Multiscale modelling for fusion and fission materials: the M4F project

The M4F project brings together the fusion and fission materials communities working on the prediction of radiation damage production and evolution and its effects on the mechanical behaviour of irradiated ferritic/martensitic (F/M) steels. It is a multidisciplinary project in which several different experimental and computational materials science tools are integrated to understand and model the complex phenomena associated with the formation and evolution of irradiation induced defects and their effects on the macroscopic behaviour of the target materials. In particular the project focuses on two specific aspects: (1) To develop physical understanding and predictive models of the origin and consequences of localised deformation under irradiation in F/M steels; (2) To develop good practices and possibly advance towards the definition of protocols for the use of ion irradiation as a tool to evaluate radiation effects on materials. Nineteen modelling codes across different scales are being used and developed and an experimental validation programme based on the examination of materials irradiated with neutrons and ions is being carried out. The project enters now its 4th year and is close to delivering high-quality results. This paper overviews the work performed so far within the project, highlighting its impact for fission and fusion materials science.This work has received funding from the Euratom research and training programme 2014-2018 under grant agreement No. 755039 (M4F project)

Les masses de sédiments marqués à injecter dans une expérience de traceurs radioactifs en sédimentologie dynamique

Lorsqu'un détecteur se trouve en présence d'une activité répartie en plusieurs entités radioactives distinctes, le taux de comptage qu'il fournit est fonction du nombre d'entités et de leur répartition dans l'espace entourant le détecteur. Les auteurs se proposent de donner une théorie générale de la variation de réponse du détecteur, et de la probabilité associée à chaque valeur de cette réponse, quand varient à activité constante, le nombre et la disposition des entités dans un volume donné. Ils appliquent la théorie ainsi développée au problème de la détection d'un nuage de traceurs dans les expériences de sédimentologie, menées au moyen de traceurs radioactifs. Dans un premier temps, le calcul est fait avec un traceur donné (192Ir) réparti à la surface du lit, pour un sable de granulométrie uniforme, pour un détecteur donné de réponse géométrique connue ; les auteurs étudient d'abord une détection statique du nuage (c'est-à-dire faite point par point) puis une détection dynamique (sonde traînée sur le lit). Ils obtiennent dans ces deux cas la masse minimale de traceur par unité d'activité qu'il est nécessaire d'immerger afin d'obtenir en limite du nuage radioactif, une fluctuation du taux de comptage due à la fluctuation de position des grains radioactifs, égale à 30 % du taux de comptage moyen. Puis les auteurs cherchent à généraliser les résultats précédents pour pouvoir déterminer cette masse minimale, quand on utilise d'autres détecteurs, d'autres traceurs, d'autres granulométries, en tenant compte du mode de décodage de l'information (numérique ou analogique) et de l'enfouissement du traceur. Ils obtiennent ainsi des formules générales tenant compte des paramètres précédents et permettant de déterminer les quantités de sable à immerger en fonction des paramètres de mesure et des caractéristiques du sédiment. Pour terminer, on examine le cas particulier de la prise des carottes. Les auteurs montrent qu'une détection dynamique, technique qu'ils préconisent, nécessite une masse de traceurs par unité d'activité dix fois plus faible qu'une détection statique, celle-ci à son tour demandant dix fois moins de traceurs qu'une étude avec des carottages

Les méthodes de bilan des taux de comptage de traceurs radioactifs appliquées à la mesure des débits massiques de charriage

The present scope of quantitative sediment bed load measurement methods is briefly reviewed (semi-empirical formulae and measurements in situ) also the principles of quantitative methods using radioactive elements (time integration methods : total counts and continuous dilution, spatial integration). A new method is presented, in which the integral (N = ρρ nds) of the number of impacts per second n given by the pick-up is calculated for a radioactive cloud representative of the particular form of motion under investigation. The operation is referred to as the "count rate balance" method of measurement. The bed load masse flow is then found by relating N to the depth of the mass in motion E, giving the following relationship : Q = ρ x l x Vm x E where Vm : mean velocity; l: width of moving mass ; ρ : bulk density of the sediment. N is related to E by the pick-up's response function f (z) with depth and the tracer's distribution function Γ (z) with depth. Hence: N = E 0 Γ (z) f (z) dz It is shown that, if ƒ(z) is a linear function, the mean buried depth of the tracer Zm, l'an be found for any value Γ (z). Such detectors have been successfully obtained for 192Ir and l82Ta down to a buried depth of 10 cms. Furthermore, by using two pick-ups with different linear response functions, an exact value can he found for Vm, independently of buried depth effect. The use of more than two pick-ups with different response functions yields additional information, which is discussed. It is found that, for all the assumptions made regarding Γ (z), the rate of flow can be calculated to within 25 %. Where the response function is exponential (which the authors observe experimentally), the rate of flow is known in terms of a factor β varying with Γ (z), ƒ (z) and E. The rate of flow is then calculated for all the assumptions made with an accuracy varying from within a few per cent for moving masses of little depth to within about 30 per cent for mass depths of up to 40 cms. The use of several pick-ups yields additional information which improves accuracy. It is also shown that the use of two pick-ups can be of considerable assistance where time Integration methods are to be applied. Result of two river experiments are described, as follows : 1) On the Stung Sen in 1963. Although the method had not been finalized at the time of obtaining the data, the rate of sediment flow during the river flood was estimated to amount to 800 tons ± 200 tons, which agreed with previous estimates. 2) On the Sienne, a coastal river in the Cotentin peninsula. The rate of flows was estimated to amount to 21 tons ± 5 tons. A systematic possible error study was done for this application, giving special consideration to errors which cannot be dealt with by calculation, for example errors in plotting field measurement data or in estimating mean count rate between two isoactives. These errors were found to be small. As these recommended methods do not require any more special equipment than is used in conventional radioactive tracer work and as they are merely a different way of interpreting measured data, the authors urge anyone using them to systematically work out "count rate balance sheers" from the results obtained, and to try to deduce the depth of moving sediment and possibly the rate of flow from them

Simulation of stage I fatigue crack gowth in a polycrystal through coupled FEM and discrete dislocation dynamics

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Tension–torsion ratcheting tests on 9Cr steel at high temperature

Because of their good thermo-physical properties, 9Cr steels are being considered as structural materials for fusion and fourth generation nuclear reactor components.However, these materials feature a particular behaviour in strain hardening, they present a very limited ductility and soften under cyclic load. This study contributes to an evaluation of the extent to which component design rules written for materials with a greater ductility and which have a better behaviour under cyclic load, are applicable to this material.This article investigates ratcheting in cyclic torsion–tension tests made on 9Cr steel specimens at 450 °C and 550 °C. The results shows that the RCC-MRx efficiency diagram does not form a lower envelope representative of the worst results for tests made using 9Cr steel. Use of this diagram in its present form would lead to estimated cumulative strains lower than experimentally observed strains. This work is a first step towards the definition of a modified efficiency diagram suited for 9Cr steel

Mechanical and microstructural stability of P92 steel under uniaxial tension at high temperature

International audience9-12%Cr creep-resistant ferritic-martensitic steels are candidates for structural components of Generation IV nuclear power plants. However, they are sensitive to softening during low cycle fatigue and creep-fatigue tests, which leads to the destabilisation of the tempered martensite microstructure, inducing a decrease in further creep resistance. To better understand the softening mechanisms in an ASTM Grade 92 steel during uniaxial deformation, tensile tests were carried out at 823 K, showing an extended and stable softening stage after some work-hardening. This behaviour may be due to mechanical instability (necking), damage or microstructural (grain size) evolution. Examination of fractured and non-fractured tensile specimens (metallography, macrohardness tests, SEM, TEM) suggests that the physical mechanisms responsible for softening are mainly grain size evolution and diffuse necking