Understanding the role of niobium on the hydrogen uptake mechanism of zirconium alloys in primary water

International audienceWhile Zircaloy-4 alloy shows a high and constant hydrogen pickup fraction over its operating life in pressurized water reactor (PWR), this fraction is lower for M5® alloy and tend to decrease for high burnup (up to 70 GWd/tU). One of the hypotheses to explain such a difference is the positive role of the 1 wt% of niobium in M5®.To study the impact of this alloying element on the hydrogen pickup fraction, the contribution of niobium either segregated at the surface or present in solid solution of the ZrO2 corrosion layer are investigated. Two batches of samples have been studied :i.Pre-oxidized samples of Zircaloy-4 and Zr-120ppmFe implanted with low energy ions in order to produce a thin layer of niobium on the surface of the oxide.ii.Model zirconium alloys with a niobium content of 0.2 and 0.4 wt% (below the niobium solubility in zirconium).Samples have been corroded in PWR primary water conditions and in 415 °C steam. In situ electrochemical impedance spectroscopy was used to monitor the impact of precipitated niobium on proton reduction kinetics at the surface of the oxide and the hydrogen pickup fraction was followed by Inert Gas Fusion. Isotopic exposure in heavy water (D2O) was used to characterize the hydrogen diffusion paths in the oxide layer (SIMS and Atom Probe Tomography) and the hydrogen trapping sites (Thermal Desorption Spectrometry). TEM investigations complete the study to characterize the effect of niobium content on the microstructure of zirconia layer and underlying alloy.Results confirm the significant effect of the niobium content on hydrogen pickup fraction) of Zr-Nb alloys corroded in steam (see figure below). Contributions of both solid solution and surface segregated niobium on the low hydrogen uptake of M5® alloy will be discussed at microscopic and macroscopic scales. Effect of niobium content on hydrogen pickup fraction of Zr-Nb alloys (Steam, 415 °C, 100 bars)M5® is a trademark of Areva NP/Framatome registered in the USA and in other countrie