Journaf of rhe Less-Common A4etals, 146 ( 1990) 241-25 1 241 THE EFFECT OF THERMAL ANNEALING KINETICS OF URANIUM ON THE HYDRIDING JOSEPH BLOCH and MOSHE H. MINTZ T&clearResearch Center-Negev, P.O. Box 9001, Beer-Sheva and Ben-Curion University ofthe Negev, Department of Nuclear Engineering, P.O. Box 65.3,Beer-Sheva (Israel) (Received January 8, 1990; in final form May 28,199O) Summary The effects of vacuum heat pretreatments on the hyd~d~g kinetics of ur~um (pure and chromium alloyed) were studied. Two pretreatment temperature regimes were correlated with changes displayed in the two stages of the hydriding reaction. The lower pretreatment regime (below about 200°C) has a drastic effect on the initial stage of the reaction (the “induction period”. However, it does not alter the rate of the subsequent massive reaction stage. This low temperature region is associated with surface modification processes, the most prominent process being gas desorption. However, the higher pretreatment regime (above about 450 “C) affects the rate of the massive reaction stage zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR (i.e. the velocity of the hydride reac- tion front) and is attributed to heat-induced changes in the microstructure of the metal (grain growth). The increased rate induced by chromimn addition is attributed to the precipitation of chromium at gram bound~ies. 1. introduction Studies of metal hydriding kinetics often require a vacuum thermal annealing prior to the hydriding reaction (an outgassing step) in order to achieve more accurate and reproducible results [ 1, 21. This step is aimed at shortening the so- called “induction period” which normally appears as an initial step in many gas-solid and solid-solid reactions (3,4]. It should be noted, however, that vacuum annealing may also induce changes in the metal microst~~t~e possibly affecting the bulk reaction rate. The kinetics of hydriding reactions are very sensitive to the parameters of the’ gas-solid system. Factors such as metal structure, sample preparation, gas purity etc. may drastically affect the kinetic behaviour of a given system [S]. Apart from its importance in nuclear technology, the U-H reaction provides a very ,convenient model system for the kinetic studies of hydride formation (4, 6-81. The simplici~ of the U-H phase diagram (i.e. the formation of a single, nearly stoichiometric UH, hydride phase and the negligible solubility of hydrogen 0022-5088/90/$3.50 0 Elsevier Sequoia/Printed in The Netherlands