Investigations of the guide channel (Fe-0.08C-18Cr-10Ni-Ti steel) of the VVER-1000 reactor fuel assembly absorbing element were carried out. The channel was irradiated in the temperature range from 284 to 322 °C and damage dose ranging from 1 to 9 dpa. In the microstructure examination the porosity was observed at temperatures of 308 °C and 310 °C at of 8 and 4.5 dpa, respectively. The dose dependencies of the mechanical property characteristics of Fe-0.08C-18Cr-10Ni-Ti steel at test temperatures of 20, 300 and 450 °C were obtained. With the damage dose increasing, a decrease in the steel plasticity and an increase in hardness is observed at all test temperatures. The obtained results were compared to those of investigations on Fe-0.12C-18Cr-10Ni-Ti steel irradiated in fast reactors. It was revealed that the temperature boundary 305–315 °C, at which the porosity appears, is nearly the same for steels irradiated in both types of reactors. However, hardening of steel irradiated in the fast reactor is higher than hardening in the thermal reactor under the same irradiation conditions (280–300 °C, 7–9 dpa).