Decommissioning is defined as the safe removal of a facility from service and reduction of residual radioactivity to a level that permits termination of the license.

Nuclear power plants cease operations for a variety of reasons.
The Authorities (Regulators) grant a license for a period of 40 years. At the end of the license period, the licensee can seek to renew the operating license of the plant for another 20 years, or can cease operations and begin the decommissioning process. Some licensees choose to cease power operations before the 40-year licensing period has been completed.
Reasons for this decision are usually financial. For example, the plant may require upgrades or repairs that are not economically justifiable, or the licensee may find other sources of power that are less expensive than nuclear generation.
In addition to financial reasons for decommissioning, the Authorities can order the licensee to cease operations for safety reasons.

As one of the conditions for an operating license, it is required to the licensee to commit to decommissioning the nuclear plant after it ceases power operations.
This requirement is based on the need to reduce the amount of radioactive material at the site in order to ensure public health and safety as well as the protection of the environment.

The regulations are written so that when a licensee announces its decision to permanently cease power operations at the nuclear power plant, the decommissioning process is automatically initiated.
However, no major decommissioning activities can take place until the licensee has provided the Authorities with specific information regarding the decommissioning process as required by the decommissioning regulations discussed later.
It is possible for the licensee to let the facility sit idle for a number of years before announcing its decision to permanently cease power operations (although the time could not extend beyond the duration of the operating license).
However, it is not in the licensee's financial interest to delay this decision since the costs required to meet the regulations at an operating plant are much greater than the costs for a decommissioning plant.

The major benefit of decommissioning for the licensee as well as the public is that the levels of radioactive material at the site are reduced to levels that permit termination of the license and use of the site for other activities, rather than leaving the radioactively contaminated material on the site so that it could adversely affect public health and safety and the environment in the future.

The major costs of decommissioning are the large financial costs involved in funding the project.
Substantial costs are incurred in the removal, treatment, and disposal of major components of the facility that are contaminated, such as pumps, valves, piping, the steam generators and the reactor vessel.
Decontamination of floors, walls and equipment also result in substantial costs. The occupational dose received by workers during decommissioning should also be considered as a cost.

At the end of the licensing period (unless renewing and extension of the licensing), the regulations require that the facility be decommissioned. The alternative to decommissioning, at the end of the licensing period, is a "no action" alternative, implying that a licensee would simply abandon or leave a facility after ceasing operations. This is not considered to be a viable alternative to decommissioning.
The objective of decommissioning is to restore a nuclear facility to such a condition that there is no unacceptable risk from the decommissioned facility to public health and safety or the environment.
In order to ensure that at the end of its life, the risk from a facility is within acceptable bounds, some action is required. If nuclear power plants were not decommissioned, they could degrade and become radiological hazards.

A number of terms (listed and defined below) are important to the understanding of decommissioning.
It is also important to gain an understanding of the units used for measuring radiation dose : rem and person-rem.

Activation products are radioactive materials that were created when stable substances were bombarded by neutrons.
For example, cobalt-60 is formed from the neutron bombardment of the stable isotope cobalt-59.
In a reactor facility, neutrons are created inside the reactor vessel during the fission process. These neutrons bombard (1) the metal around the reactor vessel, (2) the primary reactor coolant, and (3) the concrete near the reactor vessel and create activation products in these materials.

Alpha radiation is a positively charged particle ejected spontaneously from the nuclei of some radioactive elements.
It does not penetrate very far into material and it has a very short range even in air (a few centimeters). The most energetic alpha particle will generally fail to penetrate the dead layers of cells covering the skin and can be easily stopped by a sheet of paper.
Alpha particles are hazardous when an alpha-emitting isotope is inside the body.

Background radiation means the radiation that is in the natural environment, including cosmic rays and radiation from the naturally radioactive elements, both outside and inside the bodies of humans and animals.
It also includes radon (from the ground) and global fallout (as it exists in the environment from the testing of nuclear explosive devices or from past nuclear accidents, such as Chernobyl, that contribute to background radiation and that are not under the control of the licensee).

Contamination means undesired (for example, radioactive or hazardous) material that is (1) deposited on the surface of, or internally ingrained into, structures or equipment, or (2) mixed with another material.

Dose or radiation dose is a generic term that means absorbed dose, dose equivalent, effective dose equivalent, committed dose equivalent, committed effective dose equivalent (CEDE), or total effective dose equivalent (TEDE).
In the case of radiation dose, it is energy absorbed per unit mass. Dose is measured in rads.
The metric form of the rad is a gray (Gy) (1 rad = 0.01 gray). Radiation dose received by a person is measured in units called "rem," which incorporates the biological harm of the radiation dose based on the type of ionizing radiation.
A sievert (Sv) is the metric form of the rem (1 rem = 0.01 sieverts).
Greater than Class C waste is radioactive waste that is not generally acceptable for near-surface disposal. It is waste for which form and disposal methods must be different, and in general more stringent, than those specified for Class C waste.
Such waste must be disposed of in a geological repository.

Half-life is the time required for half of any quantity of identical radioactive atoms to undergo radioactive decay, so that half of the atoms in the substance are no longer emitting radiation and are no longer considered to be radioactive.

Person-rem is the sum of all the radiation dose equivalents (measured in rem) that were received by an individual or by all individuals in a population group.
For example, if 1,000 people each received 1/10^th of a rem (100 millirem), the corresponding population dose would be 100 person-rem.
Doses to an individual are usually measured in millirem. A sievert is the metric form of the rem (1 000 millirem = 1 rem = 0.01 sievert).

Radiation (ionizing radiation) means alpha particles, beta particles, gamma rays, x-rays, neutrons, high-speed electrons, high-speed protons, and other particles capable of producing ions.
Radiation, as used in this section, does not include non-ionizing radiation, such as radio or microwaves, or visible, infrared, or ultraviolet light.

Radioactive decay is the spontaneous natural process by which an unstable radioactive nucleus releases energy or particles.

Rem (see Dose).

Residual radioactivity means radioactive contamination or activation products that remain following decontamination and dismantling of the facility.

Radioactive contamination is radioactive material that is deposited on a nonradioactive surface.
The material may be deposited from the air, or it may be dissolved in water and subsequently deposited into material such as concrete. Radioactive contamination is generally located on or near the surface of materials like metal or high-density concrete or painted walls. It would travel farther into unpainted surfaces or lower density concrete. Radioactive contamination can usually be removed from surface areas by washing, scrubbing, spraying, or, in extreme cases, by removing the outer surface of the material.

Contaminated materials are transported through the facility by workers, equipment, and to some degree through the air. Although many precautions are taken to prevent the movement of contaminated material in a nuclear facility and to clean up any contaminated materials that may be found, it is most likely that contamination will occur in the reactor building, around the spent fuel pool, and around specific pieces of equipment in the auxiliary building.
The areas known to contain contamination are marked by the licensee, who routinely checks for contamination.

Activation products are radioactive materials that were created when stable substances were bombarded by neutrons. Typically these materials are the concrete and the steel that surround the fuel core. The radioactive decay of activation products is the main source of radiation exposure to plant personnel.

To decommission a nuclear power plant, the radioactive material on the site must be reduced to levels that would permit termination of the license.
This involves removing the spent fuel (the fuel that has been in the reactor vessel), dismantling any systems or components containing activation products (such as the reactor vessel and primary loop), and cleaning up or dismantling contaminated materials.
All activated materials generally have to be removed from the facility and shipped to a waste processing, storage or disposal facility. Contaminated materials may either be cleaned of contamination onsite, or the contaminated sections may be cut off and removed (leaving most of the component intact in the facility), or they may be removed and shipped to a waste processing, storage, or disposal facility.
The licensee decides how to decontaminate material, and the decision is usually based on the amount of contamination, the ease with which it can be removed, and the cost to remove the contamination versus the cost to ship the entire structure or component to a waste-disposal site.

The licensee decides how to decommission the site.
Frequently, licensees hire contractors that specialize in decommissioning sites to conduct part or most of the decommissioning. The process for decontamination and dismantling may vary from site to site.
Factors that are used to make these decisions include cost, worker exposure, availability of a waste site, and layout and structure of buildings. For example, at some sites, it may make more sense to segment the reactor vessel before removing it from the reactor building; in other cases, it would be appropriate to remove the reactor vessel intact through a hole cut in the side of the containment building and ship the reactor vessel intact.

If any major decommissioning activity does not meet the conditions specified by the regulations, the licensee is prohibited from undertaking the activity until; 1): it submits a license-amendment request that describes the proposed activity and the potential impact associated with that activity, and 2) the regulations approve the request.

The regulators have evaluated the environmental impacts of three general methods for decommissioning power facilities.

DECON : The equipment, structures, and portions of the facility and site that contain radioactive contaminants are removed or decontaminated to a level that permits termination of the license shortly after cessation of operations.

SAFSTOR : The facility is placed in a safe stable condition and maintained in that state until it is subsequently decontaminated and dismantled to levels that permit license termination.
During SAFSTOR, a facility is left intact, but the fuel has been removed from the reactor vessel, and radioactive liquids have been drained from systems and components and then processed.
Radioactive decay occurs during the SAFSTOR period, thus reducing the quantity of contaminated and radioactive material that must be disposed of during decontamination and dismantling.

ENTOMB : Radioactive structures, systems, and components are encased in a structurally long-lived substance, such as concrete.
The entombed structure is appropriately maintained, and continued surveillance is carried out until the radioactivity decays to a level that permits termination of the license.

The DECON option calls for prompt removal of radioactive material to permit restricted or unrestricted access.
The advantages of DECON include the following :

•   facility license is terminated quickly, and the facility and site become available for other purposes
•   availability of the operating reactor work force that is highly knowledgeable about the facility
•   elimination of the need for long-term security, maintenance, and surveillance of the facility, which would be required for the other decommissioning alternatives
•   greater certainty about the availability of low-level waste facilities that would be willing to accept the low-level radioactive waste
•   lower estimated costs compared to the alternative of SAFSTOR, largely as a result of future price escalation because most activities that occur during DECON would also occur during the SAFSTOR period, only at a later date.
  (It is anticipated that the later the date for completion of the decommissioning, the greater the cost).

The disadvantages of DECON include the following :

•   higher worker and public doses (because there is less benefit from radioactive decay such as would occur in the SAFSTOR option)
•   a larger initial commitment of money
•   a larger potential commitment of disposal-site space than for the SAFSTOR option
•   the potential for complications if spent fuel must remain on the site until a repository for spent fuel becomes available.

The benefits of SAFSTOR include the following :

•   a substantial reduction in radioactivity as a result of the radioactive decay that results during the storage period
•   a reduction in worker dose (as compared to the DECON alternative)
•   a reduction in public exposure because of fewer shipments of radioactive material to the low-level waste site (as compared to the DECON alternative)
•   a potential reduction in the amount of waste disposal space required (as compared to the DECON alternative)
•   lower cost during the years immediately following permanent cessation of operations
•   a storage period compatible with the need to store spent fuel onsite.

Disadvantages of SAFSTOR include the following :

•   shortage of personnel familiar with the facility at the time of deferred dismantling and decontamination
•   site unavailable for alternate uses during the extended storage period
•   uncertainties regarding the availability and costs of low-level radioactive waste sites in the future
•   continuing need for maintenance, security, and surveillance
•   higher total cost for the subsequent decontamination and dismantling period (assuming typical price escalation during the time the facility is stored).

The benefits of the ENTOMB process are primarily related to the reduced amount of work in encasing the facility in a structurally long-lived substance, and thus, reducing the worker dose from decontaminating and dismantling the facility. In addition, public exposure from waste transported to the low-level waste site would be minimized. The ENTOMB option may have a relatively low cost.
However, because most power reactors will have radionuclides in concentrations exceeding the limits for unrestricted use even after 100 years, this option may not be feasible under the current regulations.
This option might be acceptable for reactor facilities that can demonstrate that radionuclide levels will decay to levels that will allow restricted use of the site.
Three small demonstration reactors have been entombed. Currently, no licensees have proposed the ENTOMB option for any of the power reactors undergoing decommissioning.

The choice of the decommissioning method is left entirely to the licensee.
However, the regulator would require the licensee to re-evaluate its decision if the choice (1) could not be completed as described, (2) could not be completed within a defined period after the permanent cessation of plant operations, (3) included activities that would endanger the health and safety of the public by being outside of the health and safety regulations, or (4) would result in a significant impact to the environment.

A licensee need not restrict its choice of decommissioning options to either an immediate decontamination and dismantling or to a storage period of 30 to 60 years, followed by decontamination and dismantling.
Generally licensees combine the first two options. For example, after power operations stop at a facility, a licensee could use a short storage period for planning purposes, followed by removal of large components (such as the steam generators, pressurizer, and reactor vessel internals), place the facility in storage for 30 years, and eventually finish the decontamination and dismantling process.

The SAFSTOR alternative is often used at multi-unit sites when one or more of the units shuts down while others continue to operate.
This is especially true for facilities that share some systems. In this case, the staff from the operating unit(s) assist in the maintenance and surveillance of the unit that is in storage.
The choice of decommissioning options is also strongly influenced by potential uncertainties in low-level waste disposal costs and by concerns over the future availability of low-level waste sites.
The licensee's rate regulator can also influence the choice of decommissioning alternatives.

The dismantling phase typically takes between 3 to 5 years to complete, although it may take longer if there are constraints on access to low-level waste burial sites, or if the licensee decides to proceed at a slower pace for programmatic reasons.