The results of investigations of swelling and in-reactor creep of the ferrite-martensite class steels to damaging dose in the range 20-89 dpa are presented. The characteristics of radiation creep of three of the ferrite-martensite steels are close despite the differences of the chemical composition and heat-treatment. The relation B = 4.864·10 -2 + 1.45·10 -3 T describes the average modulus of radiation creep of the experimental steel in the interval 305-550°C.The transition of nuclear technologies to a new level and the development of next-generation reactor cores require structural materials for fuel elements and fuel assemblies that are capable of operating to damaging dose 150 dpa and higher. The development of such materials must be based on reliable data on the radiation damageability of prototype steel and research on the structural changes occurring under irradiation and the infl uence of these changes on the physical-mechanical properties. In this respect, ferrite-martensite class steels are the most promising materials for use in new nuclear plants. Several types of 12% chromium steel have been developed in our country for different purposes [1][2][3][4]. One of the most widely available steels is EP-450, which is used for fuel-assembly jackets and guide tubes of the safety-and-control system channels in BN-600 and -800 reactors [1, 5, 6]. The steels EI-852 and EP-823 were developed for facilities with lead-bismuth or lead coolant [2,7,8].The main advantages of ferrite-martensite steel are high resistance to vacancy swelling and low rate of radiation creep together with acceptable mechanical properties, which was shown in a post-irradiation investigation of this steel [9-13]. Nonetheless, there are not enough experimental data on radiation resistance; the greatest data defi ciency is observed for high damaging dose as well as low and high irradiation temperatures. Part of this data was obtained by irradiating the steel in BN-350. This reactor was widely used to irradiate samples of structural materials. A feature of this reactor was low sodium temperature at the core entry, which together with throttling of the assemblies or additional heating of the samples made it possible to conduct irradiation in a temperature range that is inaccessible for other reactors.Material-research assemblies irradiated in BN-350 were investigated in the hot laboratory at the FEI over a period of many years. In the present article, we present the results of investigations of swelling and radiation creep of irradiated