ABSTRACTP-Cu2-δ Se(δ=0.025) samples, self-doped through a departure from the stoichiometry(po= 4.8·1020) and cadmium-doped up to a concentration po= 5.6·1020cm−3 were studied in the course of their irradiation in a reactor up to a fluance of -6·1020 n·cm−2 of fast neutrons at a temperature of -500-550° C. The dependence of the resistivity and thermo-emf on the fast neutron fluence is described in terms of the theory of effective medium and representation of defect clusters as dielectric inclusions in a conducting matrix of p-Cu2Se(Cd) and p-Cu2_δSe samples.Defect clusters of a mean radius of -27 Å are shown to appear not only because of scattering of the fast pile neutrons on copper and selenium atoms, but also due to a nuclear reaction 112Cd(n,γ)114Cd on thermal neutrons with an efficiency of 3.43 cm−1. Increase of the conductivity in the process of the reactor irradiation of p-Cu2-δSe at small fluences of fast neutrons and appearance of a whitish deposit of selenium on the surface of samples after high fluences of fast neutrons were observed.