In the paper, principles of constructing systems and functional abilities of information support of human-operators (IOSS) of power units of nuclear power plants (NPP) and their place within the system of upper unit-level of automated process control systems (APCS) of NPP. The purpose of implementation of the NPP IOSS is preventing or decreasing the frequency and heaviness of operator errors appearing under transients and non-regular situations, when human-operators perform under the conditions of lack of the time, fast change of the actual status, and huge volumes of information, conditioning the excessively high work load. Within the frameworks of the NPP IOSS, issues of creating flexible modeling complexes are considered, enabling one to turn the system to an NPP power unit of a given type.