Littérature scientifique sur le sujet « Assembly bowing »

AbstractWeld bowing distortions are detrimental to the assembly process, where laser process parameters such as laser power, welding speed, defocusing distance and gas flow rate play a significant role in determining the weld bowing distortion. Herein, weld bowing distortions in 1-mm-thick AA5052 aluminum were measured by the digital image correlation technique following laser welding. Two mathematical response models were developed to predict the laser weld bowing distortion according to the central composite rotatable design method. The optimized process parameters for minimum bowing distortion were obtained, and the influence of the laser process parameters on the weld bowing distortions was found.

Fuel assembly bowing is a known phenomenon observed in many PWR reactors all over the world. The phenomenon is relevant to safety as it can lead to increased water gaps between assemblies which results in higher pin peaking factors. The goal of the present study is to assess the effect of assembly bowing not only for stead-state nominal conditions but also during a transient. The selected transient is the loss of one reactor coolant pump as it can be limiting especially regarding the Departure from Nucleate Boiling (DNB) safety criterion. This study focuses on an extreme case where the bowing is simulated in the core hot assembly by keeping the water gap constant over the whole core active length. The resulting cross-sections and form functions obtained from a 2d infinite lattice model are used in the nodal diffusion code DYN3D applying its pin-by-pin reconstruction method. For the transient simulation, DYN3D is coupled with the thermal-hydraulics subchannel code CTF on the SALOME platform. Several modelling options are compared: nominal geometry for neutronics and thermal-hydraulics (TH); mixed: neutronics with increased water gap, TH with nominal geometry; and increased water gap for both neutronics and TH. The results confirm that the increased water gap should be considered in both models in order to reduce the conservatism.

An analytical model is developed for determining the bowing of a component mounted on a printed wiring board (PWB) that is subjected to a bending moment. The model assumes a uniform elastic attach, between the component and the board. The elastic attach is assumed to transmit axial forces and restrain cross-sections of the component against rotation. The closed form solution to the beam equations directly determines the bowing of the component and the board. The solution is then used for computing the forces and moments, and hence, stresses in the leads that can occur in static or vibrational loading of a PWB/component assembly. The present analysis applies to electronic components with uniformly distributed leads in an array format, such as some PGA components, or to the class of components with parallel rows of leads such as a DIP or a SOIC. To demonstrate the solution and whether or not the rotational stiffness of the component leads needs to be considered, three different types of packages are analyzed.

Deformations of nuclear fuel assemblies have been observed in nuclear power plants since the mid-90s. Such deformations are generally called bowing effects. Fuel assemblies under high irradiation undergo growth and creep induced by high loading forces and low skeleton stiffness of the assemblies which gives permanent deformations and modifies moderation regions. Hence, giving an unpredicted neutron flux spectrum, power distribution, and isotopic concentrations in the burnt fuel. The aim of this thesis is to study the effects of local fuel bowing in terms of power distribution and isotopic composition changes through simulations of the reactor core.  The reactor is simulated with realistic bowing maps and previous deterministically simulated realistic fuel data from a present reactor by deploying the Monte Carlo method using the nuclear reactor code Serpent 2. Two subparts of a full reactor core with fuel from separate fuel cycles are investigated in 2D using burnup. To quantify the impact of the bowing, the change in power distribution and the induced isotopic composition change are calculated by a relative difference between a nominal case and a simulation with perturbed fuel assemblies. The results are presented in colormaps, for visualization. The isotopic composition for U235, U238, Pu239, Nd148, and Cm244 are investigated. Also, statistical uncertainty estimations in the composition of the depleted fuel are done by multiple calculations of the same geometry while changing the seed of random variables in the Monte Carlo calculation. The mean value and the standard deviation in the mass density of U235 and Pu239 are calculated for two pins together with histograms with a normal fit for each case to clarify the mathematical distribution of the calculations.  The simulations performed in this thesis have detected clear impacts of the reactor behavior in terms of power distribution and isotopic composition in the burnt fuel introduced by the bowing. Assembly perturbations of about 10 mm may locally introduce a 10 % relative difference in power density and U235 content between the nominal and the bowed case at 15 MWd/kgU burnup. The power and the isotopic composition changes agree with expectations from the bowing maps.

Les assemblages de combustible nucléaire dans le cœur d’un réacteur à eau pressurisée(REP) sont immergés dans un écoulement axial. Cet écoulement exerce une charge hydrodynamique sur les assemblages et est responsable de leur couplage et de leurs vibrations. De plus, lors d’un tremblement de terre ou d’un événement LOCA (Loss Of CoolantAccident), les assemblages de combustible sont soumis à de fortes oscillations. La charge hydrodynamique peut déformer les assemblages, générant une déformation arquée, tandis que des oscillations plus fortes, comme lors d’un événement sismique,peuvent être à l’origine d’impacts sur les assemblages. Afin de garantir l’intégrité et la sûreté du cœur du réacteur, les industries nucléaires souhaitent améliorer la connaissance phénoménologique des interactions fluide-structure à l’intérieur du cœur d’un réacteur à eau pressurisée. Les ingénieurs ont donc besoin de modèles numériques pour le comportement mécanique et de campagnes expérimentales pour les valider et définir leurs limites.L’étude présentée dans ce document est principalement divisée en trois campagnes expérimentales visant à étudier : les effets d’oscillation de l’assemblage dans un fluide au repos, les phénomènes de traînée sur les assemblages de combustible en régime permanent sous un écoulement et le comportement des oscillations des assemblages lorsqu’ils sont immergés dans un écoulement. Deux installations expérimentales sont utilisées :SBF (Shaking Bundle Facility) et Eudore. SBF accueille un assemblage fictif de pleine hauteur soumis à un écoulement axial sur une table vibrante. Grâce à des techniques optiques, le champ de vitesse du fluide et le mouvement de l’assemblage peuvent être mesurés. L’installation Eudore utilise trois assemblages réduits en ligne, soumis à un écoulement axial, avec la possibilité d’appliquer une excitation sismique à l’ensemble de la section d’essai. L’instrumentation développée sur Eudore permet de mesurer les déplacements des assemblages, le champ de vitesse du fluide et les forces d’impact.Les expériences réalisées sur Eudore sont simulées à l’aide d’un outil de calcul numérique développé au CEA, nommé FSCORE, basé sur une approche en milieu poreux. Cette approche permet d’accéder à un modèle de fluide équivalent et à un modèle de structure équivalent définis sur l’ensemble du domaine à partir de l’intégration spatiale d’équations locales. Les équations de mouvement du fluide équivalent et de la structure équivalente sont établies séparément, pour fournir un modèle couplé fluide-structure prenant en compte les contacts entre les assemblages.A l’aide d’un modèle analytique, les résultats expérimentaux obtenus sur Eudore sont utilisés pour retrouver le coefficient de traînée présent dans FSCORE. Les résultats expérimentaux et numériques sont largement discutés et montrent un bon accord
Nuclear fuel assemblies in Pressurized Water Reactor (PWR) core are immersed in anaxial flow. This flow exerts a hydrodynamic load on the assemblies, and it is responsible fortheir coupling and vibrations. Furthermore, during an earthquake or a LOCA event (LossOf Coolant Accident), fuel assemblies are subjected to strong oscillation amplitudes. The hydrodynamic load can deform the assemblies, generating assembly bow, while strongeroscillations, such in a seismic event, can be responsible for assemblies impacts. In order to ensure the reactor core integrity and safety, nuclear industries want to improve thephenomenological knowledge of fluid-structure interactions inside a PWR core. Thus, engineersneed numerical models for mechanical behavior of fuel assemblies and experimentalcampaigns to validate them and define their limits.The study presented in this document is mainly divided in three experimental campaignsand aim to investigate: the assembly oscillation effects in fluid at rest, the dragphenomena on steady state fuel assemblies under a flow and the assemblies oscillationsbehavior when immersed in a flow. Two experimental facilities are used: SBF (ShakingBundle Facility) and Eudore. SBF hosts one full-height surrogate assembly under axialflow on a vibrating table. By using optical technique, the velocity field of the fluid andassembly motion can be measured. Eudore facility uses three reduced assemblies in line,under axial flow with the possibility of applying seismic excitation to the entire test section.The instrumentation developed on Eudore makes it possible to measure the displacementsof the assemblies, velocity field of the fluid and the impact forces.The experiments performed on Eudore are simulated with a numerical calculation tooldeveloped at CEA, named FSCORE, based on a porous medium approach. This approachprovides access to an equivalent fluid model and an equivalent structure model defined overthe entire domain from the spatial integration of local equations. The equations of motionof the equivalent fluid and of the equivalent structure are established separately, to providea coupled model taking into account the contacts between assemblies.With the help of an analytical model, the experimental results obtained on Eudoreare used to retrieve the drag coefficient present in FSCORE. Experimental and numericalresults are widely discussed and show good agreement

World War II dashed the hopes and dreams of many, among them Richard Field. As President of the American Geophysical Union, Field was to host the International Geological Congress when it met in Washington, D.C. at the end of the summer of 1939, an event Field was greatly anticipating. Scientists were expected from around the globe, and the permanence —or nonpermanence —of ocean basins was high on Field’s list of topics for discussion. But as the meeting was about to begin, Adolf Hitler’s armies invaded Poland; many delegates turned around mid-voyage, and others who had already arrived in Washington quickly returned home. The following year William Bowie died; two years later Charles Schuchert died at the age of eighty-four, and Field was involved in a near-fatal car crash which effectively ended his scientific career. Bowie’s and Field’s scientific goals would be realized, however, albeit not by them. Together, they had assembled an advisory committee for gravity studies that included five subsections —navigation, geophysics, tectonics, oceanology [sic], and marine microbiology—with prominent members from acaclemia, and industry in the United States and abroad and from the U.S. Navy. Their ambitions went beyond gravity: Bowie and Field hoped to foster a global science of geophysics and oceanography to explore the three-fifths of the earth that scientists had scarcely visited by enlisting the material and financial support of the U.S. Navy and other navies. The U.S. Navy, for its part, was making plans to enlist geophysicists, both figuratively and literally. When war broke out, Harry Hess joined the naval reserve, as did many other young and aspiring geophysicists and oceanographers. In 1941, Hess was called to active duty and became the captain of an assault transport, the USS Cape Johnson. Among the ship’s tasks was the echo sounding of the Pacific basin; Hess subsequently became famous for the discovery of flat-topped sea-mounts, which he named guyots after the first professor of geology at Princeton, Arnold Guyot. I less published this discovery, but much geophysical work done during the war, and after, was classified.

The reactivity of a fast spectrum nuclear reactor core is sensitive to changes in the fuel position. The core is formed by a hexagonal array of fuel assemblies which contain the fuel elements. The main structural components of the assemblies are thinwall hexagonal ducts. The fuel elements represent negligible stiffness in the fuel assembly compared to the ducts such that the ducts determine the location of the fuel. Thermal gradients across the fuel assembly cross sections create differential thermal expansion which causes the assemblies to bow. This bowing is proportional to the power to flow ratio such that it can become an important part of the reactivity change during reactor transients such as during reactor start-up, transient overpower (TOP), and unprotected loss of flow without scram (ULOF). In addition to these short term transients, thermal and fast neutron flux gradients within the core cause the assembly ducts to swell and bow over time due to irradiation creep and swelling. These latter effects produce permanent bowing of the ducts which change the reactivity over time and more importantly affect the mechanical forces required to refuel the core as the bowing is greater that the duct-to-duct clearance. Understanding these bowing responses is important to the understanding of both the transient behavior of a fast reactor as well as the refueling loads. Through proper design of the core restraint system, the bowing response can be engineered to provide negative feedback during the above mentioned transients such that it becomes part of the inherent safety of a fast reactor. Similarly, the opposing effects of creep and swelling can be manipulated to reduce the permanent core bowing deformations. We provide a discussion of the key features of analyzing and designing a core restraint system and provide a brief survey of the past work.

In sodium-cooled fast reactor (SFR), thermal gradient is the paramount factor of assembly transient bowing, that may cause great reactivity change, accelerate wrapper vibration wear, hindering the motion of control/shutdown rods, or worse yet, threatening the integrity of assemblies. However, because of the complexity of multi-assembly contact and interaction problem, it is difficult to assess the impact of core deformation on reactor performance safety. The Core Assembly Deformation Test Facility (CADTF) is designed to perform a series of thermal bowing tests by Xi‘an Jiao Tong University (XJTU) to investigate the core deformation behaviors under thermal gradient. In this paper, a finite element model was established to simulate the mechanical response of single assembly under different flat-to-flat thermal gradient. The single assembly restrained bowing test performed in CADTF is chosen to validate the model. In the model, the measured temperature distribution as well as temperature-dependent elastoplastic and thermal expansion properties were taken into consideration. To ensure the model reliability, iterative computation is conducted by adjusting the friction coefficient of the load pads to match the calculated and measured contact force. According to the results, it can be seen that the three-dimensional displacement of assembly shows relatively good agreement with the experimental data. Therefore, it can be concluded that the model is capable of performing core deformation analysis for SFR.

A sodium cooled fast reactor is designed to attain a high burn-up core in a feasibility study on commercialized fast reactor cycle systems. In high burn-up fuel subassemblies, deformation of fuel pin due to the swelling and thermal bowing may decrease flow rate via change of flow area in the sub-assembly and influence the heat removal capability. A 2.5 times enlarged 7-pin bundle water model was applied to investigate the influence of pin bowing and wrapping wire. The test section consisted of a hexagonal acrylic duct tube and fluorinated resin pins which had a refractive index (1.336) nearly the same with water (1.333) and a high light transmission rate (93%). This enabled to visualize around the central pin through the outer pins. In this experiment, the bowing deformation of a fuel pin was simulated by compressing the central pin in the axial direction. Velocity distribution was measured by using Particle Image Velocimetry (PIV) in sub-channels around the central pin in reference and deformation conditions. The Laser Doppler Velocimetry (LDV) was used for the cross-check, the velocity distributions were consistent between PIV and LDV results. The measured velocity distribution around the wrapping wire revealed that the wire influenced to the velocity and RMS in the wide region above the pin surface.

The poolside examination is demanded essentially for the development of an advanced fuel, because it provides data about in-reactor fuel behaviors as well as product performance. Therefore, all the results of examinations are transmitted back to design groups, and evaluated with the design models and codes applied new fuel development. In general, PSE is performed a variety of examinations in assembly and single rod state, such as visual inspection, fuel assembly length growth, bowing and twist, rod-to-rod spacing, spacer grid width, fuel rod diameter, and fuel rod oxide thickness at the end of each cycle of irradiation during plant outage periods. After the completion of fuel life, selected rods are extracted from the fuel assembly for individual fuel rod measurement. In this paper, the techniques of PSE applied an advanced pressurized water reactor fuel, PLUS7™ for Optimized Power Reactors (OPR 1000s) in Korea are described.

Abstract Fuel assemblies’ deformation is an industrial issue that has been first reported in the 90’s. This phenomenon has originally been pointed out for being the explanation of IRI (incomplete rod cluster insertion). Recently, fuel assembly bowing has regained attention for its impact over several core’s management issues, including core neutronics. When deformation occurs, it tends to alter bypasses geometry around the affected fuel assembly. The water gaps’ thicknesses along the assembly’s height does not match the nominal value anymore. As a result, spacer grids can get closer of farther to the surrounding ones. The redistribution between the bypasses and the grid is then involved, depending on the bypasses’ thicknesses and the grid geometry. This unfolding effect entails differences in pressure laterally along a grid, which thus brings about a lateral hydraulic force exerting on the grid. The following paper presents a method to esteem this redistribution thanks to an hydraulic network. Hydraulic resistances can be set up according to the bypass thickness. As a result, both pressure and volumetric flow rates can be calculated to further estimate lateral forces. The approach has been validated with both CFD simulations and an experimental mock-up.

Morton Effect is a known rotordynamic phenomenon associated with fluid film bearings, where viscous heating creates a uni-directional temperature rise in the bearing journal, leading to thermal growth and subsequent bow of the rotor. This results in an unbalance distribution that exacerbates the original unbalance, increasing the heating and bow, resulting in an unstable, or self-amplifying, response. Heretofore, this phenomenon has only been reported in fluid film bearings, as it is traditionally associated with the viscous heating from shearing of the oil. There is also similar behavior associated with phenomenon named the Newkirk Effect where the same mechanics of heating, thermal growth and bowing of a shaft occurs, but the source of heating is a labyrinth rub. This paper describes an incident where such a series of interactions was experienced with a rolling-element bearing (REB). Instead of being driven by viscous shearing of the oil through the minimum film clearance, the uni-directional heating of the rotor results from unbalance and the sliding or dynamic friction of the balls on the inner race or rub of a near-by seal. Rotordynamic analysis was used to derive a correlation between measured vibration levels and temperature rise resulting in predictable bowing of the shaft in a 45,000 RPM fixed speed 250 kW microturbine having an overhung rotor supported by two rolling element bearings. Vibration response was measured with proximity probes along the rotor and temperature predictions were verified against physical evidence in the bearing races. The information gained in this effort was used to establish assembly tolerance and vibration acceptance criteria for factory testing of the turbine. This behavior has internally been described as “REB Morton Effect.” The paper describes the vibration investigation; bearing evaluation; rotordynamic modeling, analysis and verification; design and assembly corrections, and subsequent testing.