In PWR Comprehensive Study of High Burn-Up Corrosion and Growth Behavior of M5® and Recrystallized Low-Tin Zircaloy-4

Abstract

This paper is dealing with the corrosion and growth behavior of M5® and recrystallized low tin Zircaloy-4 irradiated as stress-free tubes in conditions representative of grids and guide-tubes of PWR fuel assemblies. The low-tin Zircaloy-4 tubes have reached equivalent burn-ups up to 93 GWd/tU (corresponding to a fluence of 21.0×1025 n·m−2, E>1MeV), and the M5® tubes have reached equivalent burn-ups up to 79 GWd/tU (corresponding to a fluence of 17.1×1025 n·m−2). Postirradiation growth was measured by comparing the distances between holes regularly distributed along the rod to the initial measurements. Postirradiation outer diameter oxide thicknesses were measured by eddy currents. Detailed characterizations of oxide layers have been realized through optical microscopy and scanning electron microscopy. Hydrogen uptake has been obtained through global hydrogen content measurements and metallographic examination of hydrides. Finally, the irradiation-induced microstructure of the metallic matrix has been observed by transmission electron microscopy. On these tubes, the free growth reached 1.9 % at 93 GWd/tU on Zircaloy-4 and 0.3 % at 79 GWd/tU on M5®. The external oxide thicknesses are far greater on Zircaloy-4 (∼60 μm at 93 GWd/tU) than on M5® (∼7.5 μm at 79 GWd/tU), and the oxidation rate is eight times higher on Zircaloy-4 than on M5®. Very high hydrogen content is achieved on Zircaloy-4, up to ∼1600 ppm (due to the low wall thickness and two-sided corrosion), whereas the maximum value on M5® is ∼100 ppm (despite the same geometry and corrosion conditions). On both alloys, the hydrogen pickup fraction is of the same order for these experimental empty rods as for previously analyzed fuel rod claddings, which may indicate the absence of a heat flux effect on the hydrogen uptake. Finally, potential correlation between growth, corrosion and hydrogen uptake will be discussed. Taking account of the results obtained on both experimental tubes and fuel rods, the effects of the presence or absence of heat flux through the wall thickness and of hydride rim at the metal/oxide interface are discussed, especially concerning the high burn-up corrosion acceleration on Zircaloy-4.

Heavy ion irradiation induced dislocation loops in AREVA’s M5® alloy

“…The ion irradiation induced <c>-loops' linear densities are in good agreement with data on ion irradiated Zircaloy-4 by Tournadre et al in 2011 [11], for both Zr and proton irradiation. Also, the measured linear densities agree very well with the density reported by Bossis et al in 2007 for PWR neutron irradiated low tin Zry-4 [37]. They also correspond well to high Sn and O Zry-4 irradiated in the Phénix fast breeder reactor as described by Gilbon et al, 1994 [46].…”

“…The higher magnification leads to a visibility of loop contrasts that can not be observed in the lower magnification images; thus, the linear densities measured in frames taken from the movie are higher. Figure 16: Measured <c>-loop linear densities compared to literature data by Tournadre [11], Bossis [37] and Gilbon [46] The <c>-loops show an approximately homogeneous distribution over the grains; the statistical variation can be seen by comparing the linear densities calculated from the loop size measurements at different grain positions. A change in the <c>-loop density near outstanding microstructural features like grain boundaries, precipitates or the former hydride location was not observed.…”

“…1). The Nb content of these β-Nb precipitates is 85-90 wt.-% [37,38]. To study the thickness of the irradiated layer by EBSD, a cross section of an irradiated sample was prepared and analyzed.…”

“…This is a typical method for the quantification of <c>-loop distributions [11,37,46] and has thus been used to compare the measured <c>-loop densities with literature data. For each sample, several grain regions were chosen to determine the linear density from images taken at a magnification of 30'000.…”

“…The received densities for sample A2 are shown in Fig. 16, together with literature values for heavy ion and neutron irradiated Zr-based alloys [11,37,46]. Different symbols for the measured data are used to indicate the different observed positions in the respective grain.…”