EDF operates a nuclear power generation fleet made up of 56 reactors. This fleet contains 24 reactors designed as double-walled concrete containment building. The inner concrete containment vessel has no metallic liner and is a prestressed reinforced concrete building. The inner concrete containment vessel is designed to withstand a severe accident, in terms of mechanical and sealing behaviour. The tightness of the containment is tested every 10 years, by carrying out a pressurization test and by measuring the leak rate. The leak rate is required to be below a regulatory threshold to continue operation of the concrete containment building for the next ten years. Ageing of concrete due to drying, creep and shrinkage leads to increase prestress loss and then leak rate with time. For some containment buildings, the leak rate gets closer to the regulatory threshold with time, so important coating programs are planned to mitigate and limit the leak rate under the regulatory threshold. Therefore, it is very important for EDF to have a concrete containment building leak rate prediction tool. To address this issue, an important research program around a 1/3 scale concrete containment building mock-up called "VERCORS" have been launched at EDF. The mock-up is heavily instrumented, and its materials (concrete, prestressing cables) have been widely characterized and studied. An important numerical effort has also been made to implement structural computations of the mock-up and to capitalize these computations as well as their post-processing (so as to compare automatically with the monitoring data) in what can be called a digital twin of the mock-up. This digital twin is now used to predict the leakage of VERCORS mock-up before yearly pressure test, and also to optimize the repair programs on the real containments.

Electricite de France (EDF) operates a large fleet of nuclear reactors and is responsible for demonstrating the safety of facilities, including concrete containment buildings (CCB), which are non-replaceable components. The leak-tightness of CCBs is assessed every 10 years during integrated leak-rate tests (IRLT). For double-wall containments, which have no metallic liners, the leak-tightness is strongly influenced by the degree of cracking of concrete and opening of the cracks, which mostly depends on (a) the prestress decrease due to the delayed strains of concrete and to a lesser extent due to relaxation of tendons steel, and (b) the saturation degree of the Powered by Editorial Manager (R) and ProduXion Manager (R) from Aries Systems Corporation concrete wall. Therefore, to optimize the maintenance programs on CCBs, it is important to predict the evolution of drying, creep and shrinkage strains of concrete to be able to correctly assess the pre-stress losses, and finally the air leak-tightness at a structural level during pressure tests or under accidental loadings. To improve our understanding and identify the best modelling practices on this issue, a large experimental program called VERCORS was launched in 2014. VERCORS is a 1/3 mock-up of a 1300 MWe nuclear reactor CCB. It has been widely instrumented, and its concrete thoroughly characterized. A specific attention has been paid to ensure it is consistent with real CBBs features in EDF's nuclear fleet. To complement its internal R&D efforts, EDF decided to associate external partners to this program. One of the means for this is the organization of benchmarks, where all teams are given data and information about the mock-up and are asked to quantitatively predict its behaviour. The present paper reports the organization and findings of the 2nd benchmark which was organized in 2018 and gathered several international teams around the same objective: improve the confidence in the modelling of structural behaviour as well as the leak-tightness of concrete in containment walls under pressure test loading. The benchmark has shown once again that predicting the mechanical and leakage behaviour of containment buildings is a difficult task. The benchmark also yielded interesting information about the possibility to use spatially reduced models to predict the mechanical behaviour and leakage and underlined the fact that more research must be done to better predict the localization of cracks and leakage. Some lessons have been learnt for the next benchmark: EDF will ask to clarify further the calibration methods, will give more data (including drying, creep and shrinkage at different temperatures and moisture measurements in the mock-up), and will help the participants using local leakage data by projecting the raw measurements on a regular grid, so that the local leakage models can be improved.

Préalablement à leur acquisition, les robinets installés dans les centrales nucléaires font l’objet d’une démonstration de leur aptitude à endurer des chocs thermiques d’une amplitude de 255 C, sollicitation majorante de conditions normales d’exploitation. Afin de mieux cerner la validité de démonstrations fondées sur des simulations numériques, nous confrontons l’essai en vraie grandeur d’un robinet à soupape de diamètre nominal 150 mm avec sa simulation numérique. Cet article est focalisé sur le comportement de l’assemblage boulonné du corps et du couvercle, assurant la fonction d’étanchéité, dite externe du robinet. La campagne d’essai, menée sur la boucle CYTHERE d’EDF, comprend plusieurs chocs alternés, froid puis chaud. Le robinet est équipé de 37 thermocouples distribués sur sa hauteur et dans l’épaisseur des pièces. Les douze goujons du couvercle sont équipés de jauges de déformation destinées à mesurer l’évolution de leur tension. L’essai est modélisé par des simulations multiphysiques du robinet complet en 3D, enchaînant les calculs d’écoulement, des champs de température, puis des champs de déplacement et de contraintes dans le robinet. La confrontation des résultats de calcul avec les mesures montre l’importance de la représentation des échanges thermiques résultant de l’écoulement dans les interstices laissés par les jeux fonctionnels entre les pièces internes situées sous le couvercle. Leur prise en compte est déterminante pour calculer les variations de serrage de l’assemblage lors des chocs thermiques.

Prior to their acquisition, valves installed in nuclear power plants must be proof of their ability to endure thermal shocks with an amplitude of 255 C, an upper bound of normal operating conditions. In order to better understand the validity of proofs based on numerical simulations, we confront the full scale test of a valve of nominal diameter 150 mm, with its numerical simulation. This paper is focused on the behavior of the bolted assembly of the body and the bonnet, ensuring the external sealing function of the valve. The test campaign, performed on EDF's CYTHERE loop, includes several shocks, alternatively cold and hot. The valve is equipped with 37 thermocouples distributed along its height and in the thickness of the parts. The twelve studs of the bonnet are equipped with strain gauges designed to measure the evolution of their tension. The test is modelled by multiphysics simulations of the complete valve in 3D, chaining the calculations of the flow, the temperature fields and the displacement, and stress fields in the valve. The confrontation of the calculation results with the measurements shows the importance of the thermal exchanges resulting from the flow in the interstices left by the functional clearances between the internal parts located under the lid. Their consideration is decisive in calculating the clamping variations of the assembly during thermal shocks.

VeRCoRs is a 1/3 scale mock-up of a double walled concrete containment building that has been built by EDF to study the effect of ageing on the loss of pre-stress of the post-tensioned inner wall, and its subsequent effect on the leak rate evolution. This paper briefly introduces the industrial context around the mock-up, and the important effort that has been made on the aspects of (i) material characterization, (ii) instrumentation, (iii) numerical simulations. The solutions chosen to address the issue of dealing with a large amount of experimental data and to compare them with multi-physics simulation are highlighted. Then, it allows to easily compare three successive instances of the modelling of VeRCoRs ageing and to discuss their respective forecasting capacities. In such a predictive approach, thermal and hydric boundary conditions are essentials and their simplification although necessary, should not be too important as not to deviate too much from representativeness.

For a part of the concrete containment buildings of French nuclear power plants, the leak-tightness depends vastly on the pre-stress of the building which can decrease due to concrete delayed strains. Therefore, EDF has developed a large experimental program in the early 2000 in order to study the biaxial creep of concrete. In this paper phenomenological drying, shrinkage and creep model is used to model these biaxial tests. The identification of the parameters of the drying model is first presented. Then, the parameters relative to a parasite mass-loss which occurred in the sealed tests and to the creep model are identified. The model is shown unable to reproduce correctly the biaxial strains observed in the tests. A change of the part of the model accounting for the desiccation creep is therefore proposed. Assuming that the desiccation creep Poisson's ratio is identical to the basic creep and the elasticity ones allows for a better reproduction of long-term multiaxial concrete creep.

In a nuclear power plant, the functionality of valves must be proven under severe operating conditions. Thermal shocks of approximately 225°C are part of the qualification tests, that fulfill this demonstration. Such shocks are studied here on a large globe valve (nominal diameter: 150 mm), experimentally and numerically. The experimental campaign is carried out on the testing loop “CYTHERE” of EDF. 14 thermal shocks are performed successively in pressurized cold and hot water. The tested valve is equipped with 37 thermocouples distributed over its entire height. The 12 studs of the body-bonnet flange are instrumented with strain gauges, which allow the tracking of the clamping forces evolution in the flange. A 3D numerical simulation of such a shock in the valve is performed. It is carried out by a chained multiphysics simulation: fluid, heat conduction and finally mechanics. The mechanical simulation models around 40 different parts with numerous contacts. Two versions of this simulation are presented, the second one takes into account a heat transfer taking place in clearances. A good agreement between simulation and experiment is shown for temperature in the studs. For clamping forces, only the second simulation manage to reproduce tightening variations observed experimentally at the beginning of cold and hot shocks.

Safety and life extensions of Nuclear Power Plants are one of EDF priority. To pursue this objective, an experimental mock-up of a reactor containment building at 1/3 scale was built near Paris. This mock-up named VeRCoRs (VErification Réaliste du COnfinement des RéacteurS) is highly instrumented so that its behavior is monitored from the beginning of the construction. One of the main objectives of the project is to understand leak tightness evolution under aging. Actually, at a 1/3 scale, drying effects will be nine times faster so studies of ageing speeded up. In order to verify this aging speed up, a numerical clone of VeRCoRs is developed to follow its life. This numerical model is also necessary to understand ageing of the mock-up analysing nonmeasurable quantities as stresses. Then, when VeRCoRs clone is validate, a numerical mock-up at real scale is also developed to compare clones with each other. The objective is to draw up an inventory of the prediction capacities of clones on the thermal, drying and mechanical aspects. Numerical results are compared to measurements carried out for now in terms of temperature, deformations and displacements. Finally, a first idea of VeRCoRs representativeness is drawn. Mots clefs : ageing ; prestressed ; numerical clone ; reinforced concrete 23 ème Congrès Français de Mécanique Lille, 28 Août au 1 er Septembre 2017

Iron-based hard-facing alloys, such as Norem, are considered to be good substitutes for Co-based ones where radiological activation is an issue. This work aims at studying residual stresses inside Norem02 Plasma Transferred Arc deposits. A standard test sample used by a valve manufacturer is taken as an example: it consists of a stainless steel (AISI 316L) disk with circular weld bead deposits of Norem hard-facing on both sides. Residual stresses were investigated using neutron diffraction on the ENGIN-X beam line. As expected, experimental results tends to indicate the welding overlap zones to be affected by reheating. Since previous works indicates quite limited metallurgical influence of high temperature re-heating on the deposited material, it is expected to be possible to capture this effect by further finite element modelling of the welding process, in order to study its influence on further in-service behaviour of valves.

Plasma-transferred-arc welded Norem02, an iron-based hard-facing alloy, was characterised. Its microstructure and chemical composition were investigated using optical microscopy, scanning electron microscopy (with electron probe microanalysis), electron backscattering diffraction, and X-ray diffraction. The microstructure of the as-deposit alloy consists of a dendritic austenite structure with ferrite islets at dendrites centres, with an interdendritic eutectic region containing austenite, M7C3 and M23C6 carbides and zones containing Mo-rich precipitates. Tensile behaviour of Norem02 was characterised and completed by dilatometry tests in welding process temperature range. No significant phase transformation was detectable during mechanical testing. Different heat treatment cycles of ageing at high temperatures (until 1100 degrees C) were carried out for different durations. The microstructure of Norem02 heated at 1100 degrees C was not significantly affected by a short time (15 s) treatment whereas changes were observed for longer durations (2 h), although hardness remains almost unchanged.This work tends to demonstrate that for this alloy metallurgical evolution during the welding process has very little influence on mechanical properties. (C) 2011 Elsevier B.V. All rights reserved.

Local approach to brittle fracture for low-alloyed steels is discussed in this paper. A bibliographical introduction intends to highlight general trends and consensual points of the topic and evokes debatable aspects. French RPV steel 16MND5 (equ. ASTM A508 Cl.3), is then used as a model material to study the influence of temperature on brittle fracture. A micromechanical modelling of brittle fracture at the elementary volume scale already used in previous work is then recalled. It involves a multiscale modelling of microstructural plasticity which has been tuned on experimental inter-phase and inter-granular stresses heterogeneities measurements. Fracture probability of the elementary volume can then be computed using a randomly attributed defect size distribution based on realistic carbides repartition. This defect distribution is then deterministically correlated to stress heterogeneities simulated within the microstructure using a weakest-link hypothesis on the elementary volume, which results in a deterministic stress to fracture. Repeating the process allows to compute Weibull parameters on the elementary volume. This tool is then used to investigate the physical mechanisms that could explain the already experimentally observed temperature dependence of Beremin’s parameter for 16MND5 steel. It is showed that, assuming that the hypothesis made in this work about cleavage micro-mechanisms are correct, effective equivalent surface energy (i.e. surface energy plus plastically dissipated energy when blunting the crack tip) for propagating a crack has to be temperature dependent to explain Beremin’s parameters temperature evolution.

Most safety related valves in EDF's nuclear plant must prove their ability to sustain thermal shocks of approximately 240K amplitude. This paper evaluates the simulation of a globe valve tested for thermal shocks. Since the physical test campaign showed inadequate internal sealing, the simulation focuses on the residual deformation of the hard alloy, planar seat, welded on successive body designs. This deformation is the result of the thermal loadings first induced by the welding process, then by fluid flow inside the valve.A chain of 3D simulations successively computes : a welding temperature transient in the body, the resulting strain hardening especially in the seat vicinity; temperature transients in the flow and the valve parts, and the resulting strains in the body causing a bump deformation of the seat surface. This end result agrees with measurements on the tested valve specimen.We show that inaccurate results are obtained on simpler assumptions, such as no welding, and we give insights on the dominant effect of the first hot, cold, hot transient over other profiles. Finally, the agreement we obtain on deformation predictions is toned down by an unsatisfactory sealing prediction, as well as the complexity and duration of the simulation chain compared with physical testing.

Advances in computational mechanics allow to improve safety and reliability of many components used in nuclear applications. Although valves are numerous and critical for safety considerations, many difficulties arise that highly complicate the use of numerical methods for qualification and testing. Thus, valves are mainly tested by experimental simulation in representative conditions, which is sufficient to prevent failures, but do not help to comprehensively analyze mechanisms of failure. This paper attends to demonstrate efforts made at EDF R&D to improve comprehension of valve parts loadings during operation. Thermal shock in a globe valve is represented and modeled using EDF R&D Finite Element Analysis (FEA) code (Code_Aster). Since the globe valve representation implies several parts, attention is first focused on Boundary Conditions (BC) and meshing refinements to allow calculation on a standard engineering workstation. Materials characteristics are mainly given by standard industrial codes and modeling hypothesis relies on elastic behavior of materials. Thermal BC consists in a thermal field beforehand calculated with a coupled approach between EDF computational fluid dynamics code (Code_Saturne) and its thermal code (Syrthes). Simulation results are presented. Attention is then focused on the evolution of the body-bonnet bolted flange joint tightening forces which are simulated during the thermal shock. Choices of modeling are addressed: for example, it is shown in this paper that the contact modeling hypothesis have to be discussed in order to allow good parts behavior description during transient.

The present study describes a multiscale representation of mechanisms involved in brittle fracture of a french Reactor Pressure Vessel (RPV) steel (16MND5 equ. ASTM A508 Cl.3) at low temperatures. Attention will be focused on the representation of stress heterogeneities inside the ferritic matrix during plastic straining, which is considered as critical for further micromechanical approach of brittle fracture. This representation is tuned on experimental results [1]. Modeling involves micromechanical a description of plastic glide, a mean field (MF) model and a realistic three-dimensional aggregates Finite Element (FE) simulation, all put together inside a multiscale approach. Calibration is done on macroscopic stress-strain curves at different low temperatures, and modeling reproduces experimental stress heterogeneities. This modeling allows to apply a local micromechanical fracture criterion of crystallographic cleavage for triaxial loadings on the Representative Volume Element (RVE). Deterministic computations of time to fracture for different carbide sizes random selection provide a probability of fracture for an Elementary Volume (EV) consistant with the local approach. Results are in good agreement with hypothesis made by local approach to fracture. Hence, the main difference is that no phenomenological dependence on loading or microstructure is supposed for probability of fracture on the EV: this dependence is naturally introduced by the micromechanical description.

This paper reports experimental characterisation of stress heterogeneities in a French RPV bainitic steel (16MND5) determined by X-Ray diffraction during in-situ tensile testing at low temperature (until –150°C). Results are compared successfully to simulation results, obtained by post-processing of Finite Elements computations of realistic 3D aggregates.

A new die design has been proposed for processing of materials using equal channel angular extrusion (ECAE). The proposed design is an improvement over the existing ones and has an additional attribute to process materials at high temperatures (up to 500°C). The design has been tested for different materials at different temperatures and it has shown the benefit of reducing the load requirement during pressing. This benefit is attributed to the reduction of friction and appropriate tooling.

A methodology is proposed in this paper to compute local probability of fracture of a representative volume element from microstructural and fractographic information. A microscale approach is reported for cleavage fracture modelling of bainitic reactor pressure vessel steel. A fine metallographic and micro-mechanical characterization has been performed. Morphological information has been used to generate realistic three-dimensional (3D) aggregates and a suitable calibration process has been set up to identify an accurate crystal plasticity model for A508 bainitic steel. Simulation results have been successfully compared with macroscopic stress-strain curve. Taking benefits of a fine mechanical description of local stress and strain fields in aggregates, a post-processing has been implemented to compute stress to failure values, for several realizations of an experimental carbides sizes distribution. A cumulative probability of fracture is proposed for this steel.

A flow line function is proposed to describe the material deformation in equal channel angular extrusion for a 120 degrees die. This new analytical approach is incorporated into a viscoplastic self-consistent polycrystal code to simulate the texture evolution in route A extrusion of copper and compared to experimental textures as well as to those corresponding to simple shear. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.