Post-reactor studies of experimental fuel pins with mixed nitride uranium-plutonium fuel and EP823-Sh, ChS68-ID kh.d., and EK164-ID kh.d. steel cladding, which have been irradiated in BOR-60 and BN-600 reactors, indicated the nitriding and carburization of the cladding inner layer, which leads to an increase in its microhardness and a reduction in ductility characteristics. In fuel pin claddings, made of ChS68-ID kh.d. and EK164-ID kh.d. austenitic steel, the processes of carburization and nitriding correlate with sample destruction during mechanical tests. The present article generalizes and analyzes the results of post-reactor studies concerning the carburization and nitriding of the fuel pin claddings made of ChS68-ID kh.d., EK164-ID kh.d., and EP823-Sh steel. The carburization and nitriding of the cladding inner surface was studied by electron microprobe analysis using the cross-sections of fuel pins.