How is decommissioning defined?

"Decommissioning" a nuclear power plant can be defined as the cessation of operations and the withdrawal of the facility from service, followed by its transformation into an out-of-service state and eventually, its complete removal.

Decommissioning activities are intended to place the facility in a condition that provides for the health and safety of the general public and the environment, while at the same time protecting the health and safety of the decommissioning workers.

Decommissioning involves thus all the administrative and technical operations allowing to withdraw a facility from the list of licensed facilities.

The administrative operations concern particularly the elaboration of decommissioning plans and the obtainment of authorisations and free release certificates for the facilities and the site.

The technical operations include among others the decontamination, the dismantling and the waste management. The decommissioning doesn't aim at destroying the buildings, but it liberates them from all obligations and controls corresponding to the class they belong to. It is the final objective to achieve.

Why do we need decommissioning?

About fifty years ago, governments decided to introduce the nuclear technology so as to cope with the strong need of energy. Nowadays, nuclear energy still meets the energy demand but produces also radioactive wastes that can not be directly released in the environment.

A decommissioning procedure minimizes (or in any case has this purpose) the amount of radioactive waste based on the application of strict regulations, that ensures public health and safety as well as the protection of the environment.

Nuclear power plants are designed to operate for a period of time, usually 40 years. They can also cease operations earlier for a variety of reasons (economic, safety, political, end of licensing, ...). Reasons for this decision are usually financial; for example, the plant may require upgrades or repairs that are not economically justifiable. But it can be also decided to shut down the plant for safety reasons.

The operating lifetime of nuclear facilities is thus largely and mainly determined by economic and safety considerations (even if in the background political reasons exist). Nuclear power plants are normally designed for an operating lifetime of several decades.

Assuming a 25-year lifespan (extendable by years until nowadays to 40 years), almost 300 facilities would have to be decommissioned by the year 2010. By appropriate refurbishment, replacement or upgrading of some equipment, operations at many of these plants can probably be extended well beyond this conservative estimate.

However, ultimately it becomes either technically or economically advantageous to retire a facility from operation and, if necessary, replace it with a new plant. Although decommissioning normally occurs at the end of the operating lifetime of the plant, it may also be required for various other reasons, e.g., following an accident, or for economic or political reasons.

For the most part, this paper will address only normal decommissioning.

The foreseen decommissioning operation extent - Current situation and forthcoming requirements

The development of nuclear power started in the 1950s in some countries and many research and development facilities have been constructed since then. The nuclear power plants of the first generation began to be commissioned in the late 1950s and early 1960s.

Some of these facilities and plants have already been shut down and others are reaching their original design lifetimes. The rate of retiring nuclear plants will increase markedly within a decade or so. Hence various research studies and projects on decommissioning have been performed and the countries with a nuclear programme have significantly enhanced their knowledge on the issue.

At the end of 1999, 433 nuclear power generation plants were in operation in the world.

The next figure [1] describes the age distribution of the power plants in operation at the end of 1998 (31 December).

Number of Reactors in Operation By Age (as of December 1998)
Ref.: IAEA, Nuclear Power Reactors in the World, April 1999, by courtesy

The average age of these plants is about 14 years. The figure suggests that the number of nuclear power plants reaching the age of 40 years is small before 2005, then will rapidly increase after 2010 and come to a peak around 2015. This number will stay high during the period of 2015 to 2025. Also by the end of 1992, 65 plants had already been permanently shut down, 57 of which were located in the OECD countries. Many of them had a capacity of less than 200 MWe, were commissioned before the year 1970 and were built as prototype or demonstration reactors.

However, in some countries commercial reactors have also been shut down for various reasons.

Reactors in operation and Net Electrical Power (as of December 1998)
Ref.: IAEA, Nuclear Power Reactors in the World, April 1999, by courtesy

From this picture of the general situation, it may be inferred that the requirements for decommissioning of nuclear power plants will grow during the next few decades, reaching a peak perhaps between 2015 and 2025.

However, the timing of this peak is expected to be different in different regions and countries.

Although these indications suggest a peak of decommissioning work requirements between 2015 and 2025, the operating lifetime of the plants is not the only factor affecting the timing of decommissioning. National and industrial strategies, economical, political and public opinion constraints may well influence this timing.

For example, it is conceivable that for some plants decommissioning will only be performed initially to Stages 1 and possibly 2, with Stage 3 following after many years or several decades (see below).

Finally, the last figure gives an overview of the number of reactors under construction as of December 1998. the PWR and PHWR types remain the most common used ones all over the world; a number of 36 was at that time in construction, which is even few regarding the total number of reactors in the world, but the nuclear energy is not totally concealed.

Reactors under construction and Net Electrical Power (as of Dec. 1998)
Ref.: IAEA, Nuclear Power Reactors in the World, April 1999, by courtesy

The stages of decommissioning

Although the ultimate goal of decommissioning is the complete removal of residual radioactivity and the return of the facility site and possibly of some of its premises to unrestricted use, decommissioning procedures can be implemented in progressive stages.

These stages are defined by two characteristics:

• the physical state of the plant and its equipment; and
• the surveillance necessitated by that state.

A traditional definition of the stages of decommissioning has been proposed by the IAEA [4] and is largely used worldwide. It is based on the following classification.

These three stages may be carried out by rapidly progressing from one stage to the next or carried out over a prolonged period lasting as long as 100 years or more. Although most countries intend to complete all three stages, a facility could remain at Stage 1 or Stage 2 for a relatively long period of time, or decommissioning could proceed directly from Stage 1 to Stage 3.

What are the principal decommissioning strategies?

A number of factors must be weighed and balanced when preparing the decommissioning plan for a nuclear power plant. The plan will vary with each facility and these factors must be evaluated on a case-by-case basis.

These factors have to be considered when selecting the optimum strategy for the decommissioning of the nuclear facility. These include the national nuclear policy, characteristics of the facility, health and safety, environmental protection, radioactive waste management, future use of the site, improvements of decommissioning technology that may be achieved in the future, cost and availability of funds for the project and various social considerations.

The relative importance of these factors has to be judged case by case. Besides nuclear reactors, there are in most countries large numbers of other industrial and research facilities which have been decommissioned or will require decommissioning in the next years and decades. However, no time forecasts can be made in this area, in view of the large variety of types of facilities and the associated requirements and problems of decommissioning.

Even if a lot of different strategies are existing for the decommissioning of nuclear facilities, describing the different ways and operations to reach the levels (1, 2 or 3, see above) defined by the IAEA, these strategies often are variants of the following ones:

• DECON (Decontamination)
  

  In DECON, all components and structures that are radioactive are cleaned or dismantled, packaged and shipped to a low-level waste disposal site, or they are stored temporarily on site.

  Once this task, which takes five or more years, is completed and the regulatory body terminates the plant's license, that portion of the site can be reused for other purposes.

• SAFSTOR (Safe Storage)
  

  In SAFSTOR, the nuclear plant is kept intact and placed in protective storage for tens of years (20 to 150).

  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 (the radioactive atoms "decaying" by emitting their extra energy to become non-radioactive or stable atoms.)

  If a plant is allowed to sit idle for 30 years, for example, the radioactivity from cobalt-60 will be reduced to 1/50th of its original level; after 50 years, the radioactivity will be just 1/1,000th of its original level.

  Once radioactivity has decayed to lower levels, the unit is taken apart similar to DECON.

• ENTOMB
  

  This option involves encasing radioactive structures, systems and components in a long-lived substance, such as concrete.

  The encased plant would be appropriately maintained, and surveillance would continue until the radioactivity decays to a level that permits termination of the plant's license. Most nuclear plants will have radionuclide concentrations exceeding the limits for unrestricted use even after 100 years. Therefore, special provisions will be needed for the extended monitoring period this option requires.

  To date, no facility owner have proposed the entombment option for any nuclear power plants undergoing decommissioning. This option is, in fact, similar to declaring the site as a shallow land burial site.

National nuclear strategy

Some of the questions concerning national strategy and goals for nuclear power development, which are relevant to decommissioning, include:

• Is there a national policy for the disposal of shutdown nuclear facilities?
• Is it an industry or a government responsibility?
• What is the national radioactive waste management policy?
• What are the national policies for worker and public health protection?

The answers to these questions may change with time and the decommissioning plan may need to be revised to take into account technical developments, availability of waste disposal sites, changes in health protection requirements and other factors.

Power plant characteristics

The kind of reactor, the location of the facility and the total amount of radioactivity it contains are important elements in the selection of a decommissioning strategy. The amount and location of the radioactivity are determined by the kind of reactor and its operating history. For example, a boiling water reactor (BWR) circulates steam containing radioactivity into the turbine circuit, whereas in a pressurized water reactor (PWR) the radioactivity is contained in the primary coolant system and does not contaminate the turbine circuit.

The radioactivity produced in nuclear power plants is made up of both short-lived and long-lived radionuclides, but principally short-lived isotopes which would decay in 5 to 30 years. Thus a significant reduction in radioactivity can be achieved by placing the facility in safe storage for that length of time. This delay reduces the occupational exposure of the decommissioning workers and the amount of waste needing controlled disposal.

Protection of health, safety and the environment

A primary concern in any decommissioning programme is to provide for the health and safety of the workers and to protect the general public and the environment. Public exposure and environmental impacts are expected to be minimal and well within the regulatory limits for operating facilities.

Therefore they are not likely to be significant factors in selecting a decommissioning alternative.

However, protection of the decommissioning workers is an important consideration and significant effort is made in all nuclear operations to keep the exposure as low as reasonably achievable. A cost-benefit analysis should be carried out to determine to what extent delayed dismantlement will have a positive effect. Although this depends on the physical state of the power plant, as well as available resources and equipment, it is known that a deferral of Stage 3 for 80-100 years would significantly reduce personnel management and protection costs while raising maintenance and surveillance costs.

Delaying decommissioning beyond 100 years would not achieve similar benefits, as the decay rate of, the radioactive substances is significantly slowed by then. Furthermore, surveillance beyond 100 years cannot be relied upon.

In choosing a decommissioning strategy, a radiological impact assessment should be made to determine:

• Individual and collective doses (i.e. the total dose over a population group exposed to a given source) to workers during the dismantling, including waste handling;
• Individual and collective doses to the public throughout the operation including those produced by waste disposal and activity remaining on site;
• A risk assessment of the possibility of a fire or explosion, of a possible deterioration of the installation or of the possible loss of integrity of the containment barriers, leading to the release of radionuclides to the environment.

Radioactive waste management

As indicated above, decommissioning nuclear power plants requires the removal of the fuel in the reactor prior to completion of Stage 1. Thus the first question becomes the availability of reprocessing or storage facilities for this spent fuel.

Although in some cases, this might be a determining factor, usually spent fuel management solutions are planned as part of power plant operations.

Although there is no generally applicable classification of radioactive wastes, it is often convenient to refer to low-level, intermediate-level and high-level wastes, depending on their radionuclide content, heat generation rates and methods of treatment.

The availability of disposal sites for low-and intermediate-level waste is the most important factor in several countries which are choosing decommissioning strategies. Most of the waste from nuclear power plant decommissioning is low-level waste, although a small amount of intermediate- and high-level waste is also produced.

Low- and intermediate-level decommissioning wastes can be disposed of in the same facilities that accept the waste continuously produced by the operating facilities. The decommissioning waste volume from a nuclear power plant is of the same order of magnitude as the volume of operations waste produced throughout the normal lifetime of the plant.

The volume of decommissioning waste can be substantially reduced using such techniques as surface decontamination, compaction, segmentation, special packaging and incineration.

Future use of the site

A nuclear facility site is a valuable resource, particularly for the location of replacement power or processing facilities. Among its assets are its low seismic activity, its proximity to a large supply of water, its access to an electrical distribution system and its acceptance by local residents.

If the site is to be used for other power generating or nuclear facilities, it need not be decommissioned to the same standards as for unrestricted release to the public domain.

At sites containing power plants which were constructed at different times, and therefore will reach the end of their normal operating life at different times, decommissioning may be restricted to Stage 1 until the last unit is shut down.

Decommissioning of all units would be more efficient, and maintenance, surveillance and security could be provided by personnel from the units still operating, at little or no additional cost.

Further development of decommissioning technology

All three stages of decommissioning have been carried out on small test, training and power demonstration reactors and supporting fuel cycle facilities. Thus, experience has been obtained in decommissioning the major reactor types.

Present technology is believed to be adequate for decommissioning to any of the three stages by applying the techniques from small plants to the decommissioning of commercial-size facilities.

Nevertheless, continued development in some areas to reduce worker exposure, costs and waste volumes is desirable. Such areas include the development of better equipment and methods for remote dismantlement to reduce worker exposures, the development of better methods to discriminate rapidly between radioactivity levels in waste and techniques for minimizing waste generation through treatment and volume reduction.

New equipment is expected to improve worker protection and reduce costs. Many major decommissioning projects, currently in progress or planned, will add substantially to the available knowledge. Thus some delay in dismantlement of power plants to permit the use of improved technology could be advantageous.

Cost and availability of funds

While safety considerations are given priority, the costs of decommissioning can also be a very important factor. At the end of the facility's operations, immediate dismantlement to Stage 3 would be much more costly than to remain at Stage 1.

If funds have been set aside for decommissioning while the facility is operating, the immediate cost will be less important than if the decommissioning costs must compete for funds with the owner's other capital needs.

In order to prevent this item from becoming a controlling factor, plant owners should plan to have funds set aside to cover the cost of the chosen decommissioning strategy, which in any case would at most add only a few per cent to electricity generation costs.

Social and other considerations

Certain social factors should be taken into account when evaluating the decommissioning options. For example, the chosen strategy could affect land values, the aesthetic aspects of the site, and changes in perceived risks associated with the long-term shutdown and delayed dismantlement of a plant on the site.

Another consideration, particularly important in areas where employment is scarce, is that immediate dismantlement requires a larger staff and work force than other strategies, resulting in a slower and smoother reduction of the operations staff.

Past existing decommissioning experiences

As has been stated above, extensive experience has been obtained in all three decommissioning stages. Nuclear power plants and other nuclear installations have been decommissioned in the United States, in several European countries, in Japan, ...

Although the facilities were small, several techniques such as isolation of systems, the handling of toxic as well as radioactive materials, the use of controlled explosives and other methods for pipe cutting and concrete demolition, the use of various decontamination methods, the remote segmentation of the pressure vessel and its internal components and continuous surveillance practices have all contributed to the experience gained from these past decommissioning projects.

In addition, valuable experience has been gained in decommissioning technology by the wide variety of repair operations carried out around the world. The techniques used to reduce worker exposures and costs and increase efficiency have direct application to power plant decommissioning.

Several facilities, as the result of accidents, have been decontaminated and repaired for further operations or placed in a stage of decommissioning. Such accidents cause more severe conditions than would exist at the end of the normal operating lifetime. Yet decontaminating equipment and plant areas, controlling contamination and maintaining worker exposures within regulated limits were successfully achieved.

Research is being carried out on new equipment and procedures to further reduce worker exposures and costs.

Cost(s) of "decommissioning"

The cost of decommissioning nuclear power plants is based on the following factors:

• The sequence of decommissioning stages chosen;
• The timing of each decommissioning stage; and
• The decommissioning activities accomplished in each stage.

In addition, costs depend on such country- and site-specific factors as the type of reactor, waste management and disposal practices and labor rates.

As already mentioned, a number of other factors influence the choice of decommissioning strategy and therefore the costs involved.

Total decommissioning costs include all costs from the start of decommissioning until the site is released for unrestricted use. It is assumed that each facility will eventually be decommissioned to Stage 3 because some radioactivity will continue to exceed the limits for unrestricted access to the site for a much longer period than that foreseen in Stage 2. Hence, the total cost will either be that of an immediate Stage 3 decommissioning or that of a delayed Stage 3 plus the intermediate stages.

The cost estimates are based on previous decommissioning and decontamination experience, on the cost of maintenance, surveillance and component replacements, and on the cost of similar non-nuclear work.

They are also based on a minimum storage period of 30 years after Stage 1 (this time varying also from country to country), to allow for significant decay of radioactivity, and a period of 100 years after Stage 2, to allow worker access, generally without the need for shielding or remote operations. Estimates have been made by several European countries as well as Japan, Canada and the United States.

The results, which include a 25 per cent contingency factor, showed a range of costs for an immediate Stage 3 decommissioning of between 97 and 173 Mio €. Costs for combining Stages 1 and 3 ranged from 117 to 181 Mio €. Only the United States estimated the costs of combining Stages 2 and 3, from 158 to 186 million U.S. dollars.

While these figures cannot be absolutely precise, due to differences in the original contingency factors and definitions of decommissioning stages between countries, they nevertheless show what order of magnitude actual decommissioning costs are likely to be for large power plants.

Several methods of financing these future decommissioning costs can be used depending on the circumstances of each utility and the country in which it operates.

In several countries, a fund of some type has been established or proposed to assure the availability of financing. This is usually done by estimating the cost of decommissioning at the end of the normal plant lifetime and requiring payments, either annually or on a charge per kilowatt-hour basis, to ensure that this sum is in place. This estimate is updated regularly and the charge adjusted accordingly.

The drawback to this system is that the amount estimated would not be in place if the plant were to be shut down before the end of its normal lifetime. To avoid this, a fund could be established at the start of the plant's operation which would cover the cost of decommissioning whenever it became necessary.

However, this represents a heavy burden for the utility at the moment when construction and start-up costs are already high, and thus, although it may be imposed by law, this solution is not favoured by many utilities.

Glossary

Nuclear facility

A site, equipment, factory or facility using radioactive materials.

Final shutdown

The exploitation of a nuclear facility, completely or partially, is definitely ceased for technical, economical or safety reasons, as well as the technical and administrative operations allowing to modify the operating authorisation.

Decommissioning

All the administrative and technical operations allowing to withdraw a facility from the list of licensed facilities. The administrative operations concern particularly the elaboration of decommissioning plans and the obtainment of authorisations and free release certificates for the facilities and the site; the technical operations include among others the decontamination, the dismantling and the waste management.

The decommissioning doesn't aim at destroying the buildings, but to liberate them from all obligations and controls corresponding to the class they belong to. It is the final objective to achieve.

Decontamination

All activities allowing to eliminate or reduce the contamination by using, in general, mechanical, chemical or electrochemical processes. Contrary to the dismantling, the decontamination has no influence on the structure of the components or infrastructure.

A first decontamination in situ, prior to dismantling, aims at reducing the radiological dose uptake for the operators during dismantling.

A second decontamination after dismantling aims at the free release or decommissioning of materials and waste.

Dismantling

All the operations using disassembling, cutting and demolition techniques to remove contaminated or activated materials and structures.

Depending on the radiological doses, the dismantling is carried out through direct operation, local shielding or remotely.

Demolition

All the conventional activities carried out after decommissioning and unconditioned free release of a facility or a site, in order to restore the site in its natural state.

Decommissioning plan

Conceptual study including the technical and economical analysis of the decommissioning. The initial decommissioning plan of a new operated facility evolves by regular revisions to the final decommissioning plan, becoming effective a short time before the final shutdown.

Decommissioning programme

All technical and administrative activities necessary to prepare and carry out the decommissioning.

Decommissioning funds

All financial means gathered prior to the facility's final shutdown and reserved for carrying out the decommissioning programme, the management of the generated waste included.

Decommissioning phases

Three distinctive, but no compulsory steps in the decommissioning programme, initially defined by the IAEA, leading to three decommissioning levels.

Level 3 involves the unconditioned free release of the facility or site.

Free release levels of decommissioning materials

Levels below which materials can be released without any further radioprotection control, or can be reused without or following certain conditions.

Unconditional free release

Unconditional free release allows to consider the equipments, buildings or waste as conventional. Their radioactivity level must be inferior to the unconditional free release level. This level is also called "de minimis".

Other materials with a residual activity higher than the "de minimis" level, but lower than a conditional free release level can, after treatment in especially designed facilities, be recycled on condition of reuse in the conventional or nuclear industry, for instance for metals after fusion, or for concrete after crushing. This level is also called "recycling level".

Products of which the radioactivity is higher than the recycling level are considered as radioactive waste, for which an appropriate destination is necessary.

Unconditional free release of a nuclear facility

Withdrawal of the facility from the list of licensed facilities according to national regulations. This free release allows to consider the facility, its buildings, its equipment and the site, or part of the site on which the facility is built, as non-nuclear.

Financing decommissioning

Setting up the necessary decommissioning funds sufficient to carry out the decommissioning programme.

The chosen financing technique must guarantee the disponibility of financial means to carry out the decommissioning programme according to the strategy and planning, selected in the decommissioning plan.

References

1. Decommissioning of nuclear facilities : an international perspective, O. Ilari - OECD, Nuclear Energy Agency - Paris France, From 1994 Summer school on Radioactive waste management and decommissioning - 27 june - 30 june 1994 - Jesus College Cambridge, UK.
2. The European Union's R&D in the Field of Decommissioning, R.Simon DG XII/F-5 European Commission, Brussels, Research Programme on decommissioning of Nuclear Installations (1989-94), ISBN 92-827-4016-1.
3. Decommissioning of nuclear installations in member states - Achievements and projects, A. Cregut - CEA, Marcoule France - A.R. Gregory, National power division, Central Electricity Generating Board, UK, ISBN 92-827-4016-1.
4. Safe Enclosure of Shut Down Nuclear Installations, Technical Reports Series N°. 375, pages [33-47], IAEA, Vienna 1995, ISBN 92-0-100495-8.
5. Storage with surveillance versus immediate decommissioning for nuclear reactors, Proceedings of an NEA Workshop, Paris, 22-24 October 1984.
6. Decommissioning large power reactors: strategies, K. Bragg. - NEA Newsletter, Fall 1985.
7. Building on experience: an international perspective, M.A. Feraday. - IAEA Bulletin, Winter 1985.
8. Decommissioning of nuclear facilities: feasibility, needs and costs, Report by an NEA Expert Group; OECD-Paris, 1986.
9. Safety and regulatory aspects of decommissioning, F. Luykx, CEC. Scientific Afternoon, IAEA General Conference, September 1986.
10. The methodology and technology of decommissioning, Edward G. Delaney, US Department of Energy. Scientific Afternoon, IAEA General Conference, September 1986.
11. An overview of past, present and future activities relevant to decommissioning including unplanned events, J.M. Liederman. Scientific Afternoon, IAEA General Conference, September 1986.
12. The cost and financing of the decommissioning of nuclear power plants, H.E. Thexton, NEA. Scientific Afternoon, IAEA General Conference, September 1986.
13. Nuclear power plants in the World - April 1999 Edition Reference Data Series No. 2, IAEA-RDS-2/19 (78 pp., 6 figures, 9.5 x 17.5 cm; 1999, ISBN 92-0-101599-2.

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