There and back again1 March 2002
The Nuclear Energy Agency has written a report: "Reversibility and retrievability in geologic disposal of radioactive waste." This looks into one approach to the challenging task of the management of long-lived radioactive waste that must be isolated from the human environment for many thousands of years. By Peter Brown, Claudio Pescatore and Trevor Sumerling
Radioactive waste must be managed responsibly to ensure public safety and the protection of the environment, as well as security from unauthorised interference, now and in the future. One of the most challenging tasks is the management of long-lived radioactive waste that must be isolated from the human environment for many thousands of years.
There is a consensus that engineered geologic disposal provides a safe and ethical method for the long-term management of such waste. Engineered geologic disposal involves the emplacement of waste in repositories constructed deep underground in suitable geologic media. Thus the waste is contained, and safety assured by passive barriers with multiple safety functions, so that there is no need for any further actions by future generations. The primary principles of the engineered geologic disposal concept are that waste will only be emplaced in a repository when there is a high confidence in the ultimate long-term safety, and that the long-term safety must not rely on actions following closure of the repository. This does not mean, however, that actions cannot be taken. Most repository development programmes include the possibility of post-closure activities for security and monitoring purposes.
Emplacing waste in a manner that favours its retrievability at later stages provides greater flexibility, which is useful for decision making. However, it is helpful to explore arguments for and against retrievability provisions. This enables us to gain a perspective on the benefits and possible drawbacks of enhancing retrievability.
In favour of waste retrieval
Factors that might lead or contribute to a decision to retrieve waste and weigh in favour of building provisions for retrievability are as follows:
• Technical safety concerns that are only recognised after waste emplacement and/or changes in acceptable safety standards.
• A desire to recover resources from the repository, such as components of the waste itself, or the recognition or development of some new resource or amenity value at the site.
• A desire to use alternative waste treatment or disposal techniques that may be developed in the future.
• To respond to changes in social acceptance and perception of risk, or through changed policy requirements.
Technical safety concerns or changed safety standards
The ability to retrieve waste in the event of unforeseen technical safety concerns may be the most important reason to enhance retrievability to achieve widespread confidence, even if the likelihood of the need for retrieval for this reason is very low.
The safety case for the repository should be sufficiently robust that it should not be compromised by any new technical information regarding the site and design that arise after waste emplacement.
It is possible, however, that new observations, possibly as a result of site or repository monitoring, or advances in scientific understanding will reveal unexpected characteristics or phenomena that are detrimental to the long-term safety of the repository. If the new observations and advances in scientific understanding invalidated, partly or totally, the arguments for confidence in long-term safety that supported previous licensing steps, this would be a regulatory concern. Waste may or may not need to be removed as a result of this finding. It is very unlikely that the risk of loss of containment would be such as to require urgent recovery of waste, or recovery of more than a small fraction of the waste. More likely, improvements might be made to the engineered barriers within the existing disposal system or the waste removed after an alternative disposal route had been prepared.
Changes in technical safety standards may also occur in the future that place either greater or lesser demands on repository performance.
The value of spent fuel as a resource, should energy strategies change, is often cited as a reason for keeping this material retrievable. It is unlikely that vitrified high-level waste or other long-lived wastes would one day constitute viable energy resources. Another possibility is the desire to recover elements that are rare or do not occur in nature and may have uses in future technologies.
The existence of natural resources and the amenity value of the land are important considerations when siting nuclear waste repositories. Retrieval may need to be considered due to the discovery of a previously unrecognised resource at a disposal site, or the land might acquire some new amenity value. In this case, the waste might need to be removed either because of actual safety concerns, or because of a perception that the presence of the waste was inconsistent with the use of the resource or amenity.
New waste treatment or
Research is continuing on partitioning and transmutation (P&T). It is accepted that these techniques do not offer a realistic alternative to geologic disposal, but they might be incorporated into waste management strategies in the future to reduce the waste volume for dispersal and alter its characteristics. Even so, this does not mean it will be acceptable from a safety perspective to recover waste that is already emplaced, unless the design has been made with this option in mind.
Other novel waste management or disposal techniques might be developed, although the motivation for developing such techniques is likely to be diminished if geologic disposal is successfully implemented. The arguments for and against recovery of emplaced waste are liable to be similar to those for P&T.
Social acceptance and
perception of risk
At present, the inclusion of retrievability in waste disposal programmes is favoured as a policy in several countries for ethical reasons and public confidence. People consider a technology to be safer if they know what will be done in the case of an accident, however unlikely such an accident might be. Waste retrieval can be seen as an ultimate measure to be taken in case of an unforeseen event.
Against waste retrieval
Reasons for not including retrievability provisions in repository design may be connected to factors such as the additional complexity entailed, the cost-effectiveness of a retrieval option, and long-term security concerns. They include:
• Uncertainty about negative effects, including conventional safety and radiological exposure of workers engaged in extended operations and/or associated monitoring, or marginal gains.
• The possibility of failure to seal a repository properly due to the adoption of extended or more complex operational plans to favour retrievability.
• The favouring of irresponsible attempts to retrieve or interfere with the waste during times of political and/or social turmoil when safeguards and monitoring features are no longer in place.
• A possible need for enhanced nuclear safeguards.
Negative effects on safety
Potentially negative processes may be introduced by measures to extend the open period of a repository or associated monitoring and maintenance, such as degradation of repository materials or near-field rock conditions during an extended period of open underground access ways. Any such effects would have to be assessed, and assurance reached that any detrimental influences did not significantly degrade long-term safety.
The introduction of provisions for retrievability must not be detrimental to long-term safety. Thus, for example, locating a repository at a depth that is less than optimum from a long-term safety perspective in order to facilitate retrieval is unlikely to be acceptable.
If an extended pre-closure period is implemented after emplacement of the waste in order to permit retrievability, the repository design and operation must still ensure adequate operational safety. The maintenance and monitoring necessary to ensure retrievability may lead to increased doses to workers and more prolonged hazard of conventional and mining accidents, although doses and non-radiological risks should be managed to remain within acceptable limits. Very extended times for open access to the disposal areas may imply substantial underground refurbishment programmes or even, for some types of waste and retrievability concepts, repackaging of wastes. In such cases, it would have to be considered whether any additional doses to workers were justified in terms of reduction of potential long-term doses or increased confidence in long-term safety.
Uncertainty over final
closure and sealing
A possible risk of an extended operational period associated with retrievability, such as over 100 years, is that the repository may not then be properly closed and sealed. This may occur due to the failure of organisational or financial arrangements or loss of technical capabilities. As a result, the repository access ways may be left open but not maintained or only poorly sealed. In this case, the open, perhaps collapsed, or poorly filled access ways may provide a path for the movement of groundwater, gas and contaminants and also more easy access for inadvertent or irresponsible interference with the repository.
Enhanced opportunity for irresponsible entry
Government and regulatory control should not be relied upon in the long-term. This is one of the main reasons why the geologic disposal concept has been proposed and developed: to relieve the burden on future generations to maintain and control the disposal site. Even in the span of a few decades, adherence to law and regulation can decline in a society, especially if economic conditions change or in case of political troubles and war. In such a case, a repository in which waste retrievability provisions were implemented might offer an easier target for irresponsible recovery of waste or engineered barrier materials or malicious damage than a repository without retrievability provisions.
Need for enhanced safeguards
In the case of repositories for spent fuel, any measures taken to enhance waste retrievability run counter to the objective of making the diversion of nuclear materials to military purposes as difficult as possible. According to the Treaty on the Non-Proliferation of Nuclear Weapons, safeguards negotiated with the IAEA must be applied to the management of source or special fissionable materials to prevent the use of those materials in weapons. The Treaty specifies that nuclear safeguards can only be withdrawn if the nuclear material is, in the judgement of the IAEA, "practicably irrecoverable". By making nuclear material more recoverable rather than less, retrievability provisions may necessitate an enhanced level of safeguards and oversight. For example, the level of safeguards during any extended open period of a repository would likely be much higher than would be required following final closure. Similarly, a repository designed to facilitate waste retrieval even after closure would likely require more careful monitoring than a repository not so designed, placing an undesirable burden on future generations.
Certain practical requirements must be met to assure the feasibility of retrievability of waste. These include:
• Technical understanding and capability at each stage of repository development following waste emplacement.
• R&D to develop equipment and techniques to correct any shortcomings in the present technical capability for retrieval.
• Appropriate institutional arrangements and planning to ensure the continued availability of technical and decision-making capabilities.
• Site and repository monitoring to ensure that conditions necessary for various retrieval methods are met.
The European Commission has produced a report on retrievability (Grupa et al) which discusses the technical feasibility over 13 time periods. It discusses retrieval during four broad stages of repository development distinguished by the physical ease with which retrieval might be accomplished.
A generic difficulty in repositories for spent fuel and high-level radioactive waste, at all stages, is dealing with the heat and radiation output from the waste. In some disposal concepts, there may be a period in which temperatures and radiation fields are too high to allow safe waste retrieval, forcing the delay of any desired retrieval.
The four broad stages are: during waste emplacement; after waste emplacement and before gallery backfilling; after gallery backfilling and before repository closure; and after repository closure.
R&D requirements for
During the waste emplacement period and before extensive placing of backfill, waste retrieval could usually be achieved by the reverse use of emplacement systems. In principle, the equipment could be similar to that used today in mining and decommissioning projects. Special measures will be necessary to undertake mining and retrieval operations at the high temperatures and radiation levels that will persist around spent fuel and high-level waste containers. In some geologic environments, it may be preferable to delay retrieval until the temperature and radiation have decreased.
Institutional arrangements and planning for retrieval
For waste retrieval to be feasible, institutional arrangements have to be foreseen to ensure that:
• An appropriate level of technical ability to retrieve is maintained at each stage following waste emplacement.
• The methods for retrieval are defined, including retrieval under foreseeable component failure and accident conditions.
• Periodic evaluations are made of the status of the repository, focusing on the operational safety and potential long-term safety and the appropriateness and need to proceed with the next step towards repository closure, or maintain at the current step, or reverse a step.
Site and repository monitoring
Prior to construction, and continuing up to closure, monitoring of various site and repository parameters will provide information for safety assessment. This may include confirmation of natural site conditions, understanding of the response of the natural system to the presence of the repository, and the early evolution of the engineered elements. In many designs, measures to enhance retrievability, such as an extended open period, will modify site conditions and delay or prevent the conditions that are aimed for long-term safety.
During the operational period, and any extended open period that follows it, monitoring of rock stability, the underground environment, and waste package conditions will be needed. Such monitoring is required to ensure operational safety, to detect any risk of incipient failure, and to check whether the conditions for waste retrieval according to the given methods are still met. The results of monitoring will be used to plan maintenance and refurbishment of the various systems and underground elements. The results may also contribute to decisions on when to move to the next stage towards repository closure, as they will refine estimates of how long a given stage can be maintained without significant additional expenditure on maintenance and refurbishment.
Subsequent monitoring, which may include the monitoring of backfill conditions, may be carried out to follow the early evolution of the engineered barriers, and to check that the expected evolution towards stable physical and chemical conditions is underway. It could also be used to check whether or not the conditions for waste retrieval according to the available methods are met, should the decision to retrieve the waste be taken.
Spent fuel and other waste that is rich in fissile material would be subject to both monitoring for safety and to nuclear safeguards monitoring to verify that no unlawful retrieval has taken place. During the operational period and any extended open period, this would be done by on-site administrative and surveillance measures as used at other nuclear plants. Monitoring could continue in the post-closure period using on-site and remote techniques, for example, acoustic techniques, aerial photography, and satellite imagery may be used to identify any drilling or mining activity aimed at retrieval.
In most concepts, waste retrieval will become more technically demanding as stages towards closure, such as progressive filling of disposal vaults and access ways and placing of seals, are taken. Although waste retrieval may be possible at all stages, including after closure, it is suggested retrievability should be considered mainly in the period before closure, and R&D should focus on the possibilities for retrieval in this period.
Similarly, institutional arrangements and plans for retrieval of the waste should focus on the period preceding closure. Retrieval after repository closure, although technically possible, will require substantial resources to re-establish above- and below-ground facilities and access to the waste. If the need to reverse course is carefully evaluated with appropriate stakeholders at each stage of repository development, a high level of confidence should be achieved by the time a closure decision is taken that there are no technical or social reasons for waste retrieval.