Decontamination & decommissioning
Let’s get on with it13 November 2009
Plans to build new nuclear power plants in the UK gives us an incentive to clear up old reactor sites and to re-evaluate the country’s current decommissioning plans. We have the technical capability to decommission nuclear reactors today – why wait? By David Bradbury, George Elder and Susan Hewish
The advent of the Nuclear Decommissioning Authority (NDA) in the UK represented a radical change in the organisation and management of the UK’s nuclear legacy. Previously plans for decommissioning of Magnox stations envisaged extended periods of care and maintenance, but the NDA has at least the aspiration for an accelerated programme, even if funding may not yet permit this. The renaissance of nuclear power in the UK gives an incentive to clear old reactor sites to make space for constructing new power stations. This provides an opportunity for a thorough reassessment of the technical methodology by which Magnox decommissioning takes place. The new build plans imply that there might be advantages in decommissioning being a more or less continuous activity performed as rapidly as possible to make space for new stations. Volume-reduced waste could be stored either on a small part of the site, or moved to another site. If this new approach is to be adopted, certain technical problems will need to be addressed. For example, the management of graphite moderators and measures to achieve aggressive volume reduction of wastes need to be implemented.
It would be wrong to paint an entirely negative picture of the UK’s decommissioning achievements to date. Good progress has been made. But this paper focuses on facilities which have instead been earmarked for long periods of ‘care and maintenance’ in the decommissioning plans.
Care and maintenance (sometimes called SAFESTORE) refers to the lay-up for a significant period, often several decades, of a nuclear facility that is no longer in use. The facility is kept in a passively safe condition, but the job of final clearance remains to be done at the end of the lay-up period. The driving force for a long period of care and maintenance in decommissioning is usually one or a combination of factors.
Radiation levels reduce through radioactive decay, making the task of final dismantling easier and cheaper.
If available funding is inadequate to achieve immediate decommissioning, a period of delay allows funds to be built up until they are sufficient to cover the necessary costs. Care and maintenance may also emerge as a least cost option, depending on the financial discount rate applied to future expenditures.
The lay-up period stores radioactive material safely in the absence of suitable waste disposal facilities.
Where other nuclear facilities are continuing to operate on the same site, there may be an argument for leaving individual redundant structures in storage until all operations on the site are finished.
This article argues that, apart from the final point above, none of these arguments carry as much weight as the opposing ones and that decommissioning of a facility should be completed in a continuous manner as quickly as possible to an agreed end point (which is not necessarily greenfield). There follow six reasons for completing decommissioning as soon as possible.
Facilitating new nuclear. Early decommissioning underpins the case for building new nuclear plants.
Intergenerational equity. Care and maintenance places a requirement on future generations (i.e. individuals as yet unborn) to complete the job of decommissioning facilities that are properly our own responsibility. It can be argued that we have a moral obligation not to impose such responsibilities on future generations.
Release of assets. Completion of decommissioning of old Magnox reactors could release physical space for construction of new stations. Even where new build is not contemplated many nuclear facilities occupy environmentally important or commercially valuable land (e.g. part of the Snowdonia National Park). It is in everyone’s interest to release the land for productive or amenity use.
Retention of skills. The ageing staff profile and pressure on the industry to attract and retain new recruits are well known. It would be difficult to argue that preparing and then guarding structures constitutes attractive employment for new entrants to the industry. On the other hand the technical challenge of developing and implementing technology to complete decommissioning quickly is exciting and appealing.
Benefits for ‘UK plc.’ The vibrant technical work required to complete early decommissioning is likely to enhance opportunities for UK contractors both at home and abroad. It could also be argued that focus in other countries on early decommissioning has generated skills and capabilities which are now required here. As a result, there is a risk that decommissioning jobs in this country will be displaced by skilled workers from overseas, because we have hitherto followed the wrong strategy.
Reduced costs. Relative costs of early versus late decommissioning are difficult to determine rigorously, since they depend on such factors as discount rates and future policy decisions and circumstances. There is however a general underlying parallel with radiation dose. It has been found that nuclear plants which achieve short maintenance outages often also have low occupational radiation exposure. Crudely, total dose is equal to dose rate multiplied by time, hence reduced time spent doing maintenance means less radiation dose. In the same way, reduced time spent decommissioning means less ‘hotel’ costs. Perhaps the one of the best examples of reduced cost for accelerated decommissioning is the Rocky Flats site in the USA. In 1995 closure was predicted to require 70 years and $22-36 billion. Accelerating the programme to complete by 2006 has reduced the cost to $7 billion.
Care and maintenance is typically a strategy applied to retired nuclear reactors rather than other facilities in the nuclear industry. For example it is reactors, with their neutron-activated cobalt-60 inventory, which benefit most from dose reduction through a few decades of radioactive decay. Where other types of facility have already been in care and maintenance it is usually because there have been no resources or appropriate technology available to deal with them, and that is now being addressed. Care and maintenance is rarely ‘planned’ for any facility other than a reactor.
Turning to reactors, there are examples overseas both of immediate decommissioning and care and maintenance. Water-cooled reactors prevalent in most countries other than UK have a higher core power density than the UK’s gas cooled reactors, and thus the highly radioactive parts are physically smaller and easier to dismantle, package and transport away. As stated below, the graphite moderators of the UK’s gas reactors pose special problems and have been a primary factor in the choice of a care and maintenance strategy. Interestingly, however, a graphite-moderated reactor (Fort St Vrain in the USA) was one of the first reactors to be completely decommissioned .
In the USA the reactors are split between the two strategies of care and maintenance and immediate decommissioning. The reactors in care and maintenance are principally those on sites where at least one nuclear plant is still in operation. For immediate decommissioning the time interval between shut down and completion of decommissioning can be as little as seven years . Several points are interesting from a UK perspective. The USA has low-level waste disposal facilities, but nowhere yet to take the high-level waste (spent fuel). The response has been to ship disposable waste off site, while the nuclear fuel is packaged ready for transport and placed in separate facilities on or adjacent to the site pending transport to final disposal when this is available. The unavailability of waste disposal facilities has not delayed overall progress in decommissioning.
In France there has been a change of strategy from care and maintenance to immediate decommissioning. In 2001, it decided to decommission all six of its gas-cooled reactors (Bugey 1, Chinon A1, A2, A3 and St Laurent A1, A2), which are of design rather similar to the UK Magnox plants. All of these facilities are now due to be decommissioned by 2034. A primary driver for the change in strategy has been the decision to construct further nuclear plants.
How can it be done?
Shield the waste
If nuclear waste is of a type that emits significant gamma radiation, appropriate radiation shielding must be provided for its storage, handling, transport and disposal. This can be done in one of two ways. The shielding (e.g. lead, steel or concrete) can be provided externally in the form of a shielded vault, building or transport overpack. In this case the radioactive materials themselves are packaged in a thin container which is not a significant barrier to the emitted radiation. Handling of the waste can still be safely achieved provided that at all times the waste is surrounded by appropriate shielding in one form or another. This strategy has the advantage that it does not require disposal of shielding with the waste.
Alternatively, the required shielding can act as packaging around the waste and be integral with it. A self-shielded package will not cause radiation dose to personnel unless it is broken open, and can safely be moved at any time with conventional equipment. Perhaps the greatest advantage of all is the greater safety and reduced cost of storing, handling and transporting the packages. In the United States even the highest level of waste, nuclear fuel, is often stored in shielded packages, often in the open air. This should be compared with the cranes, access locks and expensive paraphernalia required for a shielded store.
The disadvantage of the shielded package concept is the amount of shielding material that has to be disposed of together with the waste. This can be mitigated if the principles of volume reduction and recycle can be brought powerfully to bear. Why can’t the shielding be fashioned from material that is destined for disposal anyway? A good example are the radioactive waste containers manufactured by Siempelkamp, which use metal recycled through the CARLA melting facility.
If volume reduction of waste is the goal, how can it be achieved? The answer is special processing to achieve volume reduction, decontamination and recycling. The best alternative is to reuse waste material without any processing. This may sound self-evident but there are at least some examples of equipment and materials being disposed as radioactive waste while new equivalents are purchased somewhere else in the industry.
Decontamination means separating a waste form into a more radioactive and a less radioactive (or non-radioactive) fraction. There is significant opportunity for low-level radioactive waste (LLW), particularly metal, to be cleaned, released from radioactive material control and recycled through conventional scrap recycling facilities. This operation, sometimes called ‘unconditional release’ is not universally popular because of fears about radioactivity inadvertently getting past the controls and into the public domain. However, where it can be carried out safely and in an approved manner it is a very important mechanism for reducing the arisings of LLW. There are international facilities (such as Studsvik in Sweden) where operations of this type are routinely carried out on a commercial basis. The alternative of recycling the materials into new controlled uses within the nuclear industry is a little more difficult to arrange, but has almost universal support.
The principle of decontamination can also be applied to intermediate level waste (ILW), where the down-classifying of ILW to LLW by decontamination can have considerable benefits (see for example, Magnox below).
The UK does not lead the rest of the world in the application of decontamination technology, and probably lags behind. The previous culture of ‘no volume reduction–set all wastes in cement’ has worked against the development of vibrant decontamination operations, both physical and chemical. There needs to be a national effort to develop decontamination operations with mobile plants.
In France there are plans to institute a special waste site to accommodate graphite and other similar low level long lived radioactive wastes. It is unlikely that this could be duplicated in the UK on a short timescale, because the relevant national developments (such as lawmaking and public consultation) have not begun. However, alternatives do exist which could potentially be pursued on the required timescale.
There are some 90,000 tonnes of graphite in the UK nuclear legacy, mainly in the form of moderators of the Magnox and AGR reactors. This is about one-third of the world inventory.
Graphite wastes, although not particularly hazardous, pose special challenges. There are certain perceived problems in handling, storage and disposal such as those associated with Wigner energy (energy stored in the graphite matrix as a result of neutron irradiation) and the potential for fire or dust explosions. There is considerable technical confidence that these problems can be handled, but nevertheless the concerns have to be carefully answered.
However, perhaps the most important problem with graphite waste is its inventory of the isotopes carbon-14 and chlorine-36. These radioisotopes have long half-lives, and are relatively labile over geological times in a repository, which makes them a concern to repository designers. While all other radionuclides can easily be separated from graphite by processing, C-14 separation from the bulk of the graphite would require a difficult isotope separation. There is actually some evidence that the C-14 can be selectively released from graphite because it is not evenly distributed in the structure, but while this is useful it is not yet clear that it will yield a really clean separation .
C-14 is a natural constituent of the biosphere, since it is formed continuously by nuclear reactions occurring in the upper atmosphere. Its continuing presence is, of course, the basis for the technique of radiocarbon dating. Dilution of the world’s nuclear reactor inventory of C-14 with large amounts of non-radioactive C-12, e.g. that arising from burning of fossil fuels, would actually provide more assured radiological safety than trying to contain the C-14 in graphite for its long decay period. The same can also be said for Cl-36 diluted with non-radioactive chlorine. For this dilution to be effective in providing radiological protection the dilution must take place by intimate mixing of the radioactive and non-radioactive species in exactly the same chemical form. Ironically, the independent instigation of carbon sequestration schemes for fossil power provides an opportunity to do exactly that, by converting the graphite to carbon dioxide gas and transporting the gas to a sequestration scheme where it can be mixed with fossil-derived carbon dioxide. This is a new concept of ‘dilute and contain’ and combines the best features of ‘concentrate and contain’ and ‘dilute and disperse’.
Another alternative is to process the graphite into new forms of recycled product for the nuclear industry. There are many examples of new potential uses – for instance future reactor designs involving high temperature and helium coolant will require graphite in fuel and moderator components. This new graphite could in principle be made from reprocessed waste graphite. Other than uses for reactors, graphite is also used for purposes such as electrodes for vitrification of nuclear waste or for non-graphite purposes such as activated charcoal filters. Another possibility is for graphite (or carbon) to form inclusions in or form part of standard formulations for encapsulating radioactive waste (so called ‘graphite mortar’) . These alternatives are being investigated in the European CARBOWASTE programme.
Magnox waste is a good example of how processing could accomplish the previously discussed objectives. Magnox metal waste arises from the preparatory operations which took place on the power stations for sending Magnox fuel to Sellafield. It also arises at Sellafield from the de-canning operations which prepare the Magnox fuel for reprocessing. In addition to the intentional metallic waste products there is corrosion product ‘sludge’ which has come about through corrosion of the highly reactive Magnox metal. The Magnox waste at the power stations represents a significant proportion of the total ILW at those stations, although it is essentially non-radioactive.
Because of its chemical reactivity, Magnox does not constitute an ideal waste form, but the industry has established methods of encapsulating Magnox that satisfy safety concerns about long-term stability [see also p38]. Besides the difficulties of making a material as unstable as Magnox suitable for disposal, grouting all the waste leads to a volume increase – the waste itself is encased in grout, which is in a drum, which is then placed (with spaces between the drums) in a shielded store.
Chemical dissolution is a process developed in the 1980s and applied at full scale at Dungeness power station [see also p35]. It achieves volume reduction and expends the chemical reactivity of the Magnox, and can deal with both metal and sludge. Plans are now being considered to process Magnox waste on other power station sites by dissolution.
Organic materials arise quite frequently in the nuclear industry. Examples are contaminated oil and organic ion exchange resin. These could be processed by a number of techniques to remove the organic, non-radioactive content. For LLW this may involve incineration in some form. For higher levels of waste thermal processing (such as pyrolysis/steam reforming) or wet oxidation techniques can accomplish the same overall objective as incineration, but with more robust retention of radioactivity. There are full-scale commercial processes available for accomplishing this type of volume reduction [see also p37].
Neutron-activated metals constitute a significant part of the internals of Magnox reactor cores. The management of these materials raises a question of UK policy which needs to be addressed. In many countries the division between LLW (destined for shallow burial) and ILW (destined for deep burial) is based more on the longevity of the radioactivity involved rather than its absolute level. There is no logical reason for deep burial of radioactivity which will have decayed before its immediate containment has failed.
Whereas activated metals constitute ILW under existing UK rules, most of the radioactivity in activated metals is relatively short-lived, and hence would in some other countries be regarded as low level waste. There is even the possibility of recycling some of this material into specific uses (e.g. shielding blocks) where the radioactivity can decay in harmless circumstances.
Concrete and rubble can potentially be recycled, particularly for use in new structures. One particular technical issue concerns tritium. Because of the particular specific activity limits in the UK, tritium limits are unusually harsh by international standards. If risk-based standards were employed in the UK as they are elsewhere, much of the bioshield concrete of Magnox reactors would become available for recycle.
Volume reduction is key
Few people would actually wish the outcome of nuclear decommissioning to be half-dismantled structures left under guard for decades and stores of radiation-emitting drums of waste with nowhere to go. If no effort is made at volume reduction, the volume of packaged waste can exceed the volume of the structure that it comes from. That inevitably leads to the conclusion ‘you might as well leave it where it is’– and to the care and maintenance strategy.
Volume reduction offers an alternative to this. By removing materials in the legacy which do not have to be there we can make the remaining waste small, stable, safe to handle, convenient to store on site and easily transportable to a central facility or final repository if one becomes available. Most important of all we leave nothing for our descendents to do except move the waste to its final resting place (if that has not already been done).
The methods of volume reduction referred to in this article are neither comprehensive nor necessarily optimum, and there may be better alternatives available either now or in due course.
 Fort St Vrain Station History, Website, http://www.fsvfolks.org/FSVHistory_2.html
 ”Decommissioning status of US Nuclear Plants,” Nuclear Energy Institute, Website, http://www.tinyurl.com/l8qpgf
 Bradbury, D. and Wickham A.J. “Graphite Decommissioning Options for Graphite Treatment, Recycling, or Disposal, including a discussion of Safety-Related Issues” EPRI Report 1013091. http://tinyurl.com/lz3rax
 G. Tress, L. Doehring, H. Pauli, H.-F. Beer, Paul Scherrer Institute, CH-5232 Villigen, Switzerland.”Optimised Conditioning Of Activated Reactor Graphite”. Proceedings of ‘WM’02 Conference’. February 24-28, 2002, Tucson, AZ.
David Bradbury, managing director and George Elder technical director, Bradtec Decon Technologies Ltd, UK. Email: [email protected]; [email protected] Susan M. Hewish, Business Development Director – Nuclear, Hyder Consulting (UK) Ltd. Email: [email protected]Related ArticlesDissolution solution Nuclear in the UK – where did it go wrong? A UK strategy for management of LLW gets nearer Plute options
|How the NDA decommissions; a response|
The Nuclear Decommissioning Authorityâ€™s key mission is the decommissioning and clean up of the UKâ€™s civil public sector nuclear sites. We do not directly manage our sites or facilities; instead we contract out the delivery of site programmes through management and operation contracts with licensed operators, Site Licence Companies (SLCs), at each site.