Journal of the Less-Common Metals, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML 103 (1984) 163 - 171 163 THE INITIAL KINETICS OF URANIUM HYDRIDE FORMATION STUDIED BY A HOT-STAGE MICROSCOPE TECHNIQUE* J. BLOCH, F. SIMCA, M. KROUP, A. STERN, D. SHMARIAHU and M. H. MINTZ+ Nuclear Research Center, Negeu, P.O. Box 9001, Beer-Sheva (Israel) Z. HADARI Department of Nuclear Engineering, Ben Gurion University of the Negev, Beer-Sheua (Israel) (Received April 8, 1984) Summary The initial stages of the formation of uranium trihydride on the surface of uranium samples reacted with gaseous hydrogen (about 1.5 atm) were studied utilizing a hot-stage microscope. The nucleation and growth pro- cesses of the product hydride were continuously monitored with a television camera and were recorded on a videotape. The formation kinetics and the morphological characteristics of the developing hydride phase are discussed. A comparison with the kinetic results obtained in the more advanced bulk hydriding stage is made. 1. Introduction The kinetics of the reaction between gaseous hydrogen and bulk uranium have been extensively studied [l]. The reaction initiates with a stage characterized by a non-linear rate equation. The kinetic behaviour at this stage is very sensitive to the purity of the reacting gas and to the experimental conditions (temperature and pressure). Decelerating rate behaviour has been observed above about 175 “C for very pure hydrogen at ambient pressure whereas, at lower temperatures, initial S-type behaviour (i.e. accelerating rate followed by a decelerating stage) is displayed at this initial stage [ 21. For hydrogen containing traces of air impurities an induc- tion time is observed followed by a period during which the rate accelerates to a maximum. The initial non-linear stage (accelerating, decelerating or S shaped) is followed by a bulk hydriding stage which obeys the contracting- envelope rate equations [ 11. These equations describe a situation in which *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides IV, Eilat, Israel, April 9 - 13, 1984. +Also at Department of Nuclear Engineering, Ben Gut-ion University of the Negev, Beer-Sheva, Israel. 0022-5088/84/$3.00 @ Elsevier Sequoia/Printed in The Netherlands