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Transformations de phases et comportement mécanique à haute température d’un alliage de zirconium (M5Framatome) : effets de l’historique de température

Abstract : During a Loss of Coolant Accident (LOCA), a hypothetical accident scenario studied as part of the safety demonstration of pressurised water reactors, zirconium alloy nuclear fuel claddings can undergo a complex high-temperature thermal transient combined with internal pressure. This combination of stresses can lead to creep-induced ballooning and bursting of the cladding, which represents the primary containment barrier for the radioactive material. Numerous studies have revealed the behaviour of the cladding during a simple thermal cycle coupled with mechanical loading. This research work addressed the effects of thermal history during dynamic high-temperature incursions typical of LOCA conditions, on the phase transformations and mechanical properties of M5Framatome* (Zr-1%Nb) alloy cladding. Until now, phase transformations of cladding materials have mainly been studied by "indirect" techniques such as dilatometry, resistivity, and calorimetry. This work also aimed to better quantify them by developing systematic studies using in-situ Synchrotron X-ray Diffraction (SXRD), and then to refine conventional methods accordingly. Regarding the study of the αZr ↔ βZr phase transformation kinetics, cycles simulating the first temperature peaks were applied. They included a first heating up to a temperature between 870°C (middle of the two-phase αZr + βZr domain) and 1050°C (single-phase βZr domain), a cooling down to the αZr domain and a second heating up to the βZr domain. The DRXS measurements showed that the transformation kinetics upon cooling and upon the second heating were affected by the maximum temperature reached on the first heating, with a threshold effect that corresponds to the end of the αZr → βZr transformation. When cooling at 10°C.s-1 fromthe βZr domain, a deviation of 80°C from equilibrium is observed for the transformation start, whereas this deviation is negligible when cooling from the two-phase domain. The reverse transformation kinetics at the second heating, after reaching the βZr domain, starts about 20°C earlier than for a first heating, starting from the as-received microstructure. Furthermore, coupling DRXS with dilatometry measurements revealed a significant discrepancy (40°C) at the first heating between the two techniques. The causes of this discrepancy were identified. The main one seems to be a significant viscoplastic deformation, even at low stresses (< 0.3 MPa), of the small-grained,equiaxed βZr microstructure which affects the dilatometry curve at the end of the transformation. Moreover, the DRXS technique allowed monitoring and quantification of the dissolution/retention phenomena of the equilibrium (βNb) and metastable (βZr*(Nb)) minor secondary phases. Including these new results in the dilatometry data processing method strengthened the already very good agreement betweenDRXS and dilatometry for the cooling and second heating kinetics. The refinement of data processing for the indirect methods eventually allowed the determination of the phase transformation kinetics with a similar accuracy as that obtained using DRXS. The entire available experimental database was then used to identify a new phase transformation kinetic model taking thermal history effects into account. [...]
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Submitted on : Thursday, April 21, 2022 - 5:04:41 PM
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  • HAL Id : tel-03648609, version 1

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Romain Borrossi. Transformations de phases et comportement mécanique à haute température d’un alliage de zirconium (M5Framatome) : effets de l’historique de température. Matériaux. Université Paris sciences et lettres, 2021. Français. ⟨NNT : 2021UPSLM051⟩. ⟨tel-03648609⟩

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