[en] Mechanisms of pore migration and actinide redistribution on the pore surfaces in (U, Pu)O₂ nuclear fuel at constant temperature drops and chemical potential drops of oxygen bet ween ''hot'' and ''cold'' surfaces of pores are investigated. Technique for estimation of actinide concentration near the surface of a pore migrating via the evaporation-condensation mechanism is developed. An essential role of intergranular boundaries in delivery of a highly volatile component from the depth of the oxide to evaporating ''hot'' surface of a pore volatile-component-depleted due to incongruent evaporation is disclosed. Rates of pore migration and actinide concentration on the ''cold'' surface of a pore are calculated. The obtained results permit to explain a high rate of actinide redistribution at the initial moment of fuel element operation