The designing of NPP steam generators, which are complex technical systems, requires a large number of variant calculations, as result of which must be obtained both optimal integral characteristics of steam generators (total heat transfer surface, metal capacity, hydraulic circuits, etc.) and many of their local characteristics (distributions heat flux densities and temperatures, possible amplitudes of their pulsations and others). The complexity of NPP steam generators as technical systems is determined by their multi-parameter, the presence of a complex logical and functional relationship between the parameters, the presence of various restrictions on the change of parameters, the stochastic change of parameters during exploitation. The article presents methods of mathematical modeling of thermal and hydraulic processes in NPP steam generators and modular programs of their calculations in one-dimensional formulation on the example of a steam generator with countercurrent (direct current) mutual motion of the coolant and the working substance. Logical and structural schemes of computer programs of thermal and hydraulic calculations of NPP steam generators are presented.