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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. [ABSTRACT FROM AUTHOR]

The distribution and eventual stratification of hydrogen released during a hypothetical severe accident and the stability of the stratification formed in the early phase of the transient is of particular safety concern in Light Water Reactors (LWRs). The large-scale containment test facility PANDA (PSI, Switzerland) has been used to perform a series of four tests examining the erosion and break-up of stratified light-gas layers in the frame of the OECD SETH-2 project. The ultimate goal of the test program is to set-up an experimental data base of high-quality and high-density data that can challenge and validate 3D containment codes like e.g. GOTHIC, GASFLOW or MARS and validate the applicability of CFD codes like FLUENT or CFX for LWR containment problems. The test series discussed here focuses on the erosion of a stratified, helium-rich layer by horizontal steam injection at different locations below the layer. An approach with step-wise increasing complexity has been chosen to examine this problem allowing control over the rate of pressure increase and the occurrence of condensation. The step-wise approach enables a thorough understanding of the influence of different phenomena like position of steam injection, diffusion, pressurization and condensation on the behavior and erosion of the stratified layer. [ABSTRACT FROM AUTHOR]

Highlights: [•] Development of a novel fast neutron imaging system for two-phase flows in fuel bundles. [•] Novel D–D fusion neutron generator with small emitting spot for imaging. [•] Dedicated fast neutron imaging detector based on plastic scintillator and SiPM. [•] Novel proton recoil converter coupled with THGEM readout as imaging detector. [Copyright &y& Elsevier]

Highlights: [•] Annular flows with and without functional spacer in a tight lattice fuel bundle geometry are investigated. [•] Cold-neutron imaging of adiabatic flows is performed. [•] Liquid film thickness distributions are determined with high accuracy and resolution. [•] The influence of the spacer on the film thickness and liquid hold up is analyzed in details. [ABSTRACT FROM AUTHOR]

Highlights: [•] Annular flows w/wo functional spacers are investigated by cold-neutron imaging. [•] Performance of liquid film thickness measurement under different imaging modalities is evaluated and optimized. [•] MC simulations of the cold neutron imaging are performed to help quantifying the measurement accuracy. [•] Spectral, scatter corrections and minimizing the blur are necessary to avoid bias in film thickness estimate. [•] Empty channel referencing offers in general the highest accuracy and quality. [Copyright &y& Elsevier]

Abstract: The mixing of coolant streams of different temperatures in pipe junctions leads to temperature fluctuations that may cause thermal fatigue in the pipe wall. Numerous T-junction experiments are known from literature, which were performed to study the nature of thermal loads in the pipe walls occurring during the mixing of hot and cold liquid. It is common to all known experiments that the experimental boundary conditions are set to reflect cases, in which the flow velocities in both main and side branches of the T-junctions are of the same order of magnitude. In the present experiments, carried out using wire-mesh sensors, it was observed that very low flow velocities in the side branch compared to the main pipe may lead to conditions potentially severe for thermal fatigue due to the low frequency of the temperature fluctuations occurring. The T-junction presented here consists of a perpendicular connection of two pipes of 50mm inner diameter. The straight and the side branches are supplied with water of different electrical conductivities, to enable performing generic, isothermal tests on turbulent mixing with the idea to model the temperature fluctuations in thermal mixing processes. A pair of wire-mesh sensors, each with a grid of 16×16 measuring points, are used to record conductivity distributions in the downstream of the T-junction as well as directly at the junction in both branches. At very low flow rates in the side branch, a characteristic entrainment of liquid from the main branch into the side branch was found. Typically the entrainment flow in the side branch results in relatively high fluctuations at the low-frequency range. While the sensor in the main flow shows fluctuations with a power spectrum similar in character to mixing experiments with comparable flow velocities in both branches of the T-junction. The phenomenon of entrainment of water from the main branch into the side branch against the main flow direction vanishes at a certain critical velocity in the side branch. [Copyright &y& Elsevier]

Abstract: Dryout of the coolant liquid film at the upper part of the fuel pins of a boiling water reactor (BWR) core constitutes the type of heat transfer crisis relevant for the conditions of high void fractions. It is both a safety concern and a limiting factor in the thermal power and thus for the economy of BWRs. We have investigated adiabatic, air–water annular flows in a scaled-up model of two neighboring subchannels as found in BWR fuel assemblies using cold-neutron tomography. The imaging of the double suchannel has been performed at the ICON beamline at the neutron spallation source SINQ at the Paul Scherrer Institute, Switzerland. Cold-neutron tomography is shown here to be an excellent tool for investigating air–water annular flows and the influence of functional spacers of different geometries on such flows. The high-resolution, high-contrast measurements provide the spatial distributions of the coolant liquid film thickness on the fuel pin surfaces as well as on the surfaces of the spacer vanes. The axial variations of the cross-section averaged liquid hold-up and its fraction in the gas core shows the effect of the spacers on the redistribution of the two phases. [Copyright &y& Elsevier]

Abstract: In the frame of the OECD SETH project twenty-four experiments have been carried out on basic gas transport and mixing phenomena in the large-scale, multi-compartment PANDA facility. The experiments consist of several series based on the flow configuration driving the gas transport including plume and jet injection of saturated or superheated steam. A variety of test configurations with well-defined initial and boundary conditions have been investigated, including the parametric effects of the geometry of the injection, the composition and velocity of the injected mixture, the initial ambient composition, the vent location and the initial temperatures. The tests have been designed to be used for the validation of computer codes that are capable of analyzing the thermalhydraulic safety of the containment of light water reactors (LWRs). The results obtained form an extensive database especially valuable for the assessment of the capabilities of three-dimensional simulation tools. The present work reports on a specific test series of the aforementioned experimental program, the so-called Free-Plume Series featuring vertical injection of buoyant steam driving the gas transport. The experimental data is presented showing its quality and the trends in the experiments are analyzed. The phenomena whose prediction might be challenging for codes are emphasized. After the expiration of the confidentiality period, the experimental data is now available for the broad scientific community to be used for code validation purposes. [ABSTRACT FROM AUTHOR]

Abstract: In the frame of the OECD/NEA SETH project an experimental campaign has been carried out in the PANDA facility to investigate gas transport and mixing induced by a plume or a jet in the large-scale multi-compartment PANDA facility. The paper summarizes the results of the horizontal jet test series consisting of eight tests. Horizontal jets impinging on a vertical wall of one of the cylindrical PANDA containment vessels have been generated by changing various parameters, such as: type of injected fluid (steam or a mixture of steam and helium), fluid injection velocity, elevation (with respect to the containment vessel) of the injection exit, initial fluid composition in the vessels, and location of the vent line. The initial jet Froude number has been varied between 17 and 36 and in one of the test condensation occurred. The paper shows the effect of these parameters variation on the test evolution with respect to jet impingement location in the vertical curved wall and variation of impingement location as a function of buoyancy variation. Fluid mixing and stratification, characteristics of gas transport between the compartment and the effect of condensation on the overall phenomena evolution are analyzed in the paper. [Copyright &y& Elsevier]

Abstract: This paper presents the results of two tests, performed in the PANDA facility in the frame of the OECD SETH project. The Test 9 and Test 9bis were characterized by the same initial Froude number (Fro ∼1) and were performed at constant pressure in the vessels. The vessels were initially filled with air. During the tests the plumes were generated by injecting a constant amount of steam in Vessel 1 and continuous venting of a steam–air mixture from Vessel 2. In Test 9bis the temperature of the injected steam and the initial containment wall and fluid temperatures where chosen to reach later the conditions for (wall) condensation. The on-set of condensation occurred in Test 9bis at the top of Vessel 1 when, due to the steam injection the steam partial pressure increased enough to eliminate the steam superheating. The condensed droplets propagated toward the bottom of Vessel 1, where due to the lower steam partial pressure, a portion of condensate vaporized again. The evaporation had the effect of increasing the steam partial pressure and progressively new conditions for steam condensation were obtained also at the bottom of Vessel 1. The condensation in Test 9bis had the effect of mitigating the reduction of plume buoyancy (which is a function of the air-steam concentration in Vessel 1) and for this reason the overall plume trajectory, steam–air mixing, stratification and transport between the two compartments were affected. [Copyright &y& Elsevier]

Abstract: A project named SETH (SESAR Thermal Hydraulics (SESAR: Senior Group of Experts on Nuclear Safety Research)) has been performed under the auspices of 15 OECD countries, with the aim of creating an experimental database suitable to assess the 3D code capabilities in analyzing physical key phenomena relevant for LWR safety analysis. This paper presents the results of OECD/SETH Project Test 25 performed in the PANDA facility (located at PSI in Switzerland). Test 25 consists of two phases for duration of approximately 2h for each phase. During Phase 1 a steam–helium mixture (helium is used to simulate hydrogen) is released in a containment compartment initially filled with only air. This phase simulates the hydrogen released in a postulated accident due to the fuel cladding metal–water (MW) reaction. During Phase 2 only steam is released in the containment and the scenario that occurs after the MW reaction is exhausted is addressed. Temperature and gas concentration measurements obtained at several locations in the containment compartments allow the recognition of the complex stratification pattern evolution during the test period. The analysis of Test 25 carried out with the GOTHIC code is also presented in this paper. The analysis shows that the prediction of three-gas (hydrogen, steam, and air) stratification pattern in a multi-compartment geometry for scenarios characterized by the evolution of the density difference between and inside compartments and in particular of the hydrogen accumulation in a dead-end volume is a challenging task also for a code having 3D capabilities. [Copyright &y& Elsevier]

Spent fuel characterization describes the overall study of fuel rods that are extracted from a nuclear power plant at the end of their useful cycle in energy production or in case of a failure that needs to be analyzed. The knowledge of the radioactive source term of spent fuel allows, to some extent, the improvement of nuclear fuel design and the optimization of the core configuration during the different burning cycles. The same knowledge is essential for the correct handling of the fuel after discharge. The Paul Scherrer Institute (PSI) is one of the few research centers in Europe that are equipped with the appropriate facilities to perform a full-length spent fuel rod characterization. In particular, for the source term of spent fuel, different kinds of detectors are used depending on the type of radiation that is measured. The goal of this work is to investigate the possibility to build a measurement station based on two new fast neutron scintillators, the EJ-299 by Eljen Technologies and a composite ZnS(Ag) scintillator with embedded wave-length shifting optic fibers developed in-house, to measure the axial neutron profile of spent fuel rods received at the PSI Hot-Lab. To design the measurement station, we used a well-characterized UOX spent fuel sample of 40 cm with ∼70 MWd/kg and more than 10 years cooling time as neutron and gamma-ray sources and modeled the fast neutron efficiency of the detectors with MCNP6.1.1 in test conditions and in realistic configurations encountered in the hot-cell. Two methods, based on the usage of a blinding cone and of a collimator, are studied for the measurement of the fast neutron axial profile. The effect of the photon source term of the spent fuel rod on the measurement is also studied. First results show that, thanks to its gamma blindness, the composite ZnS(Ag) scintillators could be used in a specific design to determine the neutron spent-fuel axial distributions of the studied sample with a 2 cm resolution. [ABSTRACT FROM AUTHOR]

Abstract: The large-scale, multi-compartment PANDA facility (located at PSI in Switzerland) is one of the state-of-the-art facilities which is continuously upgraded to progressively match the requirements of CFD-grade experiments. Within the OECD/SETH projects, the PANDA facility has been used for the creation of an experimental database on basic containment phenomena e.g. gas mixing, transport, stratification, condensation. In the PANDA tests, these phenomena are driven by large scale plumes or jets. In the paper is presented a selection of the SETH PANDA experimental results. Examples of analytical activities performed at PSI using the GOTHIC, CFX-4 and CFX-5 codes will be used to illustrate how the spatial and temporal resolutions of the measurement grid in PANDA tests are adequate for CFD code (and advanced containment codes) assessment and validation purposes. [Copyright &y& Elsevier]

Abstract: The capability assessment of three-dimensional computational tools to predict the erosion and the break-up of stratified conditions that can build-up in a containment through the release of hydrogen during an early phase of a hypothetical severe accident is the focus of intense research worldwide. In conjunction with the OECD SETH-2 project, the GOTHIC code is assessed against experiments in which mass and/or heat sources or sinks cause mixing. This paper reports on simulation results of selected experiments where the initial helium stratification in a vessel is eroded by a vertical jet originating from an injection below the initial density interface. A 3-D coarse mesh, as well as various finer 2-D meshes, is used to simulate the evolution of the helium distribution generated by jets having different initial momentum. In general, the 3-D calculations predict too slow mixing in the vessel and a reasonable agreement between calculated and measured gas concentrations can only be achieved with a sufficiently fine mesh. These results can be explained by comparing the calculated velocity field with that measured using the PIV technique, which also provides valuable insight into the mechanisms of the interaction between the jet and the density interface. [Copyright &y& Elsevier]

Abstract: To analyze the effect of containment spray on gas mixing and depressurization, two experiments (ST3_1 and ST3_2) were performed with two interconnected vessels. These experiments were conducted in the frame of the OECD/SETH-2 project using the PANDA facility. The vessels were preconditioned such that a helium-rich layer is formed in the upper section of the first vessel, henceforth referred to as Vessel-1. In the case of the first experiment (ST3_1), the remaining volume of Vessel-1 and the entirety of the second vessel, Vessel-2, were filled with pure steam. For ST3_2, the second experiment presented here, pure steam was replaced with a steam–air mixture instead. Water was injected from the top of Vessel-1 with a spray nozzle projecting downwards. Transient behavior of system pressure, as well as global redistribution of gases is investigated. The results reveal that spray activation is very effective in containment system depressurization. Additionally it is found that the depressurization occurs at a higher rate for the systems containing more steam and less non-condensible gas. The depressurization rate gradually slows down, however, as the steam concentration decreases due to condensation, and non-condensible gases spread over the vessel system. It is also observed that the spray activation initiates the breakup of the helium-rich layer. The composition of the gas atmosphere plays a crucial role in determining the initiation time of the breakup; the presence of large amounts of non-condensible gas such as air delays the beginning of the helium layer breakup by approximately 200s. The downward component of spray momentum causes the entrainment and the recirculation of the ambient gas atmosphere. Together with the entrainment and condensation effect, spray activation influences the gas mixture density in Vessel-1 and this generates a driving force for inter-compartment flow. As a result of this, an increase of helium–rich gas mixture is observed in the regions far away from the spray, i.e., in Vessel-2. [Copyright &y& Elsevier]