The environmental impact of electricity generation sources can be generically characterized by their use of natural resources, the thermal pollution they cause, and their emissions of chemical pollutants and radionuclides. Nuclear power plants generate concern regarding their radioactive emissions, which are often poorly understood. A presentation is made of the known data on the environmental impact of pre-Gen III nuclear power plants during normal operation and as a consequence of severe accidents. The radiation doses received by the public and exposed workers as a consequence of nuclear power are compared with the ones because of natural radioactivity and medical applications. The characteristics of the new Gen III reactors which will bring significant improvements to the environmental impact of nuclear power generation are discussed in detail.