Understanding of corrosion mechanisms of zirconium alloys under irradiation : effect of ion irradiation on the corrosion rate of Zircaloy-4

摘要

Zircaloy-4 at hottest spans shows a strong acceleration in oxide growth kinetics beyond a 35 GWd/tU bumup. One of the hypotheses to explain this kinetic behavior in Pressurized Water Reactor conditions is the damage due to the neutron irradiation [1]. Neutron irradiation can damage many parts of the fuel cladding. Its effects on the metallic matrix are well characterized, but it also generates some defects in the oxide layer formed that can accelerate the corrosion by affecting transport properties in this layer. This work is dedicated to the potential effects of ion irradiation of the oxide layer and the metallic matrix on the corrosion rate of Zircaloy-4. Concerning ion irradiation impact of the oxide layer, the experimental approach chosen is first based on sequential oxidations using, after a pre-oxidation stage in light water and an irradiation step, isotopic exposure in water enriched in ~(18)O followed by Secondary Ion Mass Spectrometry analyses. This approach allows to know the average oxygen diffusion flux through the oxide. In addition, the oxide growth kinetics in light water is also studied by mass gain up to 100 days after irradiation and, at each exposure time, the presence of irradiation defects is checked by Raman Spectroscopy analyses. Irradiation with helium ions is performed to get around 0.4 dpa nuclear damage and a uniformdistribution of irradiation defects in the oxide formed on the samples. Helium ion irradiation of the oxide results in a strong increase of the oxidation rate and this increase depends on the level of nuclear damage. A simple kinetic model of corrosion mechanism taking into account irradiation is finally proposed. Secondly, to study the effect of metal irradiation damage on the corrosion rate of Zircaloy-4, several ion irradiations of Zircaloy-4 metal matrix are carried out to reproduce the evolution of the metallurgical state in reactor. The oxide growth kinetics in light water is followed by mass gain. -Dislocation-loops produced by light ion irradiation induce apparently a significant increase of the oxidation rate. These results will be discussed and compared to the in-pile corrosion behavior.