High pressure hydriding of sponge-Zr in steam-hydrogen mixtures

Abstract

Hydriding kinetics of thin sponge-Zr layers metallurgically bonded to a Zircaloy disk has been studied by thermogravimetry in the temperature range 350–400°C in 7 MPa hydrogen-steam mixtures. Some specimens were prefilmed with a thin oxide layer prior to exposure to the reactant gas; all were coated with a thin layer of gold to avoid premature reaction at edges. Two types of hydriding were observed in prefilmed specimens, viz., a slow hydrogen absorption process that precedes an accelerated (massive) hydriding. At 7 MPa total pressure, the critical ratio of H 2/H 2O above which massive hydriding occurs at 400°C is ∼ 200. The critical H 2/H 20 ratio is shifted to ∼2.5 × 103 at 350°C. The slow hydriding process occurs only when conditions for hydriding and oxidation are approximately equally favorable. Based on maximum weight gain, the specimen is completely converted to δ-ZrH 2 by massive hydriding in ∼5 h at a hydriding rate of ∼10 −6 mol H/cm 2 s. Incubation times of 10–20 h prior to the onset of massive hydriding increases with prefilm oxide thickness in the range of 0–10 μm. By changing to a steam-enriched gas, massive hydriding that initially started in a steam-starved condition was arrested by re-formation of a protective oxide scale.