HIGH-PRESSURE OXIDATION OF SPONGE-ZR IN STEAM HYDROGEN MIXTURES/

A thermogravimetric apparatus for operation in 1 and 70 arm steam-hydr ogen or steam-helium mixtures was used to investigate the oxidation ki netics of sponge-Zr containing 215 ppm Fe, Weight-gain rates, reflecti ng both oxygen and hydrogen uptake, were measured in the temperature r ange 350-400 degrees C. The specimens consisted of thin sponge-Zr laye rs metallurgically bonded to a Zircaloy disk. The edges of the disk sp ecimens were coated with a thin layer of pure gold to avoid the delete rious effect of corners. Following each experiment, the specimens were examined metallographically to reveal the morphology of the oxide and /or hydride formed. Two types of oxide, one black and uniform and the other white and nodular, were observed on sponge-Zr surfaces oxidized in steam environments at 70 atm. The oxidation rats when white-nodular oxide formed was a factor of two higher than that of black-uniform ox ide at 400 degrees C for steam contents above 1 mol%. The oxidation ra te was independent of total pressure, the carrier gas (H-2 or He) and steam content above similar to 1 mol%. The oxidation kinetics of spong e-Zr follows a linear law for maximum reaction times up to similar to 6 days, The oxidation rate in steam-hydrogen mixtures at 70 arm total pressure decreases when the steam content approaches the steam-starved region (similar to 0.5 mol% steam at 400 degrees C and similar to 0.0 2 mol% steam at 350 degrees C). Lower steam concentrations cause massi ve hydriding of the specimens, Even at steam concentrations above the critical value, direct hydrogen absorption from the gas was manifest b y hydrogen pickup fractions greater than unity.