Kinetic studies on massive hydriding of commercial zirconium alloy tubing

Kinetic studies on the massive hydriding of commercial Zircaloy-2, Zircaloy-4, and ZIRLO(TM) tubes are carried out in the temperature range 300-500 degrees C under atmospheric pressure by in situ measurements with TGA (thermo-gravimetric apparatus). The results show that hydriding kinetics follows a linear rate law after incubation time caused by the thin surface oxide film grown during inevitable prior heating. The reaction rate constants derived on the basis of the linear kinetics are 1.1 x 10(7) exp(-87.0/RT), 6.9 x 10(7) exp(-100/RT), and 1.5 x 10(6) exp(-75.3/RT) mg/dm(2) min for Zircaloy-2, Zircaloy-4, and ZIRLO(TM), respectively, where the activation energy is in kJ/mol. Slight retardations from the linear kinetic behaviors are observed at temperatures lower than 400 degrees C. It is attributed to slow diffusion of hydrogen atoms through hydride layer at the low temperatures. Despite the lack of temperature gradient, sunburst-type localized massive hydridings are unexpectedly observed mostly at the inner surfaces of specimens, even with hydrogen content of around 1000 ppm. This demonstrates the importance of surface condition in the massive hydriding of zirconium alloy tubes. (C) 1999 Elsevier Science B.V. All rights reserved.