Basic concept of safety evaluation method for decommissioning of nuclear power plants by applying a graded approach

The IAEA has proposed to apply a graded approach that takes the safety measures commensurate with the magnitude of risk in decommissioning of nuclear power plants (NPPs). In this paper, the basic concept of the safety evaluation method to support development of the decommissioning plan of NPPs by applying a graded approach was established. The severity of consequence of radiation exposure risk to the public during decommissioning was divided into following three categories. Category 1 - safety measures required. Category 2 - safety measures required for optimization of radiation protection. Category 3 - safety measures not required. A procedure of classifying each object such as equipment and structures to be dismantled into the three categories was established based on its residual radioactivity and consequence of the expected accidents. Each object to be dismantled of PWR plant was classified using a simplified method with the residual radioactivity. It is estimated that the ratio for Category 1 by weight was about 0.03%, that for Category 2 was about 16%, and that for Category 3 was over 80%. It has become applicable to specify the scope of the decommissioning activities where the safety measures should be carried out intensively and the scope of decommissioning plans for optimizing of radiation protection.