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

Abstract

This paper presents the main results of a study conducted to quantify and to model the degradation state of Zr-based fuel claddings submitted to severe accident conditions in a nuclear reactor core: high temperatures and either pure steam or air-steam mixture. Due to the progressive thickening of a dense and protective ZrO2 layer, the oxidation kinetics of Zr-based claddings in steam at high temperatures typical of nuclear severe accidents, is generally cubic or parabolic. However, for some temperature domains, this oxide layer may crack, becoming porous and non-protective anymore. In these 'breakaway' conditions, the oxidation kinetics change from (sub-)parabolic to linear or even accelerated. Additionally, 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 Zr-based claddings, successively through oxidation of Zr (forming ZrO2 layer), nitriding of Zr (forming ZrN particles) and oxidation of ZrN (forming ZrO2 and releasing nitrogen). These self-sustained chemical reactions enhance the deterioration of Zr-based claddings and of their ZrO2 layers, inducing a rise of their open porosity. To quantify this porosity, a series of two-step experiments was conducted. First, ZIRLO® cladding samples were isothermally oxidized in various conditions: in pure steam or in 50-50mol% air-steam mix, at several temperatures, and for different durations. The main thermal effects on reaction kinetics and the high impact of air on the cladding degradation are all confirmed by experimental results. Second, pioneering porosimetry measurements by Hg intrusion were realized for the first time on such corroded cladding samples. In both atmospheres, it is pointed out that 1200 and Degradation of Zr-based fuel claddings during corrosion at high temperatures 2 1250 K lead to particularly porous oxide layers, especially due to strong 'breakaway' effects. Moreover, it is confirmed that the presence of air strongly enhances the oxide cracking: cladding samples are more porous when oxidized in the air-steam mixture than under pure steam. Finally, it is observed that in all conditions, the open porous volume fraction of ZIRLO® claddings continuously rises during their corrosion process. Hence, for each experimental condition, porosity correlations are determined through linear regressions, and porosity increase rates are deduced by derivation versus time and validated against porosimetry results of cladding samples corroded in transient (nonisothermal) conditions.