Radwaste challenge14 January 2010
The technical, regulatory and logistical systems for the transport of front end nuclear materials are now well-developed. Decommissioning wastes, however, pose several tricky issues for transport that are only just starting to be addressed. By Sam Darby
More and more waste is being produced by decommissioning operations around the world. Partly these operations are efforts to clean up the legacy left by past nuclear programmes and old nuclear power plants. But these operations are also preparing existing nuclear sites for the new nuclear power plants needed to satisfy the world’s ever increasing demand for electricity.
In addition, a large variety of process wastes have been created by the nuclear fuel industry, from mining, conversion, enrichment and fuel fabrication processes, to reactor operations and reprocessing. The management of these wastes also has an impact on plans for future nuclear plants.
The focus in waste management so far has been mainly on the need to package the waste to ensure safe, interim or long-term storage. Transport of these waste streams has tended to be regarded as a separate issue. Nevertheless, transport will clearly become needed on the way to final disposal. Transport considerations should therefore become an integral part of the overall waste management strategy.
The transport of nuclear wastes is likely to increase significantly in both volume and variety. These changes will raise many new regulation and operational issues.
Many wastes arising from fuel manufacturing plants and decommissioning operations are normally stored on site initially. These wastes will eventually have to be conditioned and packaged into a form suitable for final disposal. The conditioning and packaging of wastes has to be consistent not only with the needs of storage but also those of subsequent transport operations, and finally disposal. Otherwise risks and cost are likely to be increased by multiple handling, processing and repackaging.
Unfortunately optimising waste conditioning and packaging for storage, transport and disposal equally can be difficult to achieve in practice. The time intervals between storage, waste conditioning and disposal could be in the decades of years. Sometimes wastes have to wait until the final specifications for waste conditioning and disposal are available. These delays can lead to regulatory and operational complexities as well as increased life cycle costs.
As for all potentially radioactive materials the transport of wastes at each stage will be governed by the comprehensive International Atomic Energy Agency (IAEA) regulatory regime that sets the standards for packaging and transport and which is vital to ensure safety. Compliance with the transport regulations will therefore be needed in addition to the requirements for storage and later for disposal.
In addition, the risks of radioactive waste handling, waste storage and waste transport are not necessarily the same. There may be different safety methodologies adopted in the plant and outside it. There are also different regulations (safety standards) for each type of process, as referenced by the IAEA Waste Safety Standards Committee (WASSC) and the Transport Safety Standards Committee (TRANSSC). It is therefore important to ensure that the regulations governing waste management and waste transport are harmonised as far as possible and inconsistencies avoided. With this objective in mind the IAEA set up a joint working group between the two committees to identify common safety-related issues and to suggest ways to achieve compatibility.
Intermediate nuclear fuel cycle products such as uranium ore concentrate, uranium hexafluoride and uranium dioxide, and new nuclear fuel assemblies are easily defined. So are some wastes, notably vitrified high-level waste from reprocessing, and to some extent spent nuclear fuel also, if it is designated as a waste for disposal. However many of the wastes arising from nuclear fuel cycle plants and decommissioning activities are not easily characterised. Their chemical, physical and radioactive properties vary greatly from consignment to consignment, and also within the consignment; their degree of homogeneity also varies. In particular, their radioactive content is not always easily assayed. This issue also applies to some extent to wastes from other production facilities - for example, in the manufacture of large radioactive sources for medicine and industry.
The transport of large objects, such as decommissioned reactor components, process vessels and pipework raises new challenges. Many large vessels, such as reactor heat exchangers, chemical plant and enrichment plant items, are radioactive mainly because of surface contamination, but in some geometries this cannot be conveniently assayed. Normal types of packaging are clearly not appropriate for very large objects. How can these be effectively catered for in the IAEA transport safety regulations? The way is not yet clear and new approaches need to be explored.
The IAEA transport regulations must be able to cater for these waste materials to ensure safety without imposing unjustified constraints. To what extent will it be possible to identify broad categories of waste, consider how these might be characterised and described, and frame the transport regulations to accommodate them? This important issue could potentially have a significant effect on waste management operations.
Fuel processing plants and decommissioning operations generate waste streams that could contain fissile material. These streams vary greatly. Typical examples of these wastes are plutonium-contaminated scrap equipment, cleaning materials and process wastes; low-enriched uranium oxide wastes from fuel plants; precipitates and ion exchangers from effluent treatment plants. Some contain only a small quantity of fissile material in a large quantity of non-fissile material. Because these wastes are normally recovered or disposed, they need to be stored and transported. The fissile-excepted provisions in the current transport regulations require many wastes with fissile material to be transported in fissile packages, even when there is no realistic criticality hazard even under accident conditions. These requirements can be onerous and costly and may involve repackaging in large numbers of new containers and a corresponding large number of transport operations. If such waste could qualify as fissile-excepted, excessive costs and operational problems would be avoided. Doing so would not completely eliminate the need to classify, package and transport this material as fissile waste, but it would much reduce the overall volume.
Spent nuclear fuel
There are two options for managing spent nuclear fuel from nuclear power plants. In the once-through process, the spent fuel is designated as a waste for disposal. Fuel reprocessing and recycling recover residual uranium and plutonium from the spent fuel and reused, leaving principally a high-level waste for disposal. Some countries store spent fuel for long periods before deciding which option to adopt.
In the once-through option, spent fuel may be stored for several decades until a disposal facility becomes available. Compliance with the requirements for storage over this extended period has to be demonstrated. Compliance with the requirements for transport will also be required at the time transport takes place. The most effective way of meeting these requirements is not clear, mainly because of the long time interval between storage and disposal. It is likely that the optimum strategy will depend on individual circumstances.
An increasingly popular option for the management of spent fuel is to use dual purpose casks for both transport and storage (and possibly also for disposal). This offers significant operational and economic benefits in some situations. However problems can arise, as noted above, because storage regulatory approvals are required for a long period, whereas transport approvals are valid for a relatively short period.
The whole range of nuclear fuel materials, from uranium ore concentrate through to new fuel assemblies, are now routinely and safely transported, both nationally and internationally, by all modes of transport. For decades, spent nuclear fuel has been transported by road, rail and sea without damaging public health or the environment. In recent years, high level vitrified waste has also been successfully carried internationally by both rail and sea. This excellent safety record is the result of an effective regulatory regime in combination with the professionalism of the nuclear industry and its transport service providers.
In spite of its safety record, both spent fuel and high level waste transport have on occasions given rise to public concern. There is a need to continue efforts to convince the public that this transport is a necessary, safe and secure operation.
An industry waste management group was formed in June 2009 within the World Nuclear Transport Institute (WNTI) to address the common interests of member companies in safe transport and packaging of waste, including wastes arising from decommissioning.
In conclusion, whereas the packaging and transport of nuclear fuel cycle intermediates is well-established and will be capable of dealing effectively with foreseeable developments, the packaging and transport of decommissioning and miscellaneous process wastes are not yet well developed. Some significant regulatory issues for waste packaging and transport remain. It is important that the relevant regulatory requirements should allow safety objectives to be met without imposing unnecessary operational or economic burdens. However the nuclear transport industry is experienced and robust, and with cooperation between the waste management sector, regulators and the transport industry it is confidently expected that this goal will be achieved.
Sam?Darby, waste management industry working group chair, WNTI, Remo House, 310-312 Regent St, London W1B 3AX, UKRelated ArticlesUK ships HLW back to Japan