Nuclear power plant is a station which uses controlled fission process that generate big ammount od energy, to create electric energy. Thermal energy that's used in creating steam which powers the turbine connected to electrogenerator is released inside the reactor. Depending on the year of commisioning, we can divide nuclear power plants in four generations. First two were being developed until the end of 20th century, and the others are, and will be in use until 2030. Reactors can be divided in more different types considering fuel type, moderator and cooling medium. The most common are pressure reactors and boiling reactors. Boiling reactors work on the similar principle as pressure reactors, but without steam generator because the steam is generated inside the very core. From the first and second generation reactors it is important to mention high-flow reactors, gas-cooled reactors and liquid metal cooled reactors. Through the years nuclear power plants advances with reactor preformances and also characteristics in its entirety which can be seen in increasing security and reducing plant components. Nuclear power plants don't pose much threat to the environment. New designs are becoming even safer, ie. they have less possibility of core damage and radioactive particle emission to the environment.