In this text, radioactivity and the chemical reactions of radionuclons within the different areas of a nuclear power plant are discussed. The text concentrates on commercially-operated light water reactors which currently represent the greatest faction of the world"s nuclear power capacity. In 1994, the total electricity production of such plants corresponded to approximately 17 per cent of the global production. Part A of the text is devoted to a short description of the design of different types of plants in current operation, focusing mainly on those aspects of the plants that are relevant for radionuclide chemistry and behaviour. In Part B, radiochemistry in the primary systems of the plant during normal operation state is discussed. Part C addresses accident conditions, which are different in essential ways as regards to radionuclide chemistry, from normal operating conditions.