and volatile gases from the mixture coming from the containment. Studies state that the majority of the contaminants is scrubbed within this misty flow regime. At the top of the venturi, the gas stream is injected into the pool. The pressure drop at the nozzle exit leads to the formation of smaller bubbles, thus increasing the interfacial area concentration in the pool. In this work, the flow inside a full-scale venturi scrubber has been optically analyzed using shadowgraphy with a high-speed camera. The venturi nozzle was installed in the TRISTAN facility at PSI which was originally designed to investigate the flow dynamics of a tube rupture inside a full-length scale steam generator tube bundle. The data analysis was focused on evaluating the droplet size distribution and the Sauter mean diameter under different gas flow rates and operation modes. The scrubber was operated in two different ways, submerged and unsubmerged. The aim was to include the effect on the droplet sizes of using the nozzle in a submerged operation mode.

Nuclear reactor simulation is often based on multi-group cross-section libraries. The structure and resolution of these libraries have a strong influence on the accuracy and computational time; hence, number of groups and energy structure must be carefully considered. The relationship between group structures and how they impact generated cross-sections can be a critical parameter. Common energy boundaries shared among major group structures were identified and the relative kinship among those was reconstructed in an effort to build a family tree of major group structures. Stochastic code Serpent2 [1] was employed to generate cross-sections of selected isotopes at different reactor compositions and conditions, using the investigated energy group structures. The impact on their generation was quantified by spectral weighted deviation. The 35 major energy structures were divided into three basic families. The key parameters distinguishing them were their applicability to thermal or fast reactors and their applicability in neutronic or multiphysics investigations. A sensitivity threshold of the generated cross-sections over the group structure resolution was investigated. The aim was to identify a group structure with very low dependency on the actual reactor spectrum.

For the BWRs, the dryout margin is one of the core design limitation factors. Today’s industry standard is to use a large margin to dryout and functional spacer grids with vanes to enhance the heat transfer and to reduce the fraction of entrained droplets. Difficulties for precise measurements under reactor conditions lead to a lack of knowledge on the exact effects of the spacers on the flow and suggest the use of scaled experiments. For this experiment, the goal is to provide high-resolution data for CFD code validation as well as visualizing the effects of functional spacers and the liquid film and potentially the dryout front. The Dryout Tomography Experiment (DoToX) facility at ETH Zürich is a closed loop experiment for two-phase flow investigations in a fuel bundle model using a modelling fluid. Key aspects are a single undisturbed subchannel and the surrounding four heating rods containing a liquid heating system. This setup allows for a steady state dryout without endangering the structural integrity of the facility and for the 3D reconstruction of the time averaged void distribution within the flow channel by means of an X-Ray and cold neutron Computer Tomography (CT). In this study we pay special attention to the annular flow in the upper half of the sub channel. We investigate the first results delivered by the facility. Prototypical spacer designs available in the open literature were used. We present the Liquid Film Thickness (LFT) distributions on the walls of the heating rods. Improvements towards the dryout performance as well as drawbacks of the specified spacer design are highlighted.

Near InfraRed FILM thickness PROfile (NIR-FILMPRO) is an optical technique for non-intrusive measurement of water film thickness. The technique is based on absorption of NIR light. A passband filte...