Oxide Electronic Conductivity and Hydrogen Pickup Fraction in Zr alloys

The hydrogen pick-up during cladding corrosion is a critical life-limiting degradation mechanism for nuclear fuel in existing and advanced nuclear reactors, since hydrogen ingress can cause cladding embrittlement and limit cladding lifetime. However, mechanistic knowledge of the hydrogen pick-up mechanism is still lacking. This research investigates the mechanistic links between hydrogen pick-up, oxidation rate, alloy chemistry, and microstructure on a selection of zirconium alloys including, Zircaloy-4, ZIRLO ® , Zr-2.5Nb, Zr-2.5Nb0.5Cu and model alloys. Hydrogen pick-up and oxide growth were precisely measured as a function of exposure time in an autoclave for a set of zirconium alloys with specific chemistries and microstructures [1, 2]. The results show that the instantaneous hydrogen pick-up fraction varies during corrosion, increasing just before oxide transition, when oxidation rate is the lowest [2]. Comparison between alloys has also shown that the alloying elements are closely related to the oxidation kinetics (and to the value of the corrosion exponent n) as well as to the hydrogen pick-up fraction [3]. As a result of these observations, it has been hypothesized that oxide electronic conductivity and alloying elements (either in precipitates or in solid solution) play key roles in the corrosion mechanism. Oxide doping and the state of second phase precipitate particles when absorbed into the oxide layer have been studied by X-Ray Absorption Near Edge Spectroscopy (XANES). The results show that the oxidation state of the alloying elements varies with distance from the oxide metal interface and that the oxidation of second phase precipitates is delayed relative to the Zr matrix as previously shown [4]. Consequently oxide doping varies as a function of oxide position and exposure time [5]. However, the understanding of the specific effect of oxide doping on the zirconium oxide electronic conductivity is still lacking. In the present study, the variation of the oxide electronic conductivity as function of exposure time and between alloys on various zirconium alloys presenting different oxide thickness and instantaneous hydrogen pickup fractions was measured using in-situ Electrochemical Impedance Spectroscopy (EIS). These conductivity measurements are compared with the evolution of hydrogen pickup fraction as function of exposure time to assess the possible correlation between oxide conductivity and hydrogen pickup fraction.