With nuclear power now being reappraised in light of climate change pressures, is now also the right time to reconsider the approach to waste management, ask Ian McKinley and Susie Hardie
Underground nuclear waste repositories could serve multiple waste disposal functions
In order to limit the use of fossil fuels, the rapid expansion of nuclear power is essential – not only in countries with established nuclear infrastructure, but also in “non-nuclear” developed countries and even developing countries with limited technical resources.
Although developments like SMRs may provide an option of mass-produced units that can be delivered as turnkey or supplier-operated facilities, back-end waste management is often presented as a potential show-stopper – without any actual justification. Nonetheless, given the critical need to combat climate change, this could be a good point to reassess waste management from a global perspective.
Although opponents regularly present waste as the Achilles’ Heel of nuclear power, this is technically incorrect. Indeed, a positive aspect of this energy source is that waste volumes are small and can be managed in a rigorous manner. The delays in implementing disposal of higher activity wastes have been emphasised by opponents in the past, without noting the progress in disposal of other nuclear wastes. There are many surface/near-surface facilities operating around the world and even disposal sites for military trans-uranic waste (TRU) in New Mexico, USA. There is also the fact that, to facilitate waste handling, interim storage to allow cooling by decay means that it is only in the coming decades that such repositories will be required. In this case, licences for construction have been obtained in Finland and Sweden and will soon follow in countries like France and Switzerland. Where repository siting programmes have been disrupted, for example in the UK, Germany or the USA, this can often be attributed to lack of political commitment and the NIMTOO effect – ‘not in my term of office’, rather than any fundamental technical issues or even local ‘not in my back yard’ (NIMBY) opposition.
Numerous studies have shown the practicality of geological disposal for the more radiotoxic wastes in a wide range of geological settings. This approach meets ridiculously low safety targets that are orders of magnitude below natural background levels and extremely long assessment timescales stretching 100s of thousands or even millions of years, see Figure 1. This stands in dramatic contrast to other chemotoxic wastes, which have release limits orders of magnitude above background and are assessed only for short time scales of hundreds of years or less. This approach is despite having no potential to decay away like radioactive waste – these wastes will be present for all time. Indeed, the global Climate Change threat can be attributed directly to a lack of appropriate waste management by the fossil fuel industry. Furthermore, waste pollution has now been identified by the UN as a global concern, at a level with climate change and loss of diversity, according to their Making Peace with Nature: A scientific blueprint to tackle the climate, biodiversity and pollution emergencies report. Thus, the good practice established for radwaste should also be seen from this perspective.
Figure 1: Typical radiation doses calculated for disposal of vitrified high-level waste (upper) and TRU (lower).
Need for a paradigm change
Although this was not an issue in the early days, in recent decades there has been an assumption that countries benefiting from technology that produces nuclear waste should be responsible for its safe management. This is often presented as a moral issue – but contrasts markedly with other wastes for which export to other often less-developed countries occurs on a massive scale. It is also inconsistent with the fact that waste from uranium mining is decoupled from all other radioactive wastes in the nuclear fuel cycle – with wastes the responsibility of the ore producer rather than the eventual nuclear fuel user. In addition, it is also incompatible with the treatment of naturally occurring radioactive materials (NORM), despite the fact that some industries (especially oil and gas) produce large quantities of this, which has radiotoxicities equivalent to some of the higher-activity intermediate level waste (ILW).
Many of the repository concepts developed in the more advanced programmes are over-designed. They are often purpose-built structures for limited inventories, excavated deep underground and characterised by very high-performance engineered barriers. Despite the small waste volumes involved, such facilities often have large footprints of several square kilometres and utilise large quantities of materials. These materials are often inherently valuable, for example copper, or are difficult to work with and need to be transported over large distances, like bentonite clay. Clearly, these concepts were developed before issues like sustainability and the carbon footprint were identified as concerns and hence perform poorly from an environmental impact perspective. In addition, repository implementers were originally charged only with disposal of radioactive waste and in some cases only particular waste classes. Hence there was no consideration of co-disposal of radwaste and other hazardous materials. Consequently, the potential for holistic waste management has been little considered to date.
Repositories for higher activity wastes could readily take much wider types of waste, with relatively minor modification. Of course, for this option to be realised it is advantageous to consider this before repository projects are finalised, or better still initiated. As many countries that are already close to implementing repositories may also be suppliers of SMRs to other, less advanced countries, it could also be sensible if they concomitantly assumed responsibility for all resultant decommissioning and waste disposal. In the past this option been a political hot potato, as opponents present this to the public as a country being used as a global nuclear dustbin for purely commercial reasons. If, however, this is reformulated as a commitment to reduction of CO2 releases, linked to a fair allocation of carbon credits, this may be much more acceptable. Here good and clear public communication will be key.
A holistic waste management approach
If the major nuclear countries cannot find a political path to accepting foreign waste, another option would be for smaller countries to focus on regional, shared facilities – as have already been proposed for the smaller nuclear programmes in Europe. Not only is this a cost-effective approach, it is also very sensible from an environmental and sustainability perspective. Here, the biggest challenge is to find a host for regional repositories but, again, if the focus were to be on carbon credits rather than commercial aspects, hosting a regional repository may be a popular option for countries that might otherwise struggle to meet carbon reduction targets, such as China for example.
Even if a regional repository option is not available, local solutions may well be practical if waste management is considered in a holistic manner. Although considered anathema by most repository implementers, there are good technical arguments for geological disposal of all problematic wastes and a shared facility would greatly improve the practicality (and costs) of management of a relatively small radwaste inventory. Here, it would help if international organisations would provide more pragmatic guidelines, rather than the As Low As Reasonably Achievable (ALARA) approach that helps foster design overkill.
A fundamental problem in rationalising designs results from institutional inertia and a common assumption that options that have been studied for decades must be optimal solutions. This is a version of the sunk cost fallacy. For example, the KBS-3V concept, developed for disposal of spent fuel in Scandinavian crystalline rock in the early 1980s, is still considered as a kind of gold standard for disposal of higher activity wastes in different geological settings. At the time of its development, extremely high-performance engineered barriers were incorporated to cover uncertainties in the understanding of the geological environment, whilst there was no consideration of practical implementation to high quality levels or any discussion on sustainability or environmental impacts (EIs). Whilst a slightly modified version of this vertical, in-hole emplacement concept might actually be implemented in Sweden and Finland, this may not make any sense for the boundary conditions in other national programmes. Considering only the mass of copper required for waste containers, for the reference Japanese inventory of high-level waste (HLW) for a first repository, the cost of this material alone would be ≈US$800mn (compared to ≈US$4mn for steel), without adding any benefit in terms of performance. Additionally, constructing and sealing copper canisters requires advanced technology such as friction-stir welding, which further increases costs and associated EIs.
Figure 2: Evolving repository concepts
Rethinking traditional disposal
From a starting point of questioning “traditional” disposal options, taking advantage of the current knowledge base for deep geological conditions, engineering and materials technology and safety assessment approaches, a wide spectrum of disposal concepts can be explored. Each of these concepts allows for extensive optimisation for specific boundary conditions (see Figure 2). For a purpose-built repository, taking more credit for the geological barrier leads to in-tunnel emplacement options, with steel replacing Cu as the canister material and, when prefabricated engineered barrier system (EBS) modules (PEMs) are used, allowing a higher emplacement density. This would roughly halve the required broken-out rock volume compared to a conventional concept, decrease ventilation and drainage requirements, and also reduce complexity and risks associated with implementation. Furthermore, as containers are under roughly isostatic pressure, very low-tech sealing options would be possible, such as a screwed on lid.
This approach can be extended, if disposal vaults are considered instead of tunnels, allowing yet higher emplacement densities and, possibly, emplacement of large multi-purpose containers (MPCs). This option is especially favourable if MPCs are already used for transportation and interim storage and, if not re-utilised, would ultimately end up requiring management as waste.
As emplacement density increases, heat management becomes a greater issue – in some cases leading to concepts that involve an extended open “storage” period before vaults are backfilled. However, such heat can also be considered as a positive attribute if this is utilised using heat-pump technology. This provides a source of carbon-free energy that could to some extent offset the other energy requirements for repository construction and operation.
More benefits are possible if a wider perspective on waste management is introduced. Nuclear decommissioning is a major back-end activity, producing large quantities of low-level contaminated material that is usually declared as waste. Nuclear steel could, however, be recycled to construct waste containers or MPCs – especially if all operations are remote handled and small levels of radiation from the steel are of no significance. A range of other decommissioning wastes could also be used as backfill for underground openings, rather than the specialist clays and concretes with an especially large CO2 footprint that are currently considered. Even plugs, which can play a role in isolating emplacement zones, could be replaced by nuclear-steel bulkheads – potentially with even better functionality and options for failsafe in the event of a perturbation.
Indeed, there are many other non-nuclear wastes that could greatly benefit from deep disposal as opposed to near-surface dumping, as often occurs. Even if it is too complex to incorporate such wastes into the disposal panels, repository designs commonly include a huge number of access tunnels, shafts and ramps which are simply backfilled after waste is emplaced. For example, in the reference Japanese HLW repository this could be ≈1-2 million m3. Potential wastes that could be readily included here would be those that are unlikely to significantly perturb geological barrier roles such as materials like asbestos and heavy metals.
Finally, the entire need to construct a special disposal facility can potentially be reconsidered. Disposal in disused mines has been an option considered in the past for radwaste, for example at Morsleben. It is already the basis of an international industry for disposal of chemotoxic wastes. From the viewpoint of the global fuel cycle, the particular benefits of some uranium mines being used as disposal facilities should be noted. These are not only likely to be particularly suitable from a geological point of view, having contained uranium ore for millions of years, but existing excavations could greatly reduce the carbon footprint compared to a repository built from scratch. Again, as many uranium producing nations are not doing so well with meeting carbon reduction targets, cany ontribution to reducing global climate perturbations may be an argument that facilitates public acceptance.
It should be emphasised that there is no “best” disposal option that can be applied to all applications and, to move forward, the pros and cons of different variants should be assessed for specific programme boundary conditions (see Figure 3). In addition to the concepts outlined above, further variants are possible such as disposal in deep boreholes rather than a conventional repository. However, this approach is likely to be applicable only for special conditions – such as small waste inventories and favourable boundary conditions. The very wide spectrum of options available also provides considerable flexibility for optimisation in terms of the detailed design of the disposal facility and the way in which it will be implemented.
Figure 3: Strengths and weaknesses of different radwaste disposal options
Future perspective of waste
Fundamentally, the risks from a radwaste disposal facility are trivial compared to those from global warming – which should put discussions about nuclear expansion into context. Nevertheless, there is a moral obligation to ensure that all wastes are managed in as responsible a manner as possible. Consideration of waste management in a holistic manner highlights the huge benefits that are available if we can break from existing paradigms. This will, however, require changes in the institutional culture of implementing and regulating organisations. It also requires education of decision makers so that they can consider the associated technical and socio-political issues in an unbiased manner, free from past pre-conceptions.
This article first appeared in Nuclear Engineering International magazine.