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. In addition there are many nuclear research facilities and defense related establishments that will require decommissioning.

As of September 2001, over 90 commercial power reactors, 50 fuel cycle facilities, around 100 mines and more than 250 research facilities have been retired from operation. Many are currently being or have successfully been decommissioned.

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. The next stages will depend on what type of facility is being decommissioned.

• In nuclear reactors the used nuclear fuel (and thus typically greater than 99% of the total radioactivity of the reactor) is removed from the reactor and transferred to interim or long-term storage depending on the spent fuel management policy of the utility/owner. Radioactive wastes accumulated during operation may be treated and stored/disposed of. The reactor and associated plant will be prepared for the next stage of dismantling or a period of care and maintenance as determined by the chosen decommissioning strategy.
  
• In fuel-processing facilities radioactive waste accumulated during operation may be treated and conditioned and then sent to the appropriate storage or disposal facility. Spent fuel storage pools may also be emptied. The main operations then consist of decontamination by plant wash-out using conventional or more specific reagents and enhanced when necessary by mechanical operations. Other equipment items presenting potential risks such as spreading contamination or hot spots are dismantled. The main objective is to obtain radiological conditions at the end of plant cleanout that will minimize the following aspects of the surveillance and dismantling phases (cost, waste volume and activity, radiation exposure to workers).

The next stage for all facilities is decontamination, which incorporates the removal of contamination from surfaces of facilities or equipment. Following decontamination, equipment within the facility may be 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.

• Nuclear Reactors: All equipment and buildings outside the reactor building (e.g. the turbines and turbine hall) may be dismantled. With the reactor sealed and monitored, the rest of the site can be released for re-use.
  
• Fuel-Processing Facilities: Extensive plant decontamination, partial dismantling and removal of plant systems is carried out. As a result of decontamination and dismantling of equipment, the reduced radiation levels may result in a change of status (for example in France, from a Basic Nuclear Installation to an Installation on the Environmental List) The plant can then be used for non-nuclear purposes.

Demolition and Site Clearance

• Nuclear Reactors: the final stage of reactor dismantling is completed, buildings demolished and radioactive wastes removed to storage or disposal facilities. The site may then be delicensed and released for appropriate alternative use. No further inspection or monitoring is required.
  
• Fuel-Processing Facilities: Final dismantling and demolition of the plant is carried out. Depending on the specific circumstances there may also be removal of foundations and the free release of the site for appropriate alternative use.
  
• Mines and Mills: Most of the site can be released without further constraint. Depending upon the tailings disposal method, there may be some limit placed on future land use in the tailings area. This generally represents a small fraction of the entire site.

In practice, the extent to which buildings are demolished and the site cleared, following decontamination and dismantling, is dependent on the decommissioning strategy and in some cases, the owner's purposes of re-using the site following delicensing. For example, it may be possible to re-use some buildings on site for non-nuclear purposes rather than have them demolished.

De-licensing and Release of the Site to Alternative Use

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 is then available for re-use.

Note that the time taken to complete these phases, as well as the period in between each phase, can vary considerably depending on national policy. The issue of timescales associated with the decommissioning process is discussed in the following section.

Note also that where there is more than one plant or building within the facility being decommissioned the different phases may occur at different timescales. Thus non-active buildings may move through to demolition and site clearance relatively quickly after shutdown whereas some buildings could be left for long-term storage.

Timing of the Decommissioning Approach

Decommissioning strategies can vary considerably in timescale.

Two main options are available, namely a move to early dismantling, termed "Early Site Release" (or "DECON" in the US) or deferral of one or more phases, an option called "Safestor(e)".

There is no "right" or "wrong" approach. In practice, individual national policy and the owner's strategy determines which strategy is adopted. (view a "A Decommissioning Strategy" to see which factors are taken into account when deciding which approach is suitable)

A third decommissioning option, termed "Entomb" has been defined in the US.

This option refers to the encasement of the nuclear facility in a long-lived substance, such as concrete. The facility is appropriately maintained and monitored until the radioactivity has decayed to a level which permits termination of the site licence.

In practice, however, the licensees for nuclear facilities undergoing decommissioning have not adopted the entombment option.

Further Reading:
Decommissioning Nuclear Facilities