Possible approaches to decommissioning a Nuclear Power Plant may be widely varied and the optimal choice is made on the bases of a number of parameters which, in most cases, are site specific or, at least, country specific. Therefore there is no single optimal approach for all facilities. Decommissioning process could be subdivided, in somewhat schematic way, into stages. There is no official definition of the different stages, each country using its own definitions which vary slightly to suit each case. We could mention, as an example, the IAEA Technical Reports Series N° 375, ‘’Safe Enclosure of Shut Down Nuclear Installations’’, 1995, which provides the following definitions:
"According to the definition of IAEA stages of decommissioning, the nuclear fuel or radioactive materials in the process systems as well as radioactive waters produced in normal operation is first removed by routine operation. Each of the three decommissioning stages of a nuclear plant can be defined by:

> the physical state of the plant and its equipment;

> the surveillance, inspections and tests necessitated by that state.

Stage 1

a) The first contamination barrier is kept as it was during operation but the mechanical opening systems are permanently blocked and sealed (valves, plugs etc.). The containment building is kept in a state appropriate to the remaining hazard. The atmosphere inside the containment building is subject to appropriate control. Access to the inside of the containment building is subject to monitoring and surveillance procedures.

b) The unit is under surveillance and the equipment necessary for monitoring radioactivity both inside the plant and in the area around it is kept in good condition and used when necessary and in accordance with national legal requirements. Inspections are carried out to check that the plant remains in good condition. If necessary, checks are carried out to see that there are no leaks in the first contamination barrier and the containment building.

Stage 2

a) The first contamination barrier is reduced to a minimum size (all parts easily dismantled are removed). The sealing of that barrier is reinforced by physical means and the biological shield is extended if necessary so that it completely surrounds the barrier. After decontamination to acceptable levels, the containment building and the nuclear ventilation systems may be modified or removed if they no longer play a role in radiological safety and, depending on the extent to which other equipment is removed decontaminated, access to the former containment building, if it is left standing, can be permitted. The non-radioactive parts of the plant (buildings or equipment) may be converted for new purposes.

b) Surveillance around the barrier can be relaxed but is desirable for periodic spot checks to be continued, as well as surveillance of the environment. External inspections of the sealed parts should be performed. Checks for leaks are no longer necessary on any remaining containment buildings.

Stage 3

All materials, equipment and part of the plant, the activity of which remains significant despite decontamination procedures are removed. In all remaining parts contamination has been reduced to acceptable levels. The plant is decommissioned (released) without restrictions. From the viewpoint of radiological protection, no further surveillance, inspection or tests are necessary.’ ’

Other terms that are widely used to describe the strategy adopted for the decommissioning are those that have been introduced in USA by the Nuclear Regulatory Commission (US NRC):

DECON (or one step dismantling):

In this strategy, all components and structures that are radioactive are cleaned or dismantled, packaged and transported to a low-level waste disposal site (if available) or stored temporarily on site. Once this task is completed, the facility can be used for another power plant or other purposes, without restrictions.

SAFSTOR (or Safe Storage):

In SAFSTOR, the nuclear plant is kept intact and placed in protective storage for a very long time (up to 60 years[10]), and afterwards it is dismantled. This method, which involves locking that part of the plant containing radioactive materials and monitoring it with an on-site security force, uses time as a decontaminating agent—that is, the radioactive atoms "decay" by emitting their extra energy to become non-radioactive or stable atoms. [11]Once radioactivity has decayed to low levels, the activity is the same as the one described above as DECON. All building structures and systems which are necessary for workers and public safety shall be maintained in service during the safe storage period. A pre-condition to reach the safe storage condition is that the fuel has been removed from the plant and that radioactive liquids have been drained from systems and components and then processed.

ENTOMBMENT:

The radioactive inventory is enclosed in a monolithic structure, e. g. concrete, to secure the public safety. The monolithic structure should ensure integrity for about 100 years to derive benefit from the decay of the nuclides. After the entombment period, all enclosed components are very low radioactive and the assumption should be that dismantling at that time can be performed in a “conventional” way. During entombment the plant remains under a nuclear license.

The 3 categories presented above are a crude schematization of various situations. The DECON strategy for example may imply a really “quick” decommissioning and dismantling, or a longer process, that might optimize the use of plant personnel and reduce costs associated with engulfing activities on site.

On the other side, the SAFSTOR option may really imply a simple “close and seal the door”, or a combination of immediate dismantling and safe store. In the latter case it may be considered the immediate dismantling of systems and buildings which are not, or only slightly, contaminated and a SAFSTOR strategy for the most radioactive portion of the plant. Also the safe storage period may range from 30 to more than 100 years, depending on a number of parameters and conditions that will be discussed later.

The third strategy (ENTOMBMENT) has never been applied yet to a NPP. There are several reasons for that. The first one is that the size of a NPP is too large to be simply entombed. A second reason is related to the fact that most power reactors will have radionuclides in concentrations exceeding the limits for unrestricted use even after 100 years and more and therefore this strategy cannot be successful. ENTOMBMENT is, however, a possible strategy for smaller reactors and for other small nuclear facilities.