After fastneutron irradiation, uranium-plutonium nitride U 0.8 Pu 0.2 N is shown to acquire a com� plex structure consisting of a solid solution that is based on the nitrides of uranium, plutonium, americium, neptunium, zirconium, yttrium, and lanthanides and contains condensed phases U2N3, CeRu2, BaTe, Ba3N2, CsI, Sr3N2, LaSe, metallic molybdenum, technetium, and U(Ru, Rh, Pd)3 intermetallics. The con� tents and compositions of these phases are calculated at a temperature of 900 K and a burnup fraction up to 14% (U + Pu). The change in the composition of the irradiated uranium-plutonium nitride is studied during the electron decay of metallic radionuclides. The kinetics of transformation of U 103 Ru3, 137 CsI, 140 Ba3N2, and 241 PuN is calculated.