Unavailability of nuclear power plant

The assessment of the economic risks in the design and operation of nuclear power plant related to reliability has perhaps received inadequate consideration in the past. The assessment of risk with regard to individual and societal safety has traditionally been given priority, but with the increasing tendency towards larger and more complex plant the economic risk also becomes prominent. The economic success of power plant can be measured, to some degree, by the achieved availability or by the complement of this, unavailability. This can be defined as the proportion of generating capacity that is lost because of forced outages due to plant failures and scheduled outages for maintenance and refuelling. The paper is mainly devoted to discussion of experience of forced outages and is related to currently operating water- and gas-cooled reactor stations. Some reference is also made to work on other reactor systems and on non-nuclear plant. An important initial requirement of a reliability programme is the setting of an agreed unavailability target for the plant, and methods of assessing a target and apportioning this to the main systems and subsystems are briefly discussed. This includes a review of operational experience of nuclear plant and indicates where deficiencies exist and perhaps improvements can be made. It will also be shown that the non-nuclear systems in a nuclear power plant, e.g. turbo-generators, may be significant. The importance of unavailability assessment and reliability programmes is briefly stated. Examples of the application of reliability engineering and data analysis are also presented. (author)