Decommissioning

What is Decommissioning?

Decommissioning begins immediately following final and permanent closure and continues ideally to the point of leaving a clear site where the facility had once stood. The decommissioning process incorporates some or all of the following activities: the safe management of nuclear materials held in the facility, the safe management of radioactive and other wastes, decontamination, plant dismantling, demolition and site remediation.

Which Facilities Need to be Decommissioned?

All facilities within the commercial nuclear fuel cycle will eventually need to be decommissioned following their permanent closure, this includes uranium mines, nuclear power plants, enrichment and reprocessing facilities. In addition there are many nuclear research facilities and defence-related establishments that will require decommissioning.

How is a Nuclear Facility Decommissioned?

The decommissioning of a nuclear facility is generally categorized into a number of stages.

The first stage is plant cleanout. This generally begins immediately following shutdown and involves the removal of unused nuclear fuel and other high level nuclear waste. The main objective is to obtain radiological conditions at the end of plant cleanout that will facilitate the surveillance and dismantling phases (cost, waste volume and activity, radiation exposure to workers). For a power station,  the spent fuel accounts for greater than 99% of the overall radioactivity of the plant.

The next stage for all facilities is decontamination (radiological and otherwise) of surfaces of facilities or equipment. Following decontamination, some equipment within the facility may be immediately dismantled. Likewise, equipment within the building(s) on the non-radioactive side of the facility (such as offices, turbine halls) can be removed for possible re-use, and the building(s) themselves demolished. There will still be a radiological hazard present caused by the presence of  activation products. These form in certain materials (typically impurities) as a result of exposure to high levels of neutron radiation such as is present in a reactor core. Finally, once it has been demonstrated that the site is safe and that radiation levels are below the national regulatory requirements, the operating licence is terminated and the site can be made available for other purposes. No further inspection or monitoring is required.

The time taken to complete these phases, as well as the period in between each phase, can vary considerably depending on factors such as national policy and whether or not other operative reactors/nuclear facilitiies remain on site.
 

Timing of the Decommissioning Approach

For nuclear power plants, enrichment, and reprocessing facilities the International Atomic Energy Agency has defined three options for decommissioning, the definitions of which have been internationally adopted.

• Immediate Dismantling (or Early Site Release/Decon in the US): This option allows for the facility to be removed from regulatory control relatively soon after shutdown or termination of regulated activities. Usually, the final dismantling or decontamination activities begin within a few months or years, depending on the facility.
   
• Safe Enclosure (or Safestor): This option postpones the final removal of controls for a longer period, usually in the order of 40 to 60 years. The facility is placed into a safe storage configuration until the eventual dismantling and decontamination activities occur.
   
• Entombment: This option entails placing the facility into a condition that will allow the remaining on-site radioactive material to remain  without the requirement of ever removing it totally. This option usually involves reducing the size of the area where the radioactive material is located and then encasing the facility in a long-lived structure such as concrete, that will last for a time to ensure the remaining radioactivity is no longer of concern.

There is no right or wrong approach, each having its benefits and disadvantages. In the case of immediate dismantling (or early site release), responsibility for the decommissioning is not transferred to future generations. The experience and skills of operating staff can also be utilised during the decommissioning program. Alternatively, Safe Enclosure (or Safestor) allows significant reduction in residual radioactivity, thus reducing radiation hazard during the eventual dismantling. The expected improvements in mechanical techniques should also lead to a reduction in the hazard and also costs.

Further Reading:
Decommissioning Nuclear Facilities