Safety considerations in the transition from operation to decommissioning of nuclear facilities

The stages in the lifetime of nuclear facilities are: siting, design, construction, commissioning, operation and decommissioning. The final phase, decommissioning, is a complex process involving operations such as detailed radiological surveys, decontamination and dismantling of plant equipment and systems, demolition of buildings and structures and processing of the resulting radioactive and non-radioactive waste. The number of existing nuclear power plants in the world exceeds 500. Of these, 108 are in the shutdown phase or are undergoing decommissioning. Only 13 nuclear power plants have been fully decommissioned so far. A considerable amount of experience has already been gained from the decommissioning of nuclear power plants by various Member States, but the experience achieved on the transition from operation to decommissioning of nuclear facilities is concentrated in only a few Member States. This report intends to transmit the experience gained by these to other Member States. The transition period between operation and the implementation of decommissioning involves some routine operations along with others that may be specific to the transition stage based on the type of facility. This period can be confusing and stressful to plant workers and, in addition, operational safety may be compromised. During this period a number of plant, system and organizational modifications will be necessary to meet the objectives and requirements associated with decommissioning. A change of thinking within the management and workforce is also needed to respond to these new objectives and to the different management and working practices. A number of IAEA Safety Standards apply to the decommissioning of nuclear facilities. In these publications the term decommissioning refers to the administrative and technical actions taken to allow the removal of some or all of the regulatory controls from a nuclear facility (except for a repository, for which the term closure and not decommissioning is used). These actions are carried out to achieve a progressive and systematic reduction in radiological hazards and are taken on the basis of preplanning and assessment to ensure safety during decommissioning operations. IAEA Safety Standards acknowledge that there is a large range of possible decommissioning strategies, but the standards focus on three main options: immediate dismantling, when decommissioning is carried out in one continuous operation following shutdown; safe enclosure or the deferred dismantling option, which consists of a minimum degree of early dismantling and conversion of the plant to a safe storage condition for a defined period of time before final dismantling; and entombment, when the resulting facility becomes, essentially, a waste repository. There are some important operations that are independent of the decommissioning strategy and that need to be carried out promptly after shutdown as part of the operational phase of the facility in order to achieve a large reduction in radiological hazards. The objective of this Safety Report is to provide information to Member States to help in ensuring safe management of the transition from the operational phase to the beginning of implementation of the decommissioning strategy for nuclear facilities. The transition period may be defined as the time from when a facility is permanently shut down until the decommissioning strategy begins to be implemented. This period is normally considered to be part of the operational phase. The goal during the transition period is to achieve a significant reduction in radiological hazards through the safe termination of operational activities and removal of radioactive material, and to place the facility in a stable and safe condition until the decommissioning strategy is implemented. During this period, control of any remaining spent fuel, other radioactive material or nonradioactive hazardous material should be maintained, and the safety of the workers and the public, and protection of the environment, should be ensured