Étude expérimentale et modélisation du comportement en fluage sous pression interne d’une gaine en alliage de zirconium oxydée en atmosphère vapeur

During hypothetical Loss-Of-Coolant-Accident (LOCA) scenarii, zirconium alloy fuel cladding tubes creep under internal pressure and are oxidized on their outer surface at high temperature (HT). Claddings become stratified materials: zirconia and oxygen-stabilized alpha phase, called alpha(O), are formed on the outer surface of the cladding whereas the inner part remains in the beta domain. The strengthening effect of oxidation on the cladding creep behavior under internal pressure has been highlighted at HT. In order to model this effect, the creep behavior of each layer had to be determined. This study focused on the characterization of the creep behavior of the alpha(O) phase at HT, through axial creep tests performed under vacuum on model materials, containing from 2 to 7 wt.% of oxygen and representative of the alpha(O) phase. For the first time, two creep flow regimes have been observed in this phase. Underlying physical mechanisms and relevant microstructural parameters have been discussed for each regime. The strengthening effect due to oxygen on the alpha(O) phase creep behavior at HT has been quantifiedand creep flow equations have been identified. A ductile to brittle transition criterion has beenalso suggested as a function of temperature and oxygen content. Relevance of the creep flowequations for each layer, identified in this study or from the literature, has been discussed.Then, a finite element model, describing the oxidized cladding as a stratified material, hasbeen built. Based on this model, a fraction of the experimental strengthening during creep ispredicted.Durant un scénario d’Accident par Perte de Réfrigérant Primaire (APRP), les gainages combustible en alliage de zirconium fluent sous pression interne et s’oxydent en présence de vapeur à haute température (HT). La gaine devient un matériau stratifié : des couches de zircone et de phase alpha(O) fortement enrichie en oxygène, dite alpha(O), se forment à la surface externe de la gaine, qui est alors en phase béta. L’effet durcissant de l’oxydation sur le comportementen fluage des gaines sous pression interne a été mis en évidence dans des essais de laboratoire. Pour modéliser cet effet, le comportement mécanique de chacune des couches doit être déterminé. Cette étude a porté sur la caractérisation expérimentale du comportement en fluage de la phase alpha(O). Via un procédé original, des matériaux « modèles » contenant de 2 à 7% en masse d’oxygène et représentatifs de la phase alpha(O) ont été produits puis testés enfluage axial sous vide. Pour la première fois, deux régimes de fluage ont été observés, pour lesquels les mécanismes physiques et paramètres microstructuraux influents sont discutés. L’effet durcissant de l’oxygène sur le comportement en fluage de la phase alpha a été quantifié et des lois d’écoulement viscoplastique, intégrant cet effet, ont été identifiées. Une transition ductile/fragile, fonction de la température et de la teneur en oxygène, est proposée. Après avoir discutéleur pertinence, les lois de fluage pour chacune des couches, identifiées dans cette étude ou tirées de la littérature, ont été implémentées dans un modèle par éléments finis, décrivant la gaine comme un matériau stratifié. Ce modèle permet de reproduire une partie du durcissement observé expérimentalement

Breakaway characterization of Zircaloy-4 oxidized in steam and in oxygen at high temperatures using HT- XRD analysis

International audienceHT-XRD analyses were performed on Zircaloy-4 sheet samples in steam and oxygen mixtures at high temperatures in order to investigate any possible relation between zirconia phase transformation and the breakaway oxidation. The tetragonal phase fraction decreases for temperatures less than 1000°C. At 1000°C and 1030°C, breakaway oxidation occurred, however, the tetragonal phase fraction remained constant during the isothermal oxidation showing no sudden decrease that could be attributed to the onset of breakaway oxidation. These results demonstrate that the tetragonal-to-monoclinic phase transformation in the external surface of the oxide could not be correlated to the occurrence of the breakaway oxidation

Parallel mechanism of growth of the oxide and &#945-Zr(O) layers on Zircaloy-4 oxidized in steam at high temperatures

International audienceThe kinetic behavior of Zircaloy-4 tube samples was examined during high temperature steam oxidation in order to propose a mechanism with detailed elementary steps. The study revealed that steam and hydrogen partial pressures have no effect on the reaction kinetics. Based on these experiments, the kinetic model established herein takes into account the oxide growth and the oxygen dissolution in the metal. A ratedetermining step of diffusion of oxygen vacancies in the oxide governs the kinetics of the reaction in both pre- and post-breakaway regimes meaning that the oxide growth and oxygen dissolution in the metal advance at proportional rates

Breakaway oxidation of zirconium alloys exposed to steam around 1000 °C

International audienceThe breakaway oxidation of Zircaloy-4 and M5Framatome alloys oxidized in flowing steam around 1000 °C wasstudied. The influences of oxidation temperature between 950 and 1050 °C, one-side versus two-side oxidationand chemical variations around the nominal composition of M5Framatome were investigated. Breakaway oxidationis sensitive to the oxidation temperature, the material and the oxidation type. A phenomenological mechanismfor breakaway oxidation is postulated on the basis of microstructural and microchemical analyses

Étude du phénomène de <i>breakaway</i> lors de l'oxydation du zircaloy-4 sous vapeur d'eau à haute température (>800°C)

National audienceLes alliages de zirconium sont utilisés comme matériaux de structure des crayons de combustible nucléaire pour les centrales nucléaires à eau légère en raison de leurs propriétés neutroniques, mécaniques satisfaisantes et de leur résistance à la corrosion. En conditions nominales de fonctionnement (environ 150 bar et 350°C), les tubes en alliage de zirconium sont sujets à une oxydation maîtrisée en présence d’eau pressurisée, servant de caloporteur. La démonstration de la sûreté des réacteurs nucléaires intègre l’étude des scénarios accidentels hypothétiques, tels que l’Accident de Perte de Réfrigérant Primaire (APRP). Dans ce scénario, une brèche dans le circuit primaire du réacteur peut conduire les crayons de combustible à être oxydés en milieu de vapeur d’eau à haute température (jusqu’à 1200°C). Il a été démontré [1]–[3] que dans l’intervalle de température [900-1050°C] et après un certain temps d’oxydation sous vapeur d’eau, le phénomène de breakaway peut apparaître. Il se manifeste par une accélération de la cinétique d’oxydation, par la perte du caractère protecteur de la couche d’oxyde et par l’absorption d’hydrogène du matériau. Un tel phénomène implique donc une détérioration des propriétés mécaniques de la gaine et menace son intégrité pendant et après le transitoire accidentel. Les mécanismes fondamentaux à l’origine du breakaway sont encore mal connus.Dans ce cadre, ce travail de thèse a pour objectif de caractériser les conditions d’apparitions du breakaway et d’améliorer la compréhension des mécanismes physiques sous-jacents. Pour ce faire, des essais d’oxydation sur des échantillons de Zy-4 en thermobalance symétrique ont permis d’étudier l’influence de la température, de la pression partielle de vapeur d’eau et du temps sur l’occurrence du breakaway. La couche d’oxyde formée a été caractérisée par des observations en microscopie optique et des analyses en DRX in-situ pour suivre l’évolution des phases de la zircone durant l’oxydation

Mechanical behavior at high temperatures of highly oxygen- or hydrogen-enriched α and (Prior-) β phases of zirconium alloys

International audienceMechanical behavior at high temperature of highly oxygen-or hydrogen-enriched α and (prior-) β phases of zirconium alloys ABSTRACT: During a hypothetical loss-of-coolant accident (LOCA), zirconium alloy fuel claddings can be loaded by internal pressure and exposed to steam at high temperature (HT, potentially up to 1200°C), then cooled and water quenched. A significant fraction of the oxygen reacting with the cladding during HT oxidation diffuses beneath the oxide through the metallic substrate. This induces a progressive transformation of the metallic βZr phase layer into an intermediate layer of αZr(O) phase containing up to 7 wt.% of oxygen. Furthermore, in some specific conditions, the cladding may rapidly absorb a significant amount of hydrogen during steam exposition at HT. Being a βZr-stabilizer, hydrogen would mainly diffuse and concentrate up to several thousands of wt.ppm into the inner βZr phase layer. Oxygen and hydrogen are known to modify the metallurgical and mechanical properties of zirconium alloys but data are scarce for high contents, especially at HT. However, such data are important basic components to improve the assessment of the oxidized cladding mechanical behavior and integrity during and after LOCA-like thermal-mechanical transients. This study intended to provide new, more comprehensive data on the HT mechanical behavior of the αZr(O) and the (prior-) βZr phases containing high contents of oxygen and hydrogen, respectively. Model samples, produced from M5® 5 and Zircaloy-4 cladding tubes, homogeneously charged in oxygen (≤6 wt.%) and in hydrogen (≤3000 wt.ppm) respectively, were prepared. Their mechanical behavior was determined under vacuum between 800 and 1100°C for the oxygen-enriched αZr phase, and in air between 700 and 20°C, after cooling from the βZr temperature domain, for the hydrogen-enriched (prior-) βZr phase. The αZr phase is substantially strengthened and embrittled by oxygen. Power-law and nearly linear creep regimes are observed and were modelled for stress levels beyond and below 15 MPa, respectively. The model αZr(O) material experiences a ductile-to-brittle transition at 1000-1100°C for oxygen contents between 3.4 and 4.3 wt.%. The viscoplastic behavior of the αZr(O) phase was used to evaluate the contribution of the αZr(O) layer to the HT creep behavior of an oxidized fuel cladding tube subjected to internal pressure. The model (prior-) βZr phase becomes macroscopically brittle at temperatures ≤135°C and ≤350-400°C for average hydrogen content

Creep behavior at high temperature of the oxygen stabilized zirconium alpha phase of fuel cladding tubes oxidized in loca conditions

International audienceDuring hypothetical Loss-Of-Coolant-Accident (LOCA) scenarii, zirconium alloy fuel cladding tubes are oxidized at high temperature (HT): zirconia and oxygen-stabilized alpha, called alpha(O) are formed on the outer surface of the cladding. To model the behavior of the cladding during LOCA transients, the specific behavior of each layer must be known. This study focuses on the characterization of the viscoplastic behavior of the alpha(O) phase at HT. Model alpha(O) phase specimens enriched by 2 wt% in oxygen were produced from zirconium alloys cladding specimens. Creep tensile tests were performed in the tube axial direction under secondary vacuum between 900 and 1100°C for stresses between 2 and 31 MPa. The creep resistance of the alpha(O) phase is higher than that of the beta phase by three to four decades. The HT creep behavior of oxidized cladding was then modeled by finite element simulation

Evaluation of Equivalent Cladding Reacted parameters of Cr-coated claddings oxidized in steam at 1200 °C in relation with oxygen diffusion/partitioning and post-quench ductility

International audienceCr-coated M5(Framatome) cladding materials are studied and developed within the CEA-Framatome-EDF French nuclear fuel joint program as Enhanced Accident Tolerant Fuel claddings for Light Water Reactors. The objective of this paper is to bring some insights into the relationship between Equivalent Cladding Reacted (ECR) parameters, oxygen diffusion/partitioning and Post-Quench (PQ) ductility of Cr-coated M5(Framatome) fuel claddings oxidized in steam at 1200 degrees C.The physical meaning of the ECR parameter, evaluated experimentally from the measured Weight Gain (WG) or calculated using time and temperature correlations such as the Baker-Just (BJ) or Cathcart-Pawel (CP) kinetics correlations, is discussed in the light of the benefit brought by Cr coating to oxidation resistance of cladding. As shown in this article, when applied to the Cr-coated M5(Framatome) materials, the "experimental" ECR derived from WG does not have the same physical meaning than for the uncoated cladding materials. As discussed in the paper, this is fundamentally due to the use of the ECR as a surrogate for retained ductility for uncoated claddings, and to the differences between uncoated and Cr-coated cladding in the high temperature (HT) steam oxidation processes and partitioning of the oxygen between the different layers of the oxidized cladding.It is shown in this article that Cr-coated M5(Framatome) cladding brings significant additional time-at-temperature before full embrittlement of the cladding after one-sided oxidation at 1200 degrees C and quenching, compared to uncoated materials. The oxidation times and associated Baker-Just ECR (BJ-ECR) values, above which the cladding becomes brittle after low temperature quenching, are respectively ten times and three times higher than the ones for the uncoated reference cladding. When analyzing the PQ ductility of the Cr-coated M5(Framatome) cladding using a similar methodology as the one used to derive the ECR criterion for uncoated cladding, the 1e2% ductility limit corresponds to a BJ-ECR of about 50% or higher, for a 12-15 mm-thick Cr-coated cladding tested herein