Nuclear waste management in a warming world

NUCLEAR HAS A WIDELY ACKNOWLEDGED role which should be expanded to help in phasing out fossil fuels during a time when power demand is rapidly expanding. But progress has to be made on social acceptance, both for rapid implementation of a new generation of reactors and for nuclear waste disposal — especially for longer-lived high-level waste.

In terms of climate change, a major concern is sea-level rise, an increased risk of storm surges and other flooding events. As much of our nuclear and other industrial infrastructure is sited at coastal locations, there is an urgent need to consider how to defend them against such climate effects. This is practical when the facilities are concentrated in relatively small areas.

With a little lateral thinking, waste management concepts can be developed in a way that help reduce concerns, particularly when surface facilities for deep geological disposal can be located beside existing nuclear plants. Even if not yet time-critical, it is worth initiating discussion of options now, given the long lead times for such projects.

Concept outline

Reactors and other nuclear facilities select coastal locations for their technical benefits, such as ease of access and availability of cooling water. But this puts them at risk of rising sea levels.

Ease of access and good hydrogeological conditions may also make suitable offshore host formations attractive for waste disposal. Although subsea disposal of radioactive waste is forbidden by international conventions, this applies only to deep-sea variants outside national waters and not to offshore geological repositories accessed from land. Such repositories for low and intermediate level waste already exist (for example, SFR in Sweden). Advanced plans for repositories for higher activity waste in Sweden and Finland have them located in coastal locations that could lie below the sea in the near future.

Despite superficial similarities, the option of a deep geological repository constructed offshore, in a conventional manner with onshore access, differs from oceanic sub-seabed disposal in the following ways:

• Waste disposal is in land that is within a country’s borders rather than lying under international waters;
• A system of multiple engineered barriers can be placed with rigorous quality control to assure containment at a similar level to a land-based repository;
• Waste can be retrieved with existing technology, should such a decision be made in the future. This cannot be claimed for deep ocean options.

Separation of surface waste management facilities and disposal sites has been considered elsewhere, especially when the former are on existing nuclear sites.

Figure 1 is a schematic illustration of three different repository options. Topography is generally the main driving force for freshwater flows under land, with water fluxes usually decreasing as a function of depth. Near the coastline, the higher density of seawater results in penetration of a saline wedge under land, the extent of which depends not only on the geological setting but also anthropogenic impacts such as water extraction. This simplistic representation of hydrogeology illustrates principles but cannot capture impacts of the geological settings at specific sites.

Inland and offshore disposal show a marked contrast in hydrogeological boundary conditions when compared to disposal below the coastal plain. The former tends to have higher hydraulic gradients, but longer transport distances to the biosphere and, potentially, higher dilution at the geosphere/biosphere interface if the migration plume is more dispersed. The latter has negligible hydraulic gradient and would ensure effectively no release of radionuclides into groundwater and, even if migration did occur (eg in the gas phase), there would be very high dilution of any outflow from the seabed. As hilly/mountainous terrain would represent the source for deep flow within inland catchment basins, the groundwater would be younger and less saline. Closer to the coast groundwater would be older and more saline with offshore groundwater being even older, having a salinity at least equal to that of seawater.

In general, repository construction and operation challenges would be similar for the three options. Subsea, all access would be via ramps. Access to a subsea repository would be via ramps as the option of shafts (eg for ventilation, human access) used in conventional designs would not be practical, unless there were conveniently located islands close to the coast.

Long after repository closure, knowledge of its location may well be lost, perhaps resulting in inadvertent human intrusion. Whilst this risk would be higher in a plain located repository compared to an inland hill location due to human activities, risk of human intrusion offshore would be extremely low. In the absence of intrusion, the engineered barriers and geological setting will ensure complete containment for very long periods and low levels of release thereafter. Issues to be carefully considered on a concept- and site-specific basis include perturbations, eg the formation of chemical plumes and / or mobile colloids in an advective flow system.

As radionuclide release and migration will be predominantly by diffusion offshore, such issues are of less concern. Instead the challenges are perturbations that could cause more rapid radionuclide transport – eg in a gas phase or due to thermally-driven convective water flow.

Finally, socio-political factors play a major role. The remoteness and isolation provided by the subsea variant should aid acceptance, but the safety case would have to convince key stakeholders, in particular local fishermen. Protection of the marine environment is a global concern and, even if strictly legal and with negligible health risk, any disposal option that could give rise to a release of radioactivity into the sea could cause opposition in neighbouring lands.

Long-term evolution of coastal environment

In the context of repositories for longer-lived wastes, where safety is assessed over hundreds of thousands of years, coastline changes as a consequence of climate change and glacial cycling must be taken into account.

Over the coming centuries, we expect further loss of ice sheets and hence an increase in sea level (see Figure 2). The impact of such changes of the performance of a coastal repository is very dependent on local topography and bathymetry, but the key issues are:

• Initial sea-level increase and gradual flooding of low-lying coastal plain areas. In the worst case - complete melting of the ice caps - this could cause a sea level rise of 80m.
• At some point in the future, it is assumed that the natural ice-age cycle will be re-established. The next ice age maximum would see a total decrease in sea level of ≈150m compared to the present.
• Thereafter, such cycles would repeat on a timescale of hundreds of thousands of years, with the same sea level rise and fall but the impact affected by local uplift or erosion.

A coastal deep geological repository could be implemented within the next two decades, with waste emplacement until at least the end of the century. Current models suggest sea level increase will not exceed 1-2m over this period. This should not cause significant operational problems, although stronger storm surges have to be considered in the design of surface facilities.

In the following two or three centuries, while the repository is operational or under institutional control, sea level may rise by 10m or more. It is currently impossible to preclude a ‘tipping point’ of rapid ice sheet melting. In any case, it is prudent to assume that warming will cause retreat from coastal areas and, potentially, global economic disruption. Local impacts will depend both on topography and engineered counter-measures. Even if sea level rises faster than expected, the impact on a deep repository will be limited by the slower response of deep waters to surface changes.

Thereafter, there is no scientific basis for making any kind of predictions. Human action can dominate natural climate cycles but its impact depends, for example, on global political decisions to limit emissions, geo- engineering to reduce impacts and the possibility of unknown tipping points or other black swan events.

In terms of a closed and sealed repository, the main concern would be whether the evolving salt wedge could cause significant changes to the groundwater chemistry or flow conditions. The direct impact on performance is likely to be minor as the engineered barriers should be assured over thousands of years.

If sea level rise can be limited to a few metres, the Earth’s natural cycle will tend to move towards another ice age. As above, this would also have huge impacts on civilisation (eg due to loss of land area to ice sheets, especially in the northern hemisphere). Even with active climate control and incentives this sea level rise may not be stopped, at least over the timescale of a repository. Assuming further ice ages over the next million years, the shoreline in the vicinity of a repository could retreat by tens of kilometers in some areas. Changing hydraulic gradients and hydrochemistry would eventually give rise to conditions similar to those of the wide coastal plains. Fresh water would displace marine water from shallower formations, however it is unclear if this would also take place in deeper formations. Flow paths from an inland repository might be increased, while at an offshore repository location, path lengths would decrease and fluxes of salt or freshwater around the repository would be higher.

For higher latitudes, the impact of glaciation or, at least, permafrost formation must be taken into account when assessing the hydrogeological and geochemical impact of lower sea levels.

Tailored repository concepts for an offshore setting

There are many different concepts for the geological disposal of radioactive waste that can provide sufficient performance for specific types of waste in particular geological settings. This example considers spent fuel from light-water reactors or vitrified high-level waste (HLW) from reprocessing of such fuel, but it should be applicable to other fuel and waste types resulting from future generations of fission (or even fusion) reactors.

Conventional concepts for higher activity waste generally involve single packages of waste within a metallic overpack. For typical waste inventories, small diameter emplacement tunnels from tens to hundreds of kilometres long are required. Higher density waste emplacement can be achieved by utilisation of large caverns and multi-purpose storage-transport-disposal casks (MPCs) containing about 20 waste packages. Alternatively, waste may be emplaced in channels in a massive steel monolith. Higher densities reduce the repository footprint and make emplacement logistics easier. Less broken-out rock will reduce operational hazards (mainly associated with excavation), environmental impact and cost, but heat management becomes more of a concern.

Given the clear benefits of higher density emplacement, it is worth considering how concerns resulting from the higher thermal loading could be addressed. This can involve either delayed backfilling or a mixture of active and passive cooling using heat pumps and heat pipes. The latter has the advantage of allowing earlier closure, if this is required in response to altered programme boundary conditions, and also allows radiogenic heat to be used as a resource as long as the facility is under active management. In a diffusion-dominated environment, engineered barriers can be made more cost-effective. For example, instead of a massive overpack and thick buffer, a much smaller pre-fabricated EBS module (‘mini PEM’) could be used — with any further protection provided by an enclosing steel monolith and backfill (ideally utilising material resulting from the nuclear power plant decommissioning — an example of holistic waste management).

To allow sufficient dispersal of the thermal transient after such management, disposal vaults could be well separated – for an off-shore site, this would be less of an issue as the total repository footprint is unlikely to be a concern. For a typical host rock in which structures such as major faults constrain areas with better properties, layout can be easily tailored to utilise these (see Figure 3 lower).

Moving towards holistic waste management

A geological repository provides indirect benefits in the form of excavated spoil, which could be used for coastal defence structures.

In conventional concepts, a significant proportion of such spoil is re-emplaced as backfill but, from a more holistic waste management viewpoint, it would be beneficial to backfill with lower toxicity radwaste or other chemotoxic waste. Such mixed waste disposal has been considered anathema in the past due to the technical and sociopolitical complexities, but it provides opportunities to cost-effectively introduce the option of deep emplacement of waste that traditionally has been handled by surface or near-surface disposal. Surface disposal sites in coastal settings are almost all at risk of being compromised by sea-level rise — potentially requiring additional engineering defences to be incorporated.

The discussion here focuses on nations with nuclear power and suitable coastlines, but the benefits accruing from such a project may make it very attractive as an international solution hosted by a country without nuclear power. For some low-lying islands, sea-level rise is an existential threat that would require huge financial resources to combat. Income and employment from an international repository could be attractive, with the flood defences constructed from spoil an additional benefit.

Author information: Ian McKinley, Executive consultant with McKinley Consulting; Susie Hardie, Senior project leader with CSD Engineering