Assessment of structural and transport properties in fibrous C/C composite preforms as digitized by X-ray CMT. Part I : Image acquisition and geometrical properties

International audienceRaw and partially infiltrated carbon-carbon composite preforms have been scanned by high-resolution synchrotron radiation X-ray CMT. 3D high-quality images of the pore space have been produced at two distinct resolutions and have been used for the computation of geometrical quantities : porosity, internal surface area, pore sizes, and their distributions, as well as local and average fiber directions. Determination of the latter property makes use of an originalalgorithm. All quantities have been compared to experimental data, with good results. Structural models appropriate for ideal families of cylinders are shown to represent adequately the actual pore space

Direct 3D microscale imaging of C/C composites with computed holotomography

International audienceAs part of the modelling of CVI (Chemical Vapor Infiltration), one of the processing techniques of Carbon - Carbon composites, a better understanding of the relation between fiber architecture and transport properties is required. An excellent starting point for the pore-scale modeling of heat and gas transport is the acquisition of 3D images of the porous media through Computed Micro Tomography at various densification stages. Due to the low X-ray absorption rate of light materials such as carbon, a poor image contrast is obtained with absorption tomography. On the other hand, phase contrast imaging is readily feasible using the coherence properties of modern synchrotron beams. Holotomography has been performed on these materials and it provides quantitative density images where fibers and pyrocarbon matrix deposit are easily distinguishable. Such images are appropriate for the pore-scale computation of many effective transport properties

Contribution of X-ray CMT and image processing to the modelling of pyrocarbon Chemical Vapour Infiltration

International audienceThe Chemical Vapour Infiltration (CVI) process is used to fabricate the pyrocarbon matrices of C/C composites. This process involves complex physico-chemical phenomena such as the transport of precursor, carrier, and by-product gases in the reactor and inside a fibrous preform, heat transfer, chemical reactions (pyrolysis and deposition), and the structural evolution of the preform. It is able to provide high-quality materials because the processing conditions are rather mild with respect to the fibres; however it is expensive and sometimes difficult to optimize. This process has been the object of extensive modelling efforts, because of imperative optimization needs. The present work presents an approach suited to the exploitation of computerized microtomographs of C/C composites, which features image acquisition, computation of geometrical and transport properties, and infiltration modelling, as applied to the infiltration of needled carbon fibre fabrics. Another application to the reinforcement of carbon foams is also presented, as an example of inserting this approach in a global modelling strategy

Study of zircaloy-4 cladding air degradation at high temperature

Prix de la Meilleure Communication EuropéenneInternational audienceZircaloy cladding, providing the first containment of UO2 fuel in Pressurised Water Reactors, can be exposed to air during accidental situations. This might occur during reactor operation (in case of a core meltdown accident with subsequent reactor pressure vessel breaching), under shutdown conditions with the upper head of the vessel removed, in spent fuel storage pools after accidental loss of cooling or during degraded transport situations. The fuel assemblies inadequately cooled, heat up and as a result, corrosion of Zircaloy claddings takes place. This paper is devoted to the kinetic analysis of Zy4 corroded at 850°C in 20% oxygen - 80% nitrogen partial pressure atmosphere to support the comprehension of the degradation mechanisms involved during the post-transition stage

Modelling Chemical Vapour Infiltration in C/C composites: numerical tools based on µ-CT images

ISBN 978-3-00-032049-1International audienceIn the production of high-quality Ceramic-Matrix Composites, matrix preparation is often made by Chemical Vapor Infiltration (CVI), a process which involves many phenomena such as gas transport, chemical reactions, and structural evolution of the preform. Control and optimization of this high-tech process are demanding for modeling tools.In this context, a numerical simulation of CVI in complex 3D images, acquired e.g. by X-ray Computerized Microtomography, has been developed. The approach addresses the two length scales which are inherent to a composite with woven textile reinforcement (i.e. inter- and intra-bundle), with two numerical tools.The small-scale program allows direct simulation of CVI in small intra-bundle pores. Effective laws for porosity, surface and transport properties as infiltration proceeds are produced by averaging. They are an input for the next modeling step.The second code is a large-scale solver which accounts for the locally heterogeneous and anisotropic character of the pore space. Simulation of the infiltration of a whole composite material part is possible with this program.Validation of these tools on test cases, as well as some examples on actual materials, are shown and discussed

The EC MUSA project on management and uncertainty of severe accidents: Main pillars and status

In the current state of maturity of severe accident codes, the time has come to foster the systematic application of Best Estimate Plus Uncertainties (BEPU) in this domain. The overall objective of the HORIZON-2020 project on “Management and Uncertainties of Severe Accidents (MUSA)” is to quantify the uncertainties of severe accident codes (e.g., ASTEC, MAAP, MELCOR, and AC2) when modeling reactor and spent fuel pools accident scenarios of Gen II and Gen III reactor designs for the prediction of the radiological source term. To do so, different Uncertainty Quantification (UQ) methodologies are to be used for the uncertainty and sensitivity analysis. Innovative AM measures will be considered in performing these UQ analyses, in addition to initial/boundary conditions and model parameters, to assess their impact on the source term prediction. This paper synthesizes the major pillars and the overall structure of the MUSA project, as well as the expectations and the progress made over the first year and a half of operation

Effective heat conductivities of partly infiltrated fibrous media

The present work has been motivated by the context of feeding a global model of thermal-gradient chemical vapour infiltration process for the fabrication of fibre-reinforced ceramic matrix composites: the heat conductivity has to be known in the material as a function of the pore volume distribution, at different stages of infiltration. Since the composite preform is made of woven fibres, the medium is highly anisotropic: the heat conductivity has a tensorial character which has to be retrieved. We propose an approach based on a double up-scaling. First, the fibre-scale effective longitudinal and transverse conductivities are computed for several volume fractions of matrix and fibres, using a periodic unit cell as a computational domain. The second change of scale takes precisely into account the actual architecture of the composite thanks to 3D X-ray tomographic imaging. The 3D blocks are processed in order to retrieve the local orientation of the fibres in various sub-domains; the local heat conductivities are then affected to the sub-domains, and a volume averaging procedure is applied for the determination of the global effective conductivity. Several application examples are presented and discussed

Severe accident code-to-code comparison for two accident scenarios in a spent fuel pool

International audienceSpent fuel pools (SFPs) are large structures equipped with storage racks designed to temporarily storeirradiated nuclear fuel removed from the reactor. SFP severe accidents have long been considered ashighly improbable since the accident progression is slow (in comparison with reactor core accidents)and let time to corrective operator actions. However, the accident at the Fukushima Dai-ichi NuclearPower Plants has highlighted the vulnerability of nuclear fuels that are stored in SFPs in case of prolongedloss-of-cooling accidents and consequently renewed international interest in the safety of SFPs. In thiscontext, the AIR-SFP project, funded by the Euratom 7th FP in the frame of the NUGENIA+ project, waslaunched in May 2015 with 15 participants. One of the objectives was to assess the applicability ofSevere Accident (SA) codes, which were initially developed for reactor applications, to the calculationof transients in SFPs. To reach this objective, a benchmark, including a criticality risk assessment, was carriedout. The degradation progression was computed by 14 participants with 6 different SA codes and 5have participated to the criticality risk assessment. Main results are presented as well as conclusions thathave been drawn concerning SA codes readiness to address these ‘‘beyond-scope” scenarios. 2018 Elsevier Ltd. All rights reserved

Etude 3D de préformes fibreuses : interaction entre phénomènes physico-chimiques et géométrie

Thermostructural composites are characterized by their ability of operating under high mechanical stresses and high temperatures (above 1000 ʿC), such as in spatial propulsion systems or aircraft brake disks. Carbon-carbon composites (C/C) belong to this family of materials, made of fibers linked together by a ceramic matrix. They can be manufactured by CVI (Chemical Vapor Infiltration). This processing consists in densifying a heated fibrous preform by the chemical cracking of a vapor precursor of the matrix material inside the porosity of the preform. The final quality of the composites relies on the CVI densification phase, the optimization of which is a key issue. It is thus required to assess, at initial stages and during the densification, the geometrical characteristics of the preform (porosity, internal surface area, distribution of pore sizes), its gas transport properties (gas diffusivity, permeability) and its heat transfer properties (thermal conductivity). This study aims at determining these properties from 3D images of a real C/C preform at different stages of densification. The images have been obtained by X-ray microtomography at ESRF*. An original method has then been developed to compute the geometrical characteristics and the transport properties of this material. The procedure has been validated by comparing, at each step, the results obtained numerically with experimental data.Les composites thermostructuraux sont des matériaux destinés à des applications qui requièrent des propriétés mécaniques élevées à des températures supérieures à 1000 ʿC, comme par exemple dans des systèmes de propulsion spatiale ou de freinage aéronautique. Les composites carbone-carbone (C/C) font partie de ces matériaux, constitués d'un renfort fibreux et d'une matrice céramique. Pour les élaborer, l'un des procédés est la CVI (Chemical Vapor Infiltration). Elle consiste à placer une préforme fibreuse poreuse dans une enceinte, à l'intérieur de laquelle est injecté un gaz. Le gaz peut mener à un dépôt par réactions chimiques, dans des conditions déterminées, dans la porosité de la préforme et ainsi la densifier. La qualité finale des composites dépend, entre autres, de la phase de densification par CVI, qu'il est donc primordial d'optimiser. Pour cela, il faut connaître, au stade initial et en cours de densification, les caractéristiques géométriques de la préforme (porosité, surface volumique, distribution de la taille des pores), ses propriétés de transport de gaz (diffusivité de gaz, perméabilité) et ses propriétés de transfert de chaleur (conductivité thermique). C'est la détermination de ces propriétés, calculées à partir d'images 3D de la préforme fibreuse réelle en cours de densification, qui a fait l'objet de cette étude. Les images ont été obtenues par microtomographie aux rayons X à l'ESRF*. Une méthode originale a ensuite été développée pour calculer les caractéristiques géométriques et les propriétés de transport de ces matériaux. La procédure a été validée en comparant, à chaque étape, les résultats obtenus à des mesures expérimentales