The method of multinodal treatment of nuclear transition and spatial migration of radioactive materials in complex nuclear technological systems is described. The modeling of such processes is established in a very flexible way, enabling the user to investigate a wide range of problems related to nuclear safety, i.e., to predict the radioactive source term in normal operation and under accidental conditions of any nuclear plant. The migration of radioactive material can be connected either with the flow of reactor coolant or with any of the other physicochemical processes (filtration, deposition, etc.). On the basis of this method, a modular code system - TIBSO - has been developed.