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International audience; One of the hypothetical accidents studied in the field of the safety studies of Pressurized light Water Reactors (PWR) is the Loss-Of-Coolant Accident (LOCA). In this scenario, zirconium alloy fuel claddings could undergo a high level of oxidation at high temperature in a steam environment. Cladding tubes constitute the first confinement barrier of radioelements and then it is essential that they keep a certain level of ductility after quenching to ensure their integrity. These properties are directly related to the growth kinetics of both the oxide and the sub-oxide αZr(O) layers and especially to the oxygen diffusion profiles in the residual prior-β layer after HT oxidation and final water quenching. This study was focused on the influence of a pre-annealing thermal treatment performed on first generation of 6-8 µm thick Cr-coated zirconium based specimens and on their further HT oxidation behavior. The effect of pre-annealing temperature on the structure and morphology of the Chromium coating microstructure was investigated using scanning electron microscopy. During the annealing treatment, the chromium coating experienced recrystallisation and the initial columnar grains morphology became more equiaxed. After oxidation at HT, the weight gains of pre-annealed Cr-coated specimen were generally lower than the non-annealed ones. Glow discharge mass spectrometry, Electron Probe Micro Analysis and Raman spectroscopy measurements have confirmed that the thickness of ZrO2 and αZr(O) phases were significantly reduced for some of the pre-annealed samples, indicating that the grain size and morphological evolutions of the chromium coating upon the pre-annealing thermal treatment may influence its further HT oxidation behavior.

International audience

International audience

National audience; Lors de situations accidentelles hypothetiques dans les reacteurs nucleaires a eau pressurisee, comme par exemple lors d'un Accident de Perte de Refrigerant Primaire (APRP), la gaine en alliage de zirconium des crayons combustibles peut etre exposee durant quelques minutes a de la vapeur d'eau a haute temperature (jusqu'a 1200DC) avant d'etre refroidie puis trempee a l'eau. Lors d'un transitoire de type APRP, cette gaine subit de nombreuses evolutions metallurgiques (formation d'une couche d'oxyde, diffusion d'oxygene dans le metal sous-jacent, changements de phases, ) et est soumise a des contraintes internes dont l'etat evolue au cours de l'oxydation a Haute Temperature (HT) mais aussi lors du refroidissement et de la trempe. Ces contraintes internes sont non seulement susceptibles d'avoir un effet sur la structure de l'oxyde et sur la cinetique d'oxydation du materiau, mais elles peuvent aussi nuire a la tenue mecanique de la gaine, fragilisee par son oxydation, lors du refroidissement et de la trempe apres l'oxydation a HT, ou encore avoir des consequences sur l'integrite du materiau a plus long terme. Or ces gaines constituent la premiere barriere de confinement des produits radioactifs.Des lors, il est necessaire de pouvoir estimer les contraintes internes generees au cours de l'oxydation a HT et du refroidissement au sein de ces alliages a base de zirconium (1). Ces contraintes, tres difficiles a evaluer experimentalement du fait des conditions extremes (haute temperature, atmosphere oxydante, trempe), sont a ce jour inconnues. Dans le but de progresser sur la connaissance de ces contraintes internes, des experiences in-situ de diffraction des rayons X sous rayonnement synchrotron ont ete realisees a SOLEIL, pendant l'oxydation a HT (a 700, 800 et 900DC) sous O2-He (en substitution a la vapeur d'eau) et le refroidissement ulterieur, sur des plaquettes en Zircaloy-4 (2-3). Grace au detecteur bidimensionnel XPAD S140, il a ete possible de faire des acquisitions lors du palier d'oxydation puis au cours du refroidissement, simultanement pour les phases monoclinique et quadratique de la zircone. Apres traitement des donnees 2D acquises, l'evolution des contraintes internes moyennes a alors pu etre suivie en fonction du temps pour les differentes conditions investiguees, pour les phases monoclinique et quadratique de la zircone, dont les proportions evoluent au cours du cycle thermique, pendant le palier d'oxydation a 700, 800 et 900DC et le refroidissement ulterieur (Figure 1). On observe une diminution des contraintes moyennes de compression au cours de l'oxydation et du refroidissement pour les deux phases de zircone. Les resultats mettent egalement en lumiere le fait que l'oxyde forme de structure quadratique est en moyenne toujours moins contraint que celui de structure monoclinique, et que l'oxyde (quadratique ou monoclinique) forme a 700DC reste toujours plus contraint a la fin du cycle thermique que celui formes a 800 et 900DC.

SESSION: G7 -: Materials under extreme conditions; International audience; Oxides Dispersed strengthened (ODS) stainless steels are foreseen for fuel claddingtubes in the coming generation of fission sodium cooled nuclear reactors. In spite of abody-centered matrix, those steels present a convenient creep behavior thanks to veryfine oxides dispersion. Those grades are currently obtained by Powder Metallurgy (PM).After mechanical alloying with the oxide, the powder is commonly consolidated asseamless tube by Hot Extrusion (HE). Before this operation the powder is canned andoutgazed in order to reduce the interaction with nitrogen and oxygen of ambient air. Thecontrol of the mechanical properties after extrusion is a key issue for this graderegarding service conditions. Therefore, effect of processing conditions must be takeninto account. This study focuses on microstructural and mechanical characterization ofthree 14wt%Cr ODS material elaborated with various outgazing conditions before HE.Those materials are tested in term of tenacity (using Charpy impact tests) and microhardness. In the same time microstructural characterization by X-Ray tomography revealthe occurrence of unexpected large phases whose densities is correlated withmechanical properties. EPMA characterizations are driven in order to prospect thecomposition of those phases. Precipitates on Prior Particles Boundaries (PPB) are alsostudied as they play a crucial role on the coalescence of creep damage voids. Finally,effects of outgazing conditions are summarized regarding both microstructural andmechanical properties of consolidated materials.

SESSION: G7 -: Materials under extreme conditions; International audience; Oxides Dispersed strengthened (ODS) stainless steels are foreseen for fuel claddingtubes in the coming generation of fission sodium cooled nuclear reactors. In spite of abody-centered matrix, those steels present a convenient creep behavior thanks to veryfine oxides dispersion. Those grades are currently obtained by Powder Metallurgy (PM).After mechanical alloying with the oxide, the powder is commonly consolidated asseamless tube by Hot Extrusion (HE). Before this operation the powder is canned andoutgazed in order to reduce the interaction with nitrogen and oxygen of ambient air. Thecontrol of the mechanical properties after extrusion is a key issue for this graderegarding service conditions. Therefore, effect of processing conditions must be takeninto account. This study focuses on microstructural and mechanical characterization ofthree 14wt%Cr ODS material elaborated with various outgazing conditions before HE.Those materials are tested in term of tenacity (using Charpy impact tests) and microhardness. In the same time microstructural characterization by X-Ray tomography revealthe occurrence of unexpected large phases whose densities is correlated withmechanical properties. EPMA characterizations are driven in order to prospect thecomposition of those phases. Precipitates on Prior Particles Boundaries (PPB) are alsostudied as they play a crucial role on the coalescence of creep damage voids. Finally,effects of outgazing conditions are summarized regarding both microstructural andmechanical properties of consolidated materials.

International audience

International audience; L'étude ici illustrée s'intéresse au comportement thermo - métallurgique - mécanique des alliages de Zr des tubes de gainage du combustible des Réacteurs nucléaires à Eau Pressurisée (REP), lors de transitoires à hautes températures (HT) en ambiance vapeur d'eau, simulant des conditions hypothétiques accidentelles (dites APRP). Il apparaît intéressant d'approfondir la connaissance des phénomènes métallurgiques et thermomécaniques mis en jeu lors de tels transitoires, en particulier vis-à-vis du comportement mécanique résiduel « post oxydation trempe ». Différentes teneurs en oxygène ont été incorporées dans l'alliage étudié, pour simuler la diffusion de cet élément dans le substrat métallique qui intervient lors de l'oxydation HT. Des caractérisations microstructurales par EBSD, MET, MEB... et des caractérisations mécaniques par essai de traction, mesures de constantes physiques... ont permis de décrire la microstructure et le comportement mécanique de la phase résiduelle la plus ductile – phase dite « ex-bêta » - résultant de l'oxydation HT des gaines en zircaloy-4.

To optimise the regulation of rate responsive pacemakers, the authors propose standardized tests of everyday activities. Seven tests were selected with monitoring of the heart rate (HR) by telemetry every 30 seconds: two reference tests--cycloergometry: 2 steps of 3 minutes at 30 then 60 watts and a symptoms-limited Bruce protocol on the treadmill--and 5 everyday activities, walking, going upstairs, going downstairs, squats and hyperventilation. Fifteen normal subjects (65 +/- 10 years) were used to establish normal values and 10 patients implanted with a Telelctronics Meta DDDR 1250 rate responsive pacemaker with minute ventilation sensing, were studied after choice of minimal and maximal HR on clinical criteria (age, pathology). The rate response frequency (RRF) was adjusted so that the pacemaker HR was within normal values. At the beginning of each test, the sensor was hypokinetic (p = 0.01). Hyperventilation caused hyperkinesia of the sensor (p = 0.01). A good correlation was obtained between PM patients and normal subjects on walking, going up down stairs and for squats (p = NS). On cycle ergometry and treadmill, the PM remained slightly hypokinetic (p = 0.02). The study of "ideal" RRF measured at "peak exercise" as proposed by the manufacturer was only satisfactory in 1 of the 10 cases. This study confirms the value of using everyday activities for programming rate responsive pacemakers, the limitations of automatic adjustment and the need for personalized programming.