[en] A study to improve the performance of Zircaloy fuel rods is described which is directed primarily at the development of a mathematical model of their performance. The model is to be based on rational concepts of metallurgy, mechanics and other branches of science; it will include laboratory measurements and operating data to establish appropriate values of the constants but the forms of the model will, insofar as possible, be based on fundamental concepts. Key aspects of the mechanistic model development process are (1) detailed prediction of the likely loadings imposed on the cladding by the thermal expansion of the UO₂ fuel pellets, (2) quantitative predictions of the stress and strain seen by the cladding as a response to these loadings and to the irradiation environment, (3) detailed interpretation and analysis of laboratory test results on failure of irradiated Zircaloy, and (4) development and verification of a model to predict fracture of the cladding by PCI