COVER STORY | A HOLISTIC APPROACH
PURPOSE-BUIL
BUILT Conventional - e.g. KBS-3V
...applicable even in case of geological uncertainties Right: Alternative
approaches to nuclear waste management are possible
Far right, figure 2:
Evolving repository concepts
Improved- in-tunnel PEM / steel / high density ...takes benefit from geological understanding
Improved - vault / MPCs / higher density ...takes benefit from EBS knowledge base
Optimised - vault with heat mining
...Reduces EI / carbon footprint
Integrated - waste recycled into engineered barriers
...Builds sustainability arguments
Extended - access tunnels / shafts used for other waste
...makes use of all excavated space
EXISTING MINE
V 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
24 | July 2022 |
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copper canisters requires advanced technology such as friction-stir welding, which further increases costs and associated EIs.
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 copper 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 it 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
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