Experimental study on high emissivity thicker oxide Zr-2.5Nb pressure tube material

Zirconium-alloy tubes with high emissivity are important for decay heat removal under accident conditions. During high temperature heating, the tube surface condition changes, e.g., increased oxidation (oxide thickening) or oxide dissolution (thinning of oxide layer), and as a result the tube emissivity can be affected drastically. Previous studies on Zr-2.5Nb pressure tube with thin oxide (1-2 µm), when heated under vacuum, showed that the emissivity decreased from 0.70 at 600°C to 0.35 at 800°C, and gradually decreased to 0.25 at 1000°C. The decreased emissivity was attributed to the dissolution of thin oxide layer into the metal matrix making the surface shiny. In this experimental study, Zr-2.5Nb pressure tube material with thicker oxide (11-14 µm) was evaluated. In comparison to thin oxide materials, the emissivity of thicker oxide materials decreased from 0.75 at 600°C to 0.65 at 800°C and remained at this value until 850°C, and decreased to 0.35 at 1000°C and then gradually increased to 0.39 at 1100°C. The retention of high emissivity at higher temperatures (>800°C) is largely attributed to the thicknesses of the oxide layer remaining on the surface of the thicker oxide materials. The thicker oxide feature provides a safety enhancement in some power reactor applications.