Introduction
The principle of geological disposal is to isolate the waste deep inside a suitable rock formation to ensure that no significant quantities of radioactivity ever reach the surface environment. It is the main option on which the Nuclear Decommissioning Authority (NDA) conducts research for the long-term management of radioactive waste. It is the Government's, and many other nations, preferred long-term approach. Similar deep disposal routes are being actively implemented in Finland, Sweden and the USA. It provides a vision as to how radioactive waste could be managed in the long term and so allows us to set the standards against which waste is packaged and conditioned today. Radioactive waste exists and one of the most important ethical, environmental and safety issues facing the UK is how to deal with it in the long term. The objective of responsible long-term management of such waste is to isolate it from both unwarranted human intervention and natural occurrences, so as to protect both the environment and those who live in it, from the radioactivity. For some kinds of radioactivity this requires long-term plans spanning generations.
Geological disposal is a multi-barrier, multi-phased approach, based on placing wastes deep underground, beyond disruption by man-made or natural events. This has evolved over the last few years to incorporate extended monitoring and the option of retrievability. These ideas have been incorporated as a result of extensive consultation with a range of interested groups and individuals.
Phase 1 - Packaging the waste
For enhanced safety in storage, Government policy is to package and immobilise intermediate level waste (ILW) as soon as possible. This phase is already under way, at the sites where the waste is generated. Packaging standards adopted in the UK are set by the NDA.
Waste packages provide the first of several safety barriers - physical containment.
Fig 1: Nirex standard waste containers for ILW and LLW (courtesy NDA)
Phase 2 - Interim surface storage
Existing stores for packaged waste are designed to provide a service life of 50 - 100 years or more. These provide safe and secure interim storage throughout the period when the geological disposal facility is being planned and developed. These are generally at the site of origin or site of packaging. Radioactive waste is then held in these well engineered surface stores until a decision is taken about where a geological disposal facility will be placed. (This will be the subject of local community agreement.)
Fig 2: Encapsulated product store, Sellafield (courtesy NDA)
Phase 3 - Transport
The waste packages will be transported to a national geological disposal facility. During transport, extra containment and radiation shielding will be used where necessary.
Phase 4 - Waste emplacement
The waste packages will then be put into the geological disposal facility.
A geological disposal facility
The features of a geological disposal facility will include the following:
Surface facilities
A variety of different facilities will be needed above ground, for example construction support facilities, management and administration offices, workshops and, possibly, a waste encapsulation plant and a visitor centre. There will also be a need for transport-related infrastructure, such as railway sidings and roads, to manage the arrival of waste at the facility. Transport of waste is subject to strict regulatory control, as discussed in Transport of radioactive waste .
Access to the underground vaults and disposal tunnels could be via one or more sloping underground tunnels ('drifts') and/or one or more vertical shafts. The number required will be determined by the need to provide separate access routes for personnel and waste, to segregate the construction and waste emplacement operations and to provide services such as power and ventilation. The depth at which the underground vaults and disposal tunnels will be located is likely to be somewhere between 200 and 1000 metres, but this will depend on the geology at the site in question. Given the length of time over which a facility will be expected to function, the potential local effects of some future surface change (e.g. through ice ages, erosion, etc.) will also need to be taken into account in the design.
Underground facilities for intermediate and low level wastes (ILW/LLW)
ILW/LLW wastes will typically be immobilised in a cement-based grouting material within standardised, highly engineered stainless steel or concrete-lined stainless steel containers. The waste packages will then be placed in horizontal engineered vaults or other suitable structures within the host geological environment. The waste packages can then be stored underground until the decision is taken to close the vaults. Following emplacement of the wastes the vaults would be 'backfilled' when technically required, for example with alkaline grout, specially formulated to inhibit dissolution of any soluble radionuclides, and then sealed.
Underground facilities for high level waste (HLW) and spent fuel
Because they generate heat, HLW and spent fuel (if classified as waste for disposal) require different disposal structures and layouts from ILW, LLW and other non-heat generating radioactive materials. There are a number of ways in which HLW and spent fuel could be packaged and contained, and research in this area is likely to present alternative models over the coming years. However, one method that is planned to be used in Sweden and Finland, and could potentially be applicable in the UK to stocks of HLW and spent fuel, is based on sealing the waste in copper canisters, with a cast iron internal frame for strength. These canisters are placed in individual 'deposition tunnels' and surrounded by bentonite clay, which expands on contact with water and so seals the space around the canister. Under appropriate conditions copper is extremely resistant to corrosion and in a suitable geo-chemical environment such as this the canisters can be expected to maintain their integrity for hundreds of thousands of years. Following waste emplacement, the deposition tunnels would be backfilled and sealed.
Size of a geological disposal facility
The dimensions of the underground areas of a geological disposal facility will be determined by the exact inventory for disposal, the properties of the host rock and the geometry of features within it.
If there are to be any new nuclear power stations built in the UK, the UK Government believes that it is technically possible to dispose of higher activity radioactive waste from them in a geological disposal facility (see Wastes from new nuclear power stations ). Scientific consensus and international experience suggests that despite some differences in characteristics, it would not raise such different technical issues compared with nuclear waste from legacy programmes as to require a different technical solution.
Indicative geological disposal facility dimensions have been estimated in "The Gate Process: Preliminary Analysis of Radioactive Waste Implications Associated with New Build Reactors" (United Kingdom Nirex Limited, Technical note, February 2007). These estimates were based on an inventory similar to the 'Baseline Inventory' discussed in UK Radioactive Waste Inventory . They indicate that the underground area of host rock required (i.e. the 'footprint') for an ILW/LLW disposal facility would be of the order of 1km², and for a HLW and spent fuel disposal facility (assuming that the latter were treated as a waste) would be of the order of 3km². In practice it may be possible to build a geological disposal facility over a smaller area, by building deposition tunnels or vaults on different levels. This would however depend on the geology of the site.
Fig 3: Generic co-located geological disposal facility
Construction and operations
Construction of a geological disposal facility would employ standard techniques that are used in the underground construction and nuclear industries for other major engineering projects, and have already been used to construct operational geological disposal facilities in other countries. The project will also require ongoing involvement of the scientific (and in particular the geological) community. Underground facilities would be developed in stages to enable waste emplacement operations to begin as soon as practicable once relevant permissions from the regulators have been received. Main facilities would be developed first, after which additional vaults and deposition tunnels would be constructed, equipped and commissioned as required throughout the life of a geological disposal facility. Construction and waste emplacement activities would be managed to ensure physical segregation of the two activities.
Phase 5 - Backfilling and closure
Once a geological disposal facility has been filled with waste, a process which could take many decades, the shafts and tunnels can be backfilled and sealed and the surface facilities dismantled or used for something else. There will then follow a period of post-closure institutional control and monitoring in accordance with regulatory requirements. What happens to the site will be a matter for future generations - the site could be farmed, forested, allowed to return to nature, or used for construction or other purposes, with the waste itself isolated within the multi barrier system in the geological formations hundreds of metres below the ground. Records of the location and general contents of the facility would be held permanently by The National Nuclear Archive.
Fig 4: A closed deep geological disposal facility (courtesy NDA)
A plan with options
This long-term plan will take many decades to implement and will have been developed in such a way that it progresses towards permanent disposal, but in a series of phases. The details will be agreed in partnership with the host community. In the light of future knowledge, it could be decided to move to the next phase faster, or to change the plan. In this way we take as much responsibility as we can for managing the waste we have created, but we do not tie the hands of future generations.
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