After the Fukushima nuclear accident the problem of safe interaction between a nuclear power plant and the power grid requires urgent attention. The Ukrainian power grid is the backbone for stable development of all dependent infrastructures. Nuclear power plants can be seen as elements interacting with the system for power distribution. There are five nuclear power plants (NPP) in Ukraine. The NPP safety depends on the reliability of its subsystems, components, etc., as well as on the safety levels of other power grid subsystems. There are various interdependencies among NPPs and the power grid, which impact the safety levels of both. The balance of these mutual influences is considered as a basis for the stability of any infrastructure. The change of influences could lead to violation of the balance, that in turn may lead to a change in the subsystem’s state. This paper presents an approach for formalization of different types of influences between dependent infrastructures. This approach supports the analysis of the behaviour of infrastructure subsystems and the prediction of their safety levels, considering the change of states. Two metrics are proposed to evaluate the influences: linguistic and numerical. The influence formalization enhances the understanding of risk proliferation and the assurance of NPP safety.