The main aim of decommissioning is to place facilities in such a condition that they pose no unacceptable risks to the public, to workers or to the environment. To achieve this, some action is normally required. If facilities were not decommissioned they could degrade and present an environmental hazard in the future. Simply abandoning or leaving a facility after cessation of operations is not considered to be an acceptable alternative to decommissioning. The approach to decommissioning is not always the same. Some countries have chosen to decommission their nuclear facilities as soon as they cease to generate nuclear energy (immediate dismantling), others delay the process for a number of years (deferred dismantling) while others convert their facilities into a form of waste store, after ensuring that they are safe (entombment) (IAEA, 2011c).

Irrespective of the decommissioning strategy chosen, it is necessary to ensure the protection of workers and the public. However, the potential radiation doses to workers can vary depending upon the option chosen. In the case of nuclear power plants, the removal of the fuel, process fluids and operational waste from a reactor and, if practicable, from the site, removes the main radiological and security risks presented by the facility. The remaining residual radioactive material presents a smaller, but still significant, risk to workers, the public and the environment during decommissioning. One argument for delayed dismantling in the past has been that a prolonged period of safe enclosure between the initial and final phases of decommissioning allows radioactive decay, which reduces both local dose rates to workers and the amount of radioactive waste needing disposal. Technological progress over the last 10-15 years in electronics, robotics and remote handling has considerably reduced the need for manned access to the more highly radioactive areas and, for large scale commercial operations, this has reduced the importance of radiological factors in choosing a decommissioning strategy (IAEA, 2011c).

Immediate decommissioning is normally the preferred strategy; however, it is associated with the greatest amounts of radioactive waste since there is no time for radioactive decay to occur. This is more important for some types of facility than for others; for nuclear power plants there are usually significant benefits, in terms of reduction of waste amounts and worker doses due to radioactive decay, to be obtained from deferral, while for facilities in which long-lived radionuclides are used, such as reprocessing plants, the advantages brought by delay are much fewer.

If there is no available repository for the waste from decommissioning, the options are to proceed with immediate decommissioning and to temporarily store the spent fuel and radioactive waste from decommissioning at the facility itself or at an intermediate store, pending the availability of disposal facilities, or to defer or postpone the decommissioning, thereby not creating waste, until a waste management solution is available (IAEA, 2006b).