The complete decommissioning process involves a number of stages (or activities) that shall be performed, even if the logical sequence may be changed according to the specific strategy.

Decommissioning Planning — The decommissioning planning is usually started while the plant is still operating. Some decision making process and some planning, associated with some cost evaluation shall be started well on time, since it is needed also for fund accumulation, that usually is performed during plant operation.

Post-Operation — It is the sum of the activities that are needed to maintain the safety of the plant even after the plant has been definitely shutdown. These activities are more relevant while the spent fuel is still present in the plant.

Characterization — The knowledge of the radioactive inventory in the systems, components and structures before start of decommissioning is a fundamental information to define strategy, costs, technologies and so on. Characterization is also an important process during plant dismantling in order to know exactly the content of the produced wastes. Finally, characterization is also an important, and complex activity to demonstrate that structures and systems, that have not been dismantled because no radioactive, are actually in this condition. In Table 2 a typical list of radioactive isotopes relevant to the decommissioning of NPP’s is reported.

Decontamination — It is an activity that is oriented to remove radioactivity from systems and structures, in order to release components, to reduce doses to workers and to reduce the volume of wastes. It is applied to floors, walls, piping, etc. and may be performed essentially with mechanical or chemical means.

Dismantling — It is the real demolition activity. It may be a rather simple and quick activity, using conventional tools, but it may also be a very complex activity in the case of highly radioactive parts, using remote cutting and other sophisticated tools.

Safe Storage — It is the period in which the plant is left in dormancy, waiting for the radioactive decay. The plant is not left without controls, but a number of activities are still needed and, in some cases, some maintenance and even construction activities are necessary to maintain the safety for the workers and the public.

Table 2 — Major radionuclides identified in facility characterization

2. Major factors relevant to the decommissioning strategy and planning

Optimization of a decommissioning strategy is a rather complex process. The parameters to be considered can be grouped in 3 categories: technical, economical, socio-political.