Abstract
Chemical interactions between UO 2 fuel and Zircaloy-4 cladding under isothermal and transient temperature conditions up to the melting point of zircaloy (Zry) are described. The tests were conducted in inert gas (1 to 80 bar) with 10 cm long zircaloy cladding specimens filled with UO 2 pellets. In the isothermal tests, the annealing temperature varied between 1000 and 1700°C and the annealing period between 1 and 150 min. The transient experiments were conducted from 1000°C to maximum temperatures of 1400, 1500, and 1600°C. The extent of the chemical reaction depends decisively on whether or not good contact between UO 2 and zircaloy has been established. If solid contact exists, zircaloy reduces the UO 2 to form oxygen-stabilized α-Zr(O) and uranium metal. ZrO 2 does not form. The uranium reacts with zircaloy low in oxygen to form a (U, Zr) alloy which is liquid above about 1150°C and lies between two α-Zr(O) layers. The isothermal UO 2/zircaloy reaction obeys a parabolic rate law. The growth of the reaction layers can be represented in an Arrhenius diagram.
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