Temperature gradient driven constituent redistribution in U-Zr alloys

Although the phenomenon of constituent redistribution is common in U-Pu-Zr alloys irradiated under a wide range of conditions, it has been observed in U-Zr alloys only at elevated temperatures. Redistribution is relatively rapid and is essentially complete by 5 at% burnup. Experimental observations of constituent redistribution in U-Zr fuel elements are presented and analyzed. A model based on a thermal diffusion mechanism is proposed, and its computer implementation is described. The model calculations, supported by experimental observation, indicate that the excess enthalpy of solution of the bcc gamma-phase controls the redistribution process as an additional driving force. A heat of transport of -50 to -100 kJ/mol in this phase results in the best match between calculation and experimental observations. The model predicts that constituent redistribution will be observed only when a region of the fuel operates at temperatures above 935 K.