For the needs of nuclear power engineering, a methodology of predicting the temperature of brittleness (reference temperature) $Т_0$ is developed. Within the framework of the concept of structural suitability of metals and alloys, a methodology of the stress intensity factor $K_{Q(\textrm{IC})}$ value calculation at room temperature $T_K$ (293 K) from the results of standard smooth samples testing under uniaxial tension is developed. The structural suitability of steels used in nuclear power engineering is assessed. A fundamentally important scientific position is established in the field of the material science concerning mechanical properties, $i.e.$, all structural steels can be divided into two types according to mechanical characteristics’ behaviour that differs in terms of a character of change in the dependence of the ductility indices $\psi_K$ on the strength level $\sigma_Y$, provided that the mechanical stability $K_{ms}$ or the deformation endurance (break resistance) $B_r$ are constant. This circumstance, in turn, leads to the formation of different species’ transitions at different test temperatures. Therefore, without considering these factors, any description of the relationships between complex parameters of alloys and simple (basic) characteristics as well as between these parameters themselves is not possible.