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

Abstract

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>1 MeV), 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.