Search Results (16)

A Finite Element Method (FEM) analysis of the nuclear fuel spacer grid was conducted to assess the strength of components for the safety of nuclear power plants. The fuel assembly consists of fuel rods, upper end-fitting, lower end-fitting, guide tubes, and spacer grids. Spacer grids play a critical role in maintaining the proper spacing between fuel rods within a fuel assembly and ensuring smooth coolant flow. This role becomes particularly crucial during unforeseen emergencies, such as seismic loads, where minimizing deformation caused by external forces is essential. Therefore, this study proposes FEM models of spacer grids, mesh refinement of models, and analysis of the stiffness of the spacer grids concerning the presence or absence of pellet and clad. The results revealed significantly lower shear stiffness compared to normal stiffness, indicating potential vulnerability of the fuel assembly to large loads such as those experienced during seismic events. Full article

The structural integrity of fuel assemblies hinges significantly on the effectiveness of spacer grids. In this paper, we introduce a novel approach to assess the structural robustness of the mid-spacer grid (SG) of the PLUS7 fuel assembly (FA) using 3D shell elements. Any excessive external load from seismic activity can be broken down into two perpendicular components, namely normal load and shear load. The decomposition enables easy assessment of the structural integrity of the fuel spacer grid for any external loads. From the analysis, the reaction force of normal input displacement is around 440 times greater than that for shear input displacement for the same amount of displacement. This result highlights how vulnerable the spacer grid is to shear load and this should be considered in studies of fuel assembly integrity assessment. This study also highlights the need for improved and more robust spacer grids for safer operation of NPPs. Full article

This article presents the results of an experimental study of the coolant flow in a fuel rod bundle of a nuclear reactor fuel assembly of a small modular reactor for a small ground-based nuclear power plant. The aim of the work is to experimentally determine the hydrodynamic characteristics of the coolant flow in a fuel rod bundle of a fuel assembly. For this purpose, experimental studies were conducted in an aerodynamic model that included simulators of fuel elements, burnable absorber rods, spacer grids, a central displacer, and stiffening corners. During the experiments, the water coolant flow was modeled using airflow based on the theory of hydrodynamic similarity. The studies were conducted using the pneumometric method and the contrast agent injection method. The flow structure was visualized by contour plots of axial and tangential velocity, as well as the distribution of the contrast agent. During the experiments, the features of the axial flow were identified, and the structure of the cross-flows of the coolant was determined. The database obtained during the experiments can be used to validate CFD programs, refine the methods of thermal-hydraulic calculation of nuclear reactor cores, and also to justify the design of fuel assemblies. Full article

This paper compares and discusses the methods for evaluation of the structural integrity of the mid spacer grid of nuclear fuel assembly via a finite element analysis of 3D shell elements. The structural stiffness of the spacer grid is determined by applying either force or deformation as loads onto the spacer grid for both the square load and shear load directions. This study is an extension of a single-cell strength analysis of a spacer. External events such as seismic activities that might happen in a nuclear reactor are able to transfer loads onto nuclear components in random directions, which can be broken down into square and shear loadings. The structural strength indicated by the force reaction against the input displacement load was proven to be smaller such that the same displacement square load is around 260 times greater than the shear load. Due to the weakness in shear stiffness, the maintenance of a spacer grid structure is more vulnerable against out-of-plane loads. This indicates that the shear load needs to be considered in studies of fuel assembly integrity assessment for newly developing fuel design, as well as existing fuel assembly designs. Full article

Arc sag is an important parameter in the design and operation and maintenance of transmission lines and is directly related to the safety and reliability of grid operation. The current arc sag measurement method is inefficient and costly, which makes it difficult to meet the engineering demand for fast inspection of transmission lines. In view of this, this paper proposes an automatic spacer bar segmentation algorithm, CM-Mask-RCNN, that combines the CAB attention mechanism and MHSA self-attention mechanism, which automatically extracts the spacer bars and calculates the center coordinates, and combines classical algorithms such as beam method leveling, spatial front rendezvous, and spatial curve fitting, based on UAV inspection video data, to realize arc sag measurement with a low cost and high efficiency. It is experimentally verified that the CM-Mask-RCNN algorithm proposed in this paper achieves an AP index of 73.40% on the self-built dataset, which is better than the Yolact++, U-net, and Mask-RCNN algorithms. In addition, it is also verified that the adopted approach of fusing CAB and MHSA attention mechanisms can effectively improve the segmentation performance of the model, and this combination improves the model performance more significantly compared with other attention mechanisms, with an AP improvement of 2.24%. The algorithm in this paper was used to perform arc sag measurement experiments on 10 different transmission lines, and the measurement errors are all within ±2.5%, with an average error of −0.11, which verifies the effectiveness of the arc sag measurement method proposed in this paper for transmission lines. Full article

The ongoing re-equipment of electric power systems is based on the use of smart grid technologies. Among the key tasks that are solved on this basis are increasing the capacity of power transmission lines, reducing losses, and improving power quality. To address these issues, one can use compact power transmission lines. Such lines are notable for their complex split-phase designs and close together placement of current-conducting parts, so as to keep the distance to a permissible minimum, which is achieved by the use of insulating spacers. This article reports the results of computer-aided simulations performed for a standard railroad power supply system, the traction substations of which were connected to 220 kV networks through compact overhead lines (COHLs). The purpose of the study was to calculate the values of quantitative metrics that measure power quality and energy efficiency as well as electromagnetic safety. Modeling was performed in the three-phase reference frame with the use of techniques and algorithms implemented in the Fazonord software package. We considered a power supply system with 25 kV overhead contact systems. It was assumed that the external network used three different designs of COHLs: with coaxial, linear, and sector-shaped arrangements of conductors. Based on the results obtained, we concluded that (1) when using COHLs, the voltages on bow collectors of electric rolling stock were stabilized and did not exceed permissible limits; (2) losses in the traction network were reduced; and (3) the parameters of power quality and electromagnetic safety conditions in external power supply systems of railroads were improved, if judged in terms of electric and magnetic field strengths. Out of the eight types of COHLs considered, compact lines with the three-segment and concentric arrangement of conductors had the best performance, and the use of COHLs with the vertical arrangement of conductors made it possible to reduce electric field strengths. However, the designs of such transmission lines are quite complex and entail higher construction costs. Full article

Overhead line ice-shedding causes line breakage and results in electrical faults occurring between transmission lines, which seriously affects the safe operation of the power grid. In this paper, a risk assessment method for ice-shedding on 500 kV compact overhead lines is proposed. Two ratios were used to assess the discharge risk and mechanical failure risk of a line. A 500 kV compact overhead line in southern China was taken as an example, the proposed risk assessment method was used to carry out a risk assessment of the ice-shedding line with an ice thickness of 15 mm, and the suppression effects of different spacer arrangement schemes on the ice-shedding line were analyzed. The results showed that the impact coefficient of the conductor reached 1.88, the maximum jump height of the lower phase conductor reached 22.596 m, and the interphase clearance of the conductor decreased sharply when the lower phase conductor of 500 kV case line underwent full-span ice-shedding synchronously. The ice-shedding line displayed both a discharge risk and a mechanical failure risk. When interphase spacers were installed at L/3 from both ends of the span for the upper-left phase and the lower phase, the upper-right phase, and the lower phase, the impact coefficient of the line became 1.76, which was lower than the recommended value of 1.8; there was no mechanical failure risk; and the maximum jump height became 10.266 m. This study verified the effectiveness of this spacer arrangement scheme. Full article

Because the galloping of iced conductors is one of the main disasters in the State Grid, resulting in huge economic and property losses every year, the research on relevant monitoring methods is of great significance. The existing galloping monitoring technology is mainly based on the contact detection method, which presents potential electrical hazards and power supply problems. In this paper, a conductor galloping monitoring method based on the target detection of infrared sources is put forward to overcome the shortcomings of existing methods. In other words, an infrared source label is installed on the conductor spacer, high-definition night vision infrared cameras are installed on electric power towers to take video of the infrared source labels, and the characteristic amplitude of conductor galloping is calculated by an image recognition and tracking algorithm. The practical application results indicate that the method has the advantages of non-contact detection, safety and reliability, and high detection accuracy. Full article

In practice, often devices are ordered rod structures consisting of a large number of rods. Heat exchangers, fuel assemblies of nuclear reactors, and their cores in the case of using caseless assemblies are examples of such devices. Simulation of heat and mass transfer processes in such devices in porous-body approximation can significantly reduce the required resources compared to computational fluid dynamics (CFD) approaches. The paper describes an integral turbulence model developed for defining anisotropic model parameters of a porous body. The parameters of the integral turbulence model were determined by numerical simulations for assemblies of smooth rods, assemblies with spacer grids, and wire-wrapped fuel assemblies. The results of modeling the flow of a liquid metal coolant in an experimental fuel assembly with local blocking of its flow section in anisotropic porous-body approximation using an integral turbulence model are described. The possibility of using the model of an anisotropic porous body with the integral model of turbulence to describe thermal-hydraulic processes during fluid flow in rod structures is confirmed. Full article

Piezoelectrets are artificial ferroelectrics that are produced from non-polar air-filled porous polymers by symmetry breaking through high-voltage-induced Paschen breakdown in air. A new strategy for three-layer polymer sandwiches is introduced by separating the electrical from the mechanical response. A 3D-printed grid of periodically spaced thermoplastic polyurethane (TPU) spacers and air channels was sandwiched between two thin fluoroethylene propylene (FEP) films. After corona charging, the air-filled sections acted as electroactive elements, while the ultra-soft TPU sections determined the mechanical stiffness. Due to the ultra-soft TPU sections, very high quasi-static (22,000 pC N⁻¹) and dynamic (7500 pC N⁻¹) $d_{33}$ coefficients were achieved. The isothermal stability of the $d_{33}$ coefficients showed a strong dependence on poling temperature. Furthermore, the thermally stimulated discharge currents revealed well-known instability of positive charge carriers in FEP, thereby offering the possibility of stabilization by high-temperature poling. The dependences of the dynamic $d_{33}$ coefficient on seismic mass and acceleration showed high coefficients, even at accelerations approaching that of gravity. An advanced analytical model rationalizes the magnitude of the obtained quasi-static $d_{33}$ coefficients of the suggested structure indicating a potential for further optimization. Full article

The power grid is an essential infrastructure in any country, comprising thousands of kilometers of power lines that require periodic inspection and maintenance, carried out nowadays by human operators in risky conditions. To increase safety and reduce time and cost with respect to conventional solutions involving manned helicopters and heavy vehicles, the AERIAL-CORE project proposes the development of aerial robots capable of performing aerial manipulation operations to assist human operators in power lines inspection and maintenance, allowing the installation of devices, such as bird flight diverters or electrical spacers, and the fast delivery and retrieval of tools. This manuscript describes the goals and functionalities to be developed for safe local aerial manipulation, presenting the preliminary designs and experimental results obtained in the first year of the project. Full article

In the dispersed flow film boiling regime (DFFB), which exists under post-LOCA (loss-of-coolant accident) conditions in pressurized water reactors (PWRs), there is a complex interplay between droplet dynamics and turbulence in the surrounding steam. Experiments have accredited particular significance to droplet collision with the spacer-grids and mixing vane structures and their consequent positive feedback to the heat transfer recorded in the immediate downstream vicinity. Enabled by high-performance computing (HPC) systems and a massively parallel finite element-based flow solver—PHASTA (Parallel Hierarchic Adaptive Stabilized Transient Analysis)—this work presents high fidelity interface capturing, two-phase, adiabatic simulations in a PWR sub-channel with spacer grids and mixing vanes. Selected flow conditions for the simulations are informed by the experimental data found in the literature, including the steam Reynolds number and collision Weber number ($W{e}_c=\left\{40,80\right\}$), and are characteristic of the DFFB regime. Data were collected from the simulations at an unprecedented resolution, which provides detailed insights into the continuous phase turbulence statistics, highlighting the effects of the presence of droplets and the comparative effect of different Weber numbers on turbulence in the surrounding steam. Further, axial evolution of droplet dynamics was analyzed through cross-sectionally averaged quantities, including droplet volume, surface area and Sauter mean diameter (SMD). The downstream SMD values agree well with the existing empirical correlations for the selected range of $W{e}_c$. The high-resolution data repository from the simulations herein is expected to be of significance to guide model development for system-level thermal hydraulic codes. Full article

Due to the ever-increasing demand for electricity in the one hand and the environmental constraints to use clean energy on the other hand, the global production of energy from remote renewable sources, particularly from large hydropower plants and offshore wind farms and their connection to the grid are expected to grow significantly in the future. Consequently, the demand to carry this electric power by high voltage direct current (HVDC) technology will increase too. The most suitable HVDC power transmission technology to deliver large amounts of power, exceeding a capacity of 5 GW per bipolar system over long distances with lower losses is by using compact HVDC gas insulated transmission lines (DC GIL) and gas insulated switchgears (DC GIS) with rated voltage (maximum continuous operating voltage) of ±550 kV and 5000 A which are presently under development worldwide. Among the critical challenges for the development of these HVDC gas insulated systems, there are the epoxy cast resin insulators that are used to separate gas compartments also called spacers. Indeed, thorough research studies have been and still being carried out to well understand and clarify the electrical insulation characteristics of HVDC spacers using mainly cylindrical samples and small insulator models, where useful results have been obtained and proposed for implementation in real compact gas insulated systems. However, few practical investigations have been undertaken on real size spacers (product scale) to verify such research outcomes and validate the reliability of the spacers to collect experiences or for commercial use. This paper reviews the current achievements of real size HVDC spacers development. It describes the basic electric field calculation and spacers design, the verification of the insulation performance and validation testing. It gives today’s commercially available compact HVDC GIS/GIL and finally it presents the envisaged future research and development. Full article

The aim of the numerical study was the detection of possible vortices in the upper part of the core of a Pre-Konvoi Pressurized Water Reactor (PWR) which could lead to temperature cycling. In addition, the practical application of this Computational Fluid Dynamic (CFD) simulation exists in the full 3D analysis of the coolant flow behavior in the reactor pressure vessel of a nuclear PWR. It also helps to improve the design of future reactor types. Therefore, a CFD simulation of the flow conditions was carried out based on a complex 3D model. The geometry of the model includes the entire Reactor Pressure Vessel (RPV) plus all relevant internals. The core is modelled using the porous body approach, the different pressure losses along and transverse to the main flow direction were considered. The spacer-grid levels were taken into account to the extent that in these areas no cross-flow is possible. The calculation was carried out for nominal operating conditions, i.e., for full load operation. Furthermore, a prototypical End of Cycle (EOC) power distribution was assumed. For this, a power distribution was applied as obtained from a stationary full-core calculation with the 3D neutron kinetics code DYN3D. In order to be able to adequately reproduce flow vortexes, the calculation was performed transiently with suitable Detached Eddy Simulations (DES) turbulence models. The calculation showed fluctuating transverse flow in the upper part of the core, starting at the 8th spacer grid but also revealed that no large dominant vortices exists in this region. It seems that the core acts as a rectifier attenuating large-scale vortices. The analyses included several spacer grid levels in the core and showed that in some areas of the core cross-section an upward increasingly directed transversal flow to the outlet nozzle occurs. In other areas of the core cross-section, on the other hand, there is nearly any cross-flow. However, the following limitations of the model apply: In the model all fuel elements are treated identical and cross flows due to different axial pressure losses for different FA types cannot be displayed. The complex structure of the FAs (eg. flow vanes in spacer grids) could also influence the formation of large-scale vortices. Also, the possible influence of two-phase flows was not considered. Full article

Experimental investigations on the heat transfer performance of supercritical water flowing in the subchannel of supercritical water-cooled reactor (SCWR) simulated by a triangular channel were conducted at pressures of 23–28 MPa, mass flow rates of 700–1300 kg·m⁻²·s⁻¹, and inner wall surface heat fluxes of 200–600 kW·m⁻². An 8 mm diameter fuel rod with a 1.4 pitch to diameter ratio was used. The effects of pressure, mass flow rate, and heat flux on the heat transfer performance under the resistance of a standard grid spacer were analyzed. Experimental results showed the significant positive influence of the grid spacer on the supercritical water in the subchannel. Moreover, in the presence of the grid spacer, the parameters influenced the heat transfer with different degrees of strengthening reaction. In view of the phenomenon in the tests, the rule of the supercritical heat transfer was further revealed by the comparison between empirical formulas and experimental data. This paper mainly studied the positioning grid function and the fluid flow characteristics downstream of the subchannel under the influence of the standard grid spacer and the impact mechanism of each parameter on the whole heat transfer process coefficient. Full article