Thermomechanical loading applied on the cladding tube during the pellet cladding mechanical interaction phase of a rapid reactivity initiated accident

International audienceCalculations of the CABRI REP-Na5 pulse were performed with the ALCYONE code in order to determine the evolution of the thermomechanical loading applied on the cladding tube during the Pellet-Cladding Mechanical Interaction (PCMI) phase of a rapid Reactivity Initiated Accident (RIA) initiated at 280 °C that lasted 8.8 ms. The evolution of the following parameters are reported: the cladding temperature, heating rate, strain rate and loading biaxiality. The impact of these parameters on the cladding mechanical behavior and fracture are then briefly reviewed

Temperature increase of Zircaloy-4 cladding tubes due to plastic heat dissipation during tensile tests at 0.1-10 s−1 strain rates

International audienceThis study is focused on the impact of rapid Reactivity Initiated Accident (RIA) representative strain rates (about 1 s−1 NEA, 2010) on the behavior and fracture of unirradiated cold work stress relieved Zircaloy-4 cladding tubes. Uniaxial ring tests (HT) and plane strain ring tensile tests (PST) were performed in the 0.1-10 s−1 strain rate range, at 25 °C. The local temperature increase due to plastic dissipation was measured with a high-speed infrared camera. Limited temperature increases were measured at 0.1 s−1 strain rate. Limited but not strongly localized temperature increases were measured at 1 s−1. Large temperature increase were measured at 5 and 10 s−1 (142 °C at 5 s−1 strain rate in HT tests). The local temperature increase induced heterogeneous temperature fields, which enhanced strain localization and resulted in a reduction of the plastic elongation at fracture

Hydride blisters formation, characterization and effect on the fracture of Zircaloy-4 cladding tubes under reactivity initiated accident conditions

International audienceThis work is part of the global research effort carried out at the CEA to improve the understanding of cladding failure under reactivity initiated accident (RIA) conditions, based on analytical mechanical testing techniques. The detrimental effect of hydride blisters, localized high hydrogen concentration zones, on the cladding resistance was observed in integral tests in dedicated research reactors. In order to better quantify this effect, the present study is focused on mechanical tests representative of RIA loading conditions on samples containing such blisters. A cold spot technique was developed to generate hydride blisters on Zircaloy-4 cold work stress relieved (CWSR) unirradiated pre-hydrided claddings tubes. The hydrogen concentration field in and around the blister was measured by Micro Elastic Recoil Detection Analysis (µ-ERDA) and with micro- and nano-hardness techniques. Hydrogen concentration was found to decrease from the center of the blisters toward their borders. Finally, the nature of hydride phases characterized by X-Ray diffraction was found to be exclusively ? in the blister. Hydrides blisters detrimental effect on cladding mechanical resistance was assessed at 25°C, 350°C and 480°C with different mechanical tests performed on tubular samples. A newly developed test named Controlled Loading Path Expansion Due to Compression (CLP-EDC) was developed to reach biaxiality ratio of 0.5 while the standard EDC test biaxiality is limited to -0.5. The comparison between both tests show the clear effect of increasing the biaxiality on cladding failure without blister: Standard EDC tests biaxiality level is not sufficient to induce cladding fracture at 350°C. Similar tests were performed on samples with blisters of various depths. The results show the loading biaxiality effect seems to be much limited in presence of such defects. This work provides extensive experimental characterization of hydrides blisters, and analysis of the detrimental effect of blisters on the mechanical behavior of Zircaloy-4 cladding based on a new mechanical test that have all the characteristics needed for use on irradiated materials in shielded cells in the near future. The next step is to perform numerical computations to simulate the blister induced embrittlement

Hydrogen contribution to the thermal expansion of hydrided Zircaloy-4 cladding tubes

International audienceThis study is focused on the hydrogen-induced dimensional change or "growth" of zirconium alloys. Dilatometric experiments were performed on samples taken from a unirradiated Zircaloy-4 (Zy-4) fuel cladding loaded up to 940 wppm hydrogen. Samples were taken in the axial direction of the tube or at 45° to the axial and transverse directions. The results indicate that hydrogen-induced expansion is anisotropic. Theoretical expansion calculations were carried out considering the partition of hydrogen in solid solution and hydrides together with the material crystallographic texture. Hydride-induced expansion was calculated using two different assumptions reported in the literature, namely "Pure Lattice Transformation Strains" (PLTS) and "Pure Shear Transformation Strains" (PSTS). Calculations based on the PSTS hypothesis satisfactorily predicted the anisotropy observed in the dilatometric curve. Under this assumption, the contribution of hydrides to the axial growth of high-burnup Zy-4 cladding is limited to 12%. This study shows it is important to consider the respective contribution of hydrogen in both states, together with the material crystallographic texture, to understand the dilatometric behavior of hydrided zirconium alloys

Fracture of Zircaloy-4 cladding tubes with or without hydride blisters in uniaxial to plane strain conditions with standard and optimized expansion due to compression tests

International audienceTwo optimizations of the Expansion Due to Compression (EDC) test, which induces a near uniaxial loading, were proposed and developed to reach higher biaxiality ratios (ratio between mechanical quantities in axial and in circumferential direction). The first optimization, named HB-EDC for High-Biaxiality EDC, allowed to reach transverse plane strain conditions. The second optimization, named VHB-EDC for Very High Biaxiality EDC, was designed to reach higher loading biaxiality ratios. These optimized EDC tests were performed at 25 °C, 350 °C and 480 °C on unirradiated hydrided Cold Worked Stress Relieved (CWSR) Zircaloy-4 samples. First, samples unhydrided or uniformly hydrided up to 1130 wppm were tested. Second, samples hydrided at 310 wppm with a hydride blister were tested. A large ductility reduction is induced by the increase in biaxiality level in the absence of a hydride blister or with small blisters (View the MathML source<50μm deep). The fracture strain decreases quickly with the blister depth at 25 °C, but more progressively at higher temperature. An equation that quantifies the fracture strain reduction with the blister depth is proposed. Eventually, one of the tests developed in the present study, the HB-EDC test, was proven to be a good compromise between the test complexity and the stress state reached. It is a good candidate to characterize the mechanical behaviour of irradiated cladding

Formation and characterization of hydride blisters in Zircaloy 4 cladding tubes

International audienceThis article is focused on the formation of hydride blisters in zirconium alloys an experimental and theoretical standpoint, and their characterization in terms of morphology, hydrides crystallographic phases, hardness and hydrogen concentration. An experimental setup was developed to grow hydride blisters on pre-hydrided Zircaloy-4 cladding tubes by thermo-diffusion. The thermal conditions were optimized based on thermo-diffusion calculations, that take into account the hysteresis in the hydrogen solubility limit, to obtain a high blister growth rate. Micro-X-ray Diffraction (XRD), nano-hardness and Elastic Recoil Detection Analysis (ERDA) showed that the blisters contain a hydrogen gradient, with pure δδ-hydride phase close to the external surface over one third of the blister depth. Thermo-diffusion calculations showed these half thickness blisters should grow in only a few days in PWR conditions. Eventually, the Diffusion Equilibrium Threshold (DET) was defined as a criterion that limits the blister growth, and emphasizes that the hysteresis in the hydrogen solubility limit in zirconium must be taken into account to model hydrogen thermo-diffusion in zirconium alloys

Fracture of Zircaloy-4 fuel cladding tubes with hydride blisters

International audienceThe influence of hydride blister on cold worked stress relieved Zircaloy-4 cladding tubes fracture toughness at room temperature was studied using hoop tensile tests and plane strain tensile tests. The experimental macroscopic fracture stress and strain values and an elastic-plastic finite element analysis of the mechanical tests with the CAST3 M code were used to calculate the J-integral and estimate the fracture toughness for several blister depths from 120 to 240 μm

Formation de blisters d'hydrures et effet sur la rupture de gaines en Zircaloy-4 en conditions d'accident d'injection de réactivité

Our aim is to study the cladding fracture with mechanical tests more representative of RIA conditions, taking into account the hydrides blisters, representative strain rates and stress states. To obtain hydride blisters, we developed a thermodiffusion setup that reproduces blister growth in reactor conditions. By metallography, nanohardness, XRD and ERDA, we showed that they are constituted by 80% to 100% of delta hydrides in a Zircaloy-4 matrix, and that the zirconium beneath has some radially oriented hydrides. We modeled the blister growth kinetic taking into account the hysteresis of the hydrogen solubility limit and defined the thermal gradient threshold for blister growth. The modeling of the dilatometric behavior of hydrided zirconium indicates the important role of the material crystallographic texture, which could explain differences in the blister shape. Mechanical tests monitored with an infrared camera showed that significant local heating occurred at strain rates higher than 0.1/s. In parallel, the Expansion Due to Compression test was optimized to increase the biaxiality level from uniaxial stress to plane strain (HB-EDC and VHB-EDC tests). This increase in loading biaxiality lowers greatly the fracture strain at 25°C and 350°C only in homogeneous material without blister. Eventually, a fracture criterion of unirradiated Zircaloy-4 cladding tube taking into account the blister depth is proposed.Ce travail vise à étudier la rupture du gainage avec des essais mécaniques plus représentatifs des conditions RIA, en prenant en compte les blisters d'hydrures ainsi que le niveau élevé de biaxialité du chargement mécanique et des vitesses de déformation. Nous avons formé par thermodiffusion en laboratoire des blisters similaires à ceux observés sur des gaines de Zircaloy-4 irradiées en réacteur. Les caractérisations par métallographie, nanodureté, DRX et ERDA ont montré qu'un blister est constitué d'hydrures delta dont la concentration dans la matrice varie entre 80% et 100%, et que la matrice sous-jacente contient des hydrures radiaux. Nous avons modélisé la cinétique de croissance des blisters avec l'hystérésis de la limite de solubilité de l'hydrogène,puis défini le gradient thermique seuil permettant leur formation. Notre étude sur le comportement dilatométrique du zirconium hydruré montre le rôle important de la texture cristallographique du matériau, ce qui peut expliquer des différences de morphologie des blisters. En parallèle, des essais suivis par caméra infrarouge ont montré que des vitesses de déformation supérieures à 0,1/s induisent des échauffements locaux importants qui favorisent la localisation précoce de la déformation. Enfin, nous avons optimisé l'essai d'Expansion Due to Compression pour atteindre un niveau de biaxialité de déformation plane (essais HB-EDC et VHB-EDC), ce qui réduit fortement la déformation à rupture à 25°C et 350°C, mais seulement en l'absence de blisters. Un critère de rupture est proposé pour rendre compte de la baisse de ductilité des gaines en Zircaloy-4 non irradiées en présence de blisters

Hydride Blister Formation and Induced Embrittlement Zircaloy-4 Cladding Tubes in Reactivity Initiated Conditions

Ce travail vise à étudier la rupture du gainage avec des essais mécaniques plus représentatifs des conditions RIA, en prenant en compte les blisters d'hydrures ainsi que le niveau élevé de biaxialité du chargement mécanique et des vitesses de déformation. Nous avons formé par thermodiffusion en laboratoire des blisters similaires à ceux observés sur des gaines de Zircaloy-4 irradiées en réacteur. Les caractérisations par métallographie, nanodureté, DRX et ERDA ont montré qu'un blister est constitué d'hydrures delta dont la concentration dans la matrice varie entre 80% et 100%, et que la matrice sous-jacente contient des hydrures radiaux. Nous avons modélisé la cinétique de croissance des blisters avec l'hystérésis de la limite de solubilité de l'hydrogène,puis défini le gradient thermique seuil permettant leur formation. Notre étude sur le comportement dilatométrique du zirconium hydruré montre le rôle important de la texture cristallographique du matériau, ce qui peut expliquer des différences de morphologie des blisters. En parallèle, des essais suivis par caméra infrarouge ont montré que des vitesses de déformation supérieures à 0,1/s induisent des échauffements locaux importants qui favorisent la localisation précoce de la déformation. Enfin, nous avons optimisé l'essai d'Expansion Due to Compression pour atteindre un niveau de biaxialité de déformation plane (essais HB-EDC et VHB-EDC), ce qui réduit fortement la déformation à rupture à 25°C et 350°C, mais seulement en l'absence de blisters. Un critère de rupture est proposé pour rendre compte de la baisse de ductilité des gaines en Zircaloy-4 non irradiées en présence de blisters.Our aim is to study the cladding fracture with mechanical tests more representative of RIA conditions, taking into account the hydrides blisters, representative strain rates and stress states. To obtain hydride blisters, we developed a thermodiffusion setup that reproduces blister growth in reactor conditions. By metallography, nanohardness, XRD and ERDA, we showed that they are constituted by 80% to 100% of delta hydrides in a Zircaloy-4 matrix, and that the zirconium beneath has some radially oriented hydrides. We modeled the blister growth kinetic taking into account the hysteresis of the hydrogen solubility limit and defined the thermal gradient threshold for blister growth. The modeling of the dilatometric behavior of hydrided zirconium indicates the important role of the material crystallographic texture, which could explain differences in the blister shape. Mechanical tests monitored with an infrared camera showed that significant local heating occurred at strain rates higher than 0.1/s. In parallel, the Expansion Due to Compression test was optimized to increase the biaxiality level from uniaxial stress to plane strain (HB-EDC and VHB-EDC tests). This increase in loading biaxiality lowers greatly the fracture strain at 25°C and 350°C only in homogeneous material without blister. Eventually, a fracture criterion of unirradiated Zircaloy-4 cladding tube taking into account the blister depth is proposed