A complex structure is shown to form in the uranium–plutonium nitride U0.8Pu0.2N irradiated by fast neutrons. It consists of a uranium-based solid solution; plutonium, zirconium, yttrium, and lanthanide nitrides; and individual condensed phases such as U2N3, BaTe, CeRu2, LaSe, Rh3Te2, USe, Ba3N2, CsI, Sr3N2, metallic molybdenum, and U(Ru,Rh,Pd)3 intermetallic compounds. The amount and composition of these phases are calculated at temperatures of 900 and 1900 K in the process of depletion to 18% heavy atoms (U + Pu). The variation of the composition of the irradiated uranium–plutonium nitride is studied upon the electron decay of metallic radionuclides. The kinetics of the transformations of 89Sr3N2 and 90Sr3N2 to 89YN +89Y and 90ZrN +90Zr, respectively, is calculated.