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    Investigation and Modeling of the Degradation of Zirconium-Based Fuel Cladding during Corrosion in Steam and Air-Steam Mixtures at High Temperatures

    Published: 2018

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    This paper presents the main results of a study conducted to quantify and to model the degradation state of zirconium-based fuel claddings submitted to severe-accident conditions in a nuclear reactor core: high temperatures and either pure steam or an air-steam mixture. Due to the progressive thickening of a dense and protective ZrO2 layer, the oxidation kinetics of zirconium-based claddings in steam at high temperatures typical of severe nuclear accidents are generally cubic or parabolic. However, for some temperature domains, this oxide layer may crack, becoming porous and no longer protective. In these “breakaway” conditions, the oxidation kinetics change from (sub)parabolic to linear or even accelerated. In addition, the temperature increase can lead core materials to melt and to relocate down to the vessel lower head, threatening its integrity. If it fails, and for specific conditions, air ingress may take place into the reactor. Hence, oxygen and nitrogen both react with zirconium-based claddings successively through the oxidation of zirconium (forming a ZrO2 layer), nitriding of zirconium (forming zirconium nitride particles), and the oxidation of zirconium nitride (forming ZrO2 and releasing nitrogen). These self-sustained chemical reactions enhance the deterioration of zirconium-based claddings and their ZrO2 layers, inducing an increase in their open porosity. To quantify this porosity, a series of two-step experiments was conducted. First, ZIRLO™ cladding samples were isothermally oxidized in pure steam or in a 50:50 mol% air-steam mixture at several different temperatures and durations. The main thermal effects on reaction kinetics and the high impact of air on the cladding degradation were all confirmed by experimental results. Second, pioneering porosimetry measurements by mercury intrusion were realized for the first time on such corroded cladding samples. In both atmospheres, it was pointed out that 1,200 and 1,250 K led to particularly porous oxide layers, especially due to strong breakaway effects. Moreover, we confirmed that the presence of air strongly enhances the oxide cracking: cladding samples were more porous when oxidized in the air-steam mixture than under pure steam. Finally, we observed that in all conditions, the open porous volume fraction of ZIRLO claddings continuously rose during their corrosion process. Hence, for each experimental condition, porosity correlations were determined through linear regressions, and porosity increase rates were deduced by derivation versus time and validated against porosimetry results of cladding samples corroded in transient (nonisothermal) conditions.


    severe nuclear accident, zirconium-based fuel cladding, thermochemical reactions, degradation phenomena, corrosion experiments, porosimetry measurements, porosity increase rates

    Author Information:

    Haurais, Florian
    EDF R&D, Département SINETICS, Palaiseau,

    Beuzet, Émilie
    EDF R&D, Département SINETICS, Palaiseau,

    Steinbrück, Martin
    Karlsruhe Institute of Technology, Institute for Applied Materials, Eggenstein-Leopoldshafen,

    Simoni, Éric
    Institut de Physique Nucléaire d'Orsay, Université Paris-Sud, Université Paris-Saclay,

    Ambard, Antoine
    EDF R&D, Département MMC, Écuelles,

    Torkhani, Mohamed
    EDF R&D, Département SINETICS, Palaiseau,

    Committee/Subcommittee: B10.02

    DOI: 10.1520/STP159720160033