Abshact, The formation and evolution of defect microstructures in inadiated materials is analysed in he framework of a dynamical model for the evolution of the two fundamental defects of irradiated microsmctures, namely vacancy and interstitial clusters. The effects of irradiation on materials is described by dynamical equations for W O mobile atomic Size species (vacancies and interstitial atoms), and two basic immobile elements of the microstructure (vacancy and interstitial clusters). It is

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... hown that uniform vacancy and interstitid loop distributions may become unsfable during irradiation and that they will form large-scale spatially organized distributions, in a specific range of irradiation and material conditions. The selection and stability of the resulting microsmctures studied in the quasi-static approximation and in the weakly non-linear regime around the bifurcation point It is shown that. after transients corresponding to three-dimensional BCC patterns, the final pattern should correspond to planar wall structures in agreement with experimental observations.