Toward a Better Understanding of Dimensional Changes in Zircaloy-4: What is the Impact Induced by Hydrides and Oxide Layer?

In pressurized water reactors, new operating conditions (higher burnup, new chemistry, etc.) can have an effect on the dimensional stability of the fuel assembly skeleton. Previous studies have shown that the fuel assembly growth is, among others parameters (free growth, creep), strongly driven by corrosion. Oxide layer and hydrides precipitation could both induce an increase of the dimensional parameters. The scope of the present study is, regardless of irradiation effect, to quantify and to understand the separate effects of hydrogen and oxide layers on the Zircaloy-4 dimensional changes. Experimental works have been performed in laboratory on stress relieved annealed (SRA) and recrystallized (RXA) Zircaloy-4 strips. First, the hydrogen impact on dimensional changes has been studied without the effect of the oxide layer. The measurements were performed at room temperature on strips previously pre-hydrided by the gaseous charging method. The hydrogen content of the samples was between 100 ppm and 2000 ppm. Results indicate a linear correlation between hydrogen content and length variation. RXA material is more affected by the hydrogen effect than the SRA material. Nevertheless, in comparison with data issued from out-of-reactor measurement, the impact of hydrides is not sufficient (with irradiation growth) to explain the post-irradiation examinations (PIE) results. To understand these differences, the oxide layer contribution must be quantified. Second, the impact of the oxide layer was therefore studied on RXA Zircaloy-4 strips. Corrosion tests have been performed in autoclave at 360°C in primary water (2 ppm Li-1000 ppm B–H₂) on as-received and pre-hydrided materials. To obtain thicker oxide layer within a shorter duration, samples have been also oxidized in furnace at 415°C. Moreover, as no significant hydriding occurs during oxidation in air, we are able to characterize properly the specific effect of the oxide layer. As for hydrides’ effect, an increase of strain is observed as the oxide thickness becomes thicker. The contributions of hydrides and oxide layer are then discussed with regard to the metallurgical properties of the alloy. Finally, all these results are compared with PIE observations. Free growth, hydride precipitation, and oxide thickness seem to be the three main parameters to explain the dimensional changes in Zircaloy-4 observed in reactor.