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.

原子能机构建议采用分级方法，采取与核电厂退役风险大小相称的安全措施。

本文建立了采用分级方法支持核电厂退役计划发展的安全评价方法的基本概念。退役期间对公众的辐射暴露风险后果的严重性分为以下三类。

第1类-必需的安全措施。

第2类-优化辐射防护所需的安全措施。

第3类-不需要安全措施。

根据其剩余放射性和预期事故的后果，建立了将每个物体（例如设备和建筑物）分解为三类的程序。压水堆工厂要拆除的每个物体都采用简化的方法对剩余的放射性进行了分类。据估计，类别1的重量比约为0.03％，类别2的重量比约为16％，类别3的重量比超过80％。

规定应集中执行安全措施的退役活动的范围以及优化辐射防护的退役计划的范围已变得适用。