During operation of water-water reactors including KLT-40 type reactors widely used nowadays on icebreakers and floating power units (FPU), the issue of the duration of safe operation of steam generators is acute. The problem is connected with the formation of microcracks in the steam generator water and steam piping during normal operation at power, through which radioactive nitrogen 16N7 enters the steam piping, the content of which in the steam is just a sign of leakage failure of the steam generator second circuit water and steam piping. This effect, further named "leaks", was considered earlier in a number of works of the authors under conditions of cyclic operation of the steam generator water-steam mode. In the present work the main attention is paid to mathematical and physical analysis of the mechanisms causing the formation of microcracks in the steam pipeline of a steam generator arising in the steam pipeline region in which water and steam pressures are balanced, and their effective length is estimated. Calculations of temperature distribution along the steam-pipe thickness in the steam-water transition region at periodic "slamming" of this region with water are carried out. It is shown that depending on the time period of steam-water oscillations, the radial temperature distribution along the tube thickness shows a significant difference in the distribution at a constant temperature on the outer surface of the tube, and the maximum temperature difference is observed in the area of the inner surface of the tube. The distribution of mechanical stresses arising in the tube metal is characterized by a linear dependence of stresses on temperature, the maximum value of which corresponds to the maximum temperature difference characteristic of the area of the inner surface of the tube. This allows us to state that the formation of cracks starts exactly from the area of the inner surface of the tube of the steam generator in the specified transitional area of the tube.