The supersaturation of point-defect solutions is markedly maintained by irradiation. These solutions of vacancies and interstitials are metastable, and their decay results in the formation of new phase nuclei, the interstitial and vacancy clusters. The crystal response to the interstitial supersaturation can be described by means of the chemical potential of interstitials, which is connected with the interstitial flux by Onsager's relations. If di-interstitials and other interstitial clusters are taken into account in the chemical potential, then under appropriate interstitial supersaturation small positive deviations of interstitial concentration from a spatially homogeneous value will not be damped by spatial diffusion. On the contrary, these deviations will grow in time. This means that the spatially homogeneous interstitial solution appears to be unstable with respect to small interstitial cluster formation. Interstitial cluster nucleation can be both spatially homogeneous and very heterogeneous. In the latter case, the nucleation of dislocation walls can take place. The corresponding spatially inhomogeneous solution results naturally from the interstitial diffusion equation when small interstitial complexes are taken into account in the interstitial chemical potential.