The results of the study of the temperature regimes of the evaporator tubes in the area around the critical pressure of one-phase and twophase are given in the paper. The studies carried out in the field of single-phase critical pressure environment can be classified into two groups according to their physical nature: 1) heat exchange process at large values of not heating the liquid up to the pseudocritical temperature at the inlet of the evaporator pipe; 2) heat exchange process at small values of at the pipe entrance. It is appropriate to use the proposed method in order to precalculate the temperature of the screen tubes in partial load modes of steam generators. With the help of this method, we can determine the reliability of the steam generator in this mode by checking the probability of occurrence of a deteriorated mode in the evaporator tubes in a given mode; The change of the temperature regime of the pipes in the area around the two-phase critical pressure is subject to a general regularity; Equations are proposed to calculate the temperature of the pipe wall in the degraded regime and the enthalpy of the fluid at the boundary of the degraded regime.