TEMPERATURE-GRADIENT DRIVEN CONSTITUENT REDISTRIBUTION IN U-ZR ALLOYS

Although the phenomenon of constituent redistribution is common in U-P u-Zr alloys irradiated under a wide range of conditions, it has been o bserved in U-Zr alloys only at elevated temperatures. Redistribution i s relatively rapid and is essentially complete by 5 at% burnup. Experi mental observations of constituent redistribution in U-Zr fuel element s are presented and analyzed. A model based on a thermal diffusion mec hanism is proposed, and its computer implementation is described. The model calculations, supported by experimental observation, indicate th at the excess enthalpy of solution of the bcc gamma-phase controls the redistribution process as an additional driving force. A heat of tran sport of -50 to -100 kJ/mol in this phase results in the best match be tween calculation and experimental observations. The model predicts th at constituent redistribution will be observed only when a region of t he fuel operates at temperatures above 935 K.