[en] The main mechanism of deformation of a fast reactor fuel pin clad under loading is in-pile creep. Depending upon the value of stress and temperature in a clad, two mechanisms of deformation may be realized: intergranular creep and intragranular dislocation creep, respectively at low stresses and at higher stresses. Intergranular creep is accompanied either by formation of growth of pores along grain boundaries (at T_irr>0.5 T_melt) or by arising and development of cracks at grain triad joints (at higher temperatures). Formation of pores and cracks occurs as the stress exceeds a certain threshold value. The level of failure stresses in the course of reactor operation has been computed in relation to behaviour of an austenitic steel as a fuel pin clad material. (author)