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Etude expérimentale de l'oxydation des alliages de zirconium à haute pression de vapeur d'eau et modélisation des mécanismes

Abstract : The corrosion of the cladding materials used for the fuel rods is one of the limiting factor of their lifetime in light water reactors. In this field, the aim of the nuclear industry is today to increase the time and the number of cycles and to submit the claddings in zirconium alloys to higher corrosive conditions. In this way, new alloys devoted to replace the standard Zircaloy-4, for instance Nb containing alloys, have been recently developed and licensed and show better corrosion resistance. A better understanding of the corrosion mechanisms of the zirconium alloys is necessary to predict the corrosion behaviour of these materials.
In this work, the oxidation rate of model alloys of two metallurgic families has been studied in steam in a pressure range between 100 millibars and 100 bars. The Zircaloy type alloys contain as alloying elements oxygen and/or tin and/or iron and chromium. For the Zr-Nb family, three niobium contents have been studied, respectively 0.2, 0.4 and 1 weight percent of niobium. Our objectives were to understand the variations of the reactivity between the low pressure and the high pressure range, in quantifying the dependency of the corrosion rate with the steam pressure and the alloying element concentrations. The segregation process of the niobium at the surface has also been studied on the Zr-Nb alloys.
During this work, a magnetic suspension thermobalance has been developed and used to follow in-situ the corrosion rate at high pressure of water vapour. The oxide layers have been characterized by many techniques, macro and microphotoelectrochemistry, XRD, FEG-SEM, XPS, HR-TEM and SIMS.
For the Zircaloy type alloys, we have confirmed the major role of the intermetallic precipitates Zr(Fe,Cr)2 on the corrosion resistance. Unlike the standard Zircaloy-4, for which the oxidation rate does not depend on the pressure of the water vapour and is thus limited by the vacancy diffusion in the oxide layer, we have shown that the rate of the pure zirconium increase drastically with the steam pressure and could be controlled by an interface reaction, associated to a degradation of the oxide favoured by the increase of the oxidant pressure. Finally, we have proposed an oxidation mechanism of the Zr(Fe,Cr)2 precipitates, during which the iron is dissolved in the oxide around the precipitate and probably stabilises by this way the quadratic zirconia. This process could ensure the integrity of the oxide layer and annihilate the degradation of the layer.
Concerning the Zr-Nb alloys and specially the Zr-1%Nb, the oxidation rate is dependant on the steam pressure. Moreover, the segregated niobium content grows with the pressure of water vapour, the niobium concentration in the alloy and is not observed under oxygen. We have proposed a corrosion mechanism able to describe the oxidation and the segregation processes. Finally, a rate limiting step of the diffusion of adsorbed hydroxide ions in the external oxide layer is consistent with our experimental results.
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https://tel.archives-ouvertes.fr/tel-00341747
Contributor : Andrée-Aimée Toucas <>
Submitted on : Tuesday, November 25, 2008 - 7:33:53 PM
Last modification on : Wednesday, June 24, 2020 - 4:18:29 PM
Long-term archiving on: : Monday, June 7, 2010 - 8:26:49 PM

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  • HAL Id : tel-00341747, version 1

Citation

Yacoub Dali. Etude expérimentale de l'oxydation des alliages de zirconium à haute pression de vapeur d'eau et modélisation des mécanismes. Sciences de l'ingénieur [physics]. Ecole Nationale Supérieure des Mines de Saint-Etienne, 2007. Français. ⟨tel-00341747⟩

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