Dynamics and wear of fragments of a lower part of a fuel assembly with free fuel elements

A procedure and findings of investigations of the dynamic characteristics and the wear of fuel elements in the framework of the development of a perspective design of fuel elements without fastening in a lower support grid are presented. At the first phase of the work, natural vibration frequencies and the dynamic rigidity of fuel elements are determined in various design variants of lower spans. Resonance frequencies of spans of fuel elements are revealed by minimums of the dynamic rigidity. At the second phase, investigations of vibrational wear within contact of fuel elements with cells of spacer grids are performed on models of a lower part of a fuel assembly. Four embodiments of the lower part of the fuel assembly are investigated. As a result of the application of high vibrational loads, the wear of fuel element cans and grid cells is observed in more than a half of contact assembles of fuel elements with spacer grids. The most and least wear-resistant designs of the lower part of the fuel assembly are determined by results of tests.     

Dynamic characteristics of the mockup of the fuel assembly in coolant flow

During exploitation, fuel assemblies are subjected to vibration loads caused by vibrations of intracase devices, pressure pulsations, acoustic waves, and turbulent flow of the coolant. Vibrations of the fuel assembly can be the cause of damage to its details, particularly, fuel elements and spacer lattices, because of their wear. Therefore, one requirement for the project of the fuel assembly of AES-2006 reactor installations is vibration strength and resistance throughout the service life of the fuel assembly in the reactor. The vibration strength and fuel assembly strength are substantiated by computational-experimental methods using the results of the experimental modal analysis of the full-scale mockup for verification.     

Vibrational wear of pipe-spacer pair in steam generator with lead coolant

The vibrational wear at the junctions of heat-exchanger pipe and spacer grids is studied experimentally, so as to confirm the shock resistance of hot-water generators in fast reactors with lead coolant. Endurance tests of tube and spacer samples are conducted. Metallographic and profilometric data on samples subjected to vibrational wear are presented. The wear mechanism is described. A preliminary estimate is obtained for the pipe life in the steam generator.     

Comparative study on rod fretting behavior of different spacer spring geometries

The resistance to fretting wear is one of the most important parameters of fuel assembly behavior in the reactor. To choose and optimize the designs with regards to this aspect, it is necessary to carry out lots of model tests. This paper describes the results of some of them carried out to determine the fuel rod fretting behavior for different support conditions. The tests were carried out in a special autoclave in water and at room temperature, with the test excitation being applied by an electromagnetic force excitation system. The test objects consisted of a test rod made of zirconium alloy, supported in three spacer grid cells with convex spring profiles of different geometries (i.e. different radii of curvature). The grid-to-rod diametral gap measured 0.1 mm at all locations. Due to these conditions of support, both impact and sliding motions can simultaneously occur at the interface between the contact surfaces. The dimensions of the fretting marks found after testing for 100 h were measured using mechanical instruments and an interferometer. The results showed that fretting increases the more, the smaller the radius of curvature of the convex support elements. This is due, on the one hand, to the smaller contact surface and, on the other, to certain features of the spring design. Given that the shapes of the fretting marks can vary widely, evaluation of fretting on the basis of fretting depth alone is insufficient. Instead, it is important to determine the entire geometry or shape of the fretting mark and thus the wear volume. The conclusions of this study are limited to the explored parameters and must not be extrapolated to any operational product without further investigations.     

Resistance to fretting corrosion of fuel rods in a VVER-440 water-moderated reactor in contact with spacers

The wear resistance of fuel assemblies in the VVER-440 water-moderated reactor is investigated, with small fuel-rod diameter. The experimental apparatus, the model of the fuel rods, and the similarity conditions are described; the possibility of applying the experimental results to regular fuel rods is noted.     

Single-phase computational fluid dynamics applicability for the study of three-dimensional flow in 5′ 5 rod bundles with spacer grids

Spacer grids play an important role in pressurized water reactor(PWR) fuel assembly in that they have significant influence on the thermal-hydraulic characteristics of the reactor core.But so far,the numerical studies are performed without regarding dimple and spring of spacer grids,just considering mixing vane.Moreover,these studies use k-ε turbulence model without considering the suitability of the other turbulence models upon the different spacer grids flow.A study is carried out to understand the 3-D single-phase flow in AFA-2G 5×5 rod bundles with spacer grids based on numerical method.In order to investigate the suitability of different turbulence models,k-ε model and k-ω model,the influence of different parts of spacer grid on the fluid flow is also predicted.By using second-order upwind scheme,hybrid grids technique,and improved SIMPLEC algorithm,the Reynolds averaged mass conservation and momentum conservation equations are solved,and the pressure and velocity field of flow are obtained.The numerical simulation results are compared with experiment results and the agreement is satisfactory.The simulation results show the influences of the spring,dimple and mixing vane,and the different characteristics of the k-ε model and k-ω model.Comparing with the experiment results,the simulation results suggest that the k-ω model is suitable for the simulation of the rod bundle flow with spacer grids;the spring and dimple are the main causes of the pressure loss in the spacer grid channel.The friction coefficient of the channel with spring and dimple is 1.5 times the coefficient of the channel with the vane.These results are beneficial to enhance the simulation ability of spacer grids flow and optimization design ability of spaces grid.     

Seismic stability tests for models of fuel assemblies

A procedure and results of seismic stability tests of full-scale models for fuel assemblies of the water-water energetic reactor VVER-1000 are presented. In the course of tests, kinematic loading of fuel assembly supports has been carried out in accordance with accelerograms of the motion of fuel assembly supports during an earthquake, which was obtained by computational methods. Based on the results of measurements of the vibrational response of fuel assemblies, transfer characteristics between the response of fuel assemblies and applied action (support acceleration) are determined. The lack of a resonant response of fuel assemblies at seismic loads is found.     

Wear with low-lubricity fuels II. Correlation between wear maps and pump components

Increasingly severe refinery processes remove many of the compounds necessary for effective lubrication with diesel fuels. No widely recognized lubricity test or standard currently exists relating to the needs of the fuel injection system on compression ignition equipment. The Ball-on-Cylinder Lubricity Evaluator (BOCLE) is commonly used by the US Air Force to measure aviation turbine fuel lubricity. However, the tribological requirements of fuel-lubricated components in aviation may not coincide with the needs of ground vehicles. This study uses the wear-mapping technique developed in the preceding paper (“Development of a wear mapping technique”) to indicate the significance of the BOCLE test methodology in this context. In particular, the work highlights the effects of various contact parameters on fuel-lubricated wear that are not revealed by the BOCLE results.

Where possible, both the BOCLE and wear map data are compared with the results of full-scale pump stand tests detailed in the previous paper. Additional nonstandard bench wear tests were performed to further evaluate the contact conditions present within the operating pump. The wear mechanisms of each fluid were determined to be strong functions of both metallurgy and contact stress. Moreover, the onset and severity of each wear mechanism appear to be controlled by different fuel properties. As a result, the relative lubricity observed between fuels depends on the test conditions chosen so it is unlikely that fuel lubricity may be uniquely defined by a single bench wear test procedure.     

An ultrasonic method for the detection of leaking fuel elements in nuclear reactors

An ultrasonic method for the detection of leaking fuel elements in the structure of the fuel assembly of BBЭP-1000 water-water power reactors is considered. The method is intended for the detection of water passing through a cladding flaw into a fuel element. The calculation and experimental study data for the acoustic channel of the method and the results of testing on radiated fuel assemblies from the Kalinin and Balakovo Nuclear Power Plants are given. The high sensitivity of the developed method is shown experimentally. The boundaries of its applicability are determined.     

On-line evaluation of electrode wear by servo gun in resistance spot welding

More galvanized steels are being used in auto body assembly to improve safety and fuel efficiency of automobiles. However, spot welding these materials will result in a short electrode life due to serious and inconsistent electrode wear. Electrode radial wear during the electrode life test is the main factor leading to unacceptable nugget size. This paper firstly studied the feedback characteristics of servo guns to detect electrode axial wear online based on the developed system of an integrated servo gun with robot. Then a mathematical model of a spherical electrode was established to evaluate radial wear by detecting axial wear online at different electrode wear stages. The model was verified by experiments for welding different galvanized steels. It was shown that evaluation results of the model agreed well with the experiment results. The model revealed that a smaller radius of electrode face at beginning welds would result in a higher electrode wear rate. The method of evaluating electrode wear online based on the model also offered a guidance to determine the suitable time for tip dressing in real production.     

Wear Mechanisms at High Temperatures. Part 1: Wear Mechanisms of Different Fe-Based Alloys at Elevated Temperatures

To extend the lifetime of the sinter grate used to crush the sinter cake into smaller pieces for steel fabrication, a study was undertaken to investigate which wear processes are primarily responsible for limiting the lifetime of the sinter grate. Several wear processes could be identified. The sinter temperature which is up to 800 °C causes temperature-induced material ageing and oxidation. The falling of the sinter cake onto the sinter grate causes high impacts, erosion and abrasive wear. There is enormous economic pressure, which makes the most cost-efficient solution the most attractive one, not the technically “best” coating material; thus, Fe–Cr–C hardfacing alloys are mostly used. In view of the above, four different alloys which are promising for this application were studied with regard to their wear resistance. Each wear mechanism was investigated in a special test tribometer. Fatigue wear caused by multiple impacts and abrasion was tested in the high-temperature continuous impact abrasion test. Materials behaviour in heavy single impacts was evaluated in the single impact test. Characterisation of microstructure and wear behaviour was performed by optical microscopy and scanning electron microscopy. The results obtained with the help of the different measurement techniques were linked and set into comparison to calculate the volumetric wear of the specimen. Aim of this work was to investigate the influence of the material parameters such as macrohardness, hard phase content, microstructure coarseness on the wear resistance in impact loading and abrasive applications at high temperatures. Results also indicate that the matrix ability to bind carbides at high temperature as well as the matrix hardness at high temperatures strongly influence the wear resistance in the different tests. Those material parameters get correlated to the wear rates in different material demands. The test results indicate that at higher temperatures material fatigue becomes a major wear-determining factor which makes the matrix hardness and the matrix ability to bind carbides at high temperatures very important. Especially, in abrasive wear, a certain content of hard phases is also necessary to keep the wear to a lower level. It could also be shown that in impact loading applications, a coarse microstructure is a disadvantage.     

Materials selection to excavator teeth in mining industry

Hard alloys are normally used as materials for excavators teeth in mining industry. In most cases these alloys do not have enough anti-wear properties and coatings are employed as a good alternative. The objective of this work is to test the abrasive wear resistance of several cast irons alloyed with different elements. Laboratory tests based on the ASTM G105-89 standard were compared to tests carried out under real working conditions of excavator teeth in mines.

The experimental results show an acceptable correspondence between laboratory and field tests. To complete the laboratory research, hardness and microhardness measurements and optical micrographs were performed to identify the mechanism of wear. As a result of the experimental work, an economic evaluation of materials was also performed.     

基于摄影测量的乏燃料组件池边检查系统设计

乏燃料组件池边检查是核电站燃料组件必不可少的重要检测项目。针对国内外池边检查技术发展现状的不足,设计了一套基于摄影测量的乏燃料组件池边检查系统,结合接触式与非接触式测量方法,能够快速对组件的整体尺寸及变形进行检测,并能实现对燃料棒长度、直径、棒间隙与氧化膜厚度的检测。针对关键检测元件CCD相机在辐射环境下寿命短的问题,设计了一种耐辐射相机防护装置,通过一次反射减少进入相机的辐射剂量,可用普通CCD相机代替昂贵的耐辐射相机。详细介绍了燃料组件长度、燃料棒与上下管座间隙、格架宽度、组件弯曲及扭曲的视觉测量实现方法,具有理论和实际的可行性,以及重要的应用价值。     

微结构表面展开轮摩擦磨损特性与仿真研究

为了提高AVIKO系列钢球检测系统中展开轮的服役寿命及检测精度，在增大表面摩擦因数的同时减少磨损，将表面微结构应用于展开轮，研究其摩擦磨损特性。利用激光在试件周向外表面加工三种不同直径参数的凹坑微结构，并用自主设计的摩擦磨损试验机进行试验，得到摩擦因数与磨损量，通过数值模拟得到应力分布图和磨损深度图，与光滑表面试件进行对比和分析。结果表明，在干滑动摩擦条件下，微结构表面试件均比光滑表面试件的摩擦因数更大且磨损量更小，微结构表面通过改变应力分布和分散磨损点位置减少了磨损并提高了耐磨性。所建立的数值模拟磨损模型可用于预测微结构表面磨损深度，为微结构表面展开轮的寿命预测提供了基础理论。     

Effect of Carbon Coating on Scuffing Performance in Diesel Fuels

Low-sulfur and low-aromatic diesel fuels are being introduced in order to reduce various types of emissions in diesel engines to levels in compliance with current and impending U.S. federal regulations. The low lubricity of these fuels, however, poses major reliability and durability problem for fuel injection components that depend on diesel fuel for their lubrication. In the present study, the authors evaluated the scuff resistance of surfaces in regular diesel fuel containing 500 ppm sulfur and in Fischer-Tropsch synthetic diesel fuel containing no sulfur or aromatics. Tests were conducted with the high frequency reciprocating test rig (HFRR) using 52100 steel balls and H-13 tool-steel flats with and without Argonne's special carbon coatings. Test results showed that the sulfur-containing fuels have about 20% higher scuffing resistance than does fuel without sulfur. The presence of the carbon coating on the flat increased scuffing resistance in both regular and synthetic fuels by about ten times, as measured by the contact severity index at scuffing. Coating removal was observed to be a necessary, but not sufficient condition for scuffing failure in tests conducted with coated surfaces. The loss of coating from the surface occurred by the two distinct mechanisms of spalling and wear.     

Impact wear resistance of WC/Hadfield steel composite and its interfacial characteristics

A composite coating of WC/Hadfield steel was fabricated through centrifugal casting process to improve the impact wear resistance of Hadfield steel under the conditions of low or medium impact energy. The interfacial structure between WC ceramic particle and the steel matrix was analyzed with scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The impact wear tests at different impact energy were carried out on a MLD-10 type impact wear rig to investigate the wear-resistant properties of three kinds of composites with different WC particle sizes. For comparison, the wear tests of Hadfield steel were also carried out under the same conditions. The results show that WC particles are partially dissolved in the steel during centrifugal casting. The elements W, C and Fe in steel react to form new carbides such as Fe₃W₃C or M₂₃C₆, which precipitate around former WC particles forming fine particles during subsequent solidification. So the interface between WC particles and Hadfield steel matrix is a strong metallurgical bonding. The composite reinforced with smaller WC particles has better impact wear resistance than that of Hadfield steel regardless of impact energy level. Whereas, the composite reinforced with larger WC particles has better impact wear resistance property than that of Hadfield steel when the impact energy is small but an opposite result is gained when the impact energy is higher. So, it is very essential to choose suitable size of WC particles as reinforcement in Hadfield steel to make the composite material more durable in the service conditions.     

Wear with low-lubricity fuels I. Development of a wear mapping technique

Use of low-lubricity fuels such as Jet A-1 is now relatively common in military ground equipment, while severe refinery processes such as hydrotreating are removing reactive components from commercially available diesel fuels. Under critical conditions, durability problems with fuel-sensitive equipment emerge with the use of Jet A-1. Consequently, a bench wear test that accurately reflects the environment within the fuel injection system is needed. However, a better understanding of the primary wear mechanisms present in the fuel injection system and their relationship with potential bench wear tests is first required.

Wear maps have previously been used to study ceramic materials and to define the wear mechanisms in metallic contacts. This technique systematically produces a data base according to a self-consistent methodology and allows the effects of each contact parameter to be precisely described. This paper, the first of two, details the initial test development required to ensure that the data base is completed according to a single well-defined procedure. The second paper, “Correlation between wear maps and pump components”, uses the test methodologies and results developed to elucidate the wear mechanisms present in fuel-lubricated contacts and to assist in bridging the gap between laboratory tests and practical applications.     

介稳奥氏体锰钢耐冲击磨粒磨损性能及磨面组织

利用冲击磨粒磨损试验结合扫描电镜、透射电镜和穆斯堡尔谱等分析手段研究了介稳奥氏体锰钢的耐磨性和磨损机制。结果表明,介稳奥氏体锰钢在某一冲击功下具有最佳耐磨性。在磨损过程中磨面诱发生产两类马氏体,无碳马氏体和合金马氏体。最耐磨时的磨面组织为合金奥氏体和无碳马氏体的混合组织。     

Stiffness analysis of new spacer grid model for metal 3D additive manufacturing

Currently, the Republic of Korea operates a total of 28 nuclear power plants, including those under construction. Although every part of a nuclear power plant is important for safe operation, most attention is paid to the nuclear fuel assembly. The assembly is divided into fuel rods containing fuel assembly fuel and spacer grids. The spacer grid is an important component for core cooling by keeping the fuel rods protected and spaced apart. The spacer grid is produced from a strap bar, which is the base plate, and by spot welding eight edges of the strap bar. However, the spot welding machine has a large capacity and power supply such that the welding quality varies depending on the weld metal and surface conditions. As a result, the welding point may be defective. Therefore, this study aims to analytically verify the performance of a newly designed spacer grid for metal 3D AM as well as present analytical criteria for product development.

     

Hydrodynamic load impact on vibrations of fuel element clusters in water-moderated water-cooled power reactor (VVER) fuel assemblies

For the first time, the hydrodynamic loads on fuel element clusters are specified by measuring pressure pulsations in the gap between external line of the fuel elements and the internal surface of the hexahedral casing in VVER-440 fuel elements in the initial hydrodynamic part close to the lower support grid and in the region of steady flow. It is shown that the formation conditions of vortex flow structure at the fuel assembly input influence substantially the hydrodynamic load levels and spectral distribution and the corresponding vibration response of the fuel element clusters in the heat carrier longitudinal flow. The findings may be used to analyze vibration processes in the fuel assemblies of water-cooled nuclear reactors.