1.1 The GDF Concept

Geological disposal involves the emplacement and isolation of HAWs in an underground repository (a GDF), housed deep inside a suitable rock formation (see Figure 2). For UK purposes, the definition of geological disposal is ‘‘burial underground (200-1000 m) of radioactive waste in a purpose built facility with no intention to retrieve’’.1,4,5 Geological disposal facilities utilise a multiple barrier concept, whereby several engineered barriers are intended to work together with the host geology to contain and retard the radionuclides that are present in radioactive wastes. The components of a multiple barrier GDF typically include (see Table 1):

from nuclear fuel processing is vitrified to make it into an insoluble waste form. In the case of ILWs, grout encapsulation buffers the pH of the waste to hyperalkaline conditions under which a range of radi­ologically significant radionuclides (including the actinides) are pre­dicted to be poorly soluble.

(ii) The waste container. The conditioned waste form is encapsulated in a container prior to disposal, creating a waste package. For example, grouted ILW is encapsulated in steel to provide mechanical stability. Furthermore, when the GDF evolves, steel corrosion creates reducing conditions that should retard the mobility of some radionuclides, par­ticularly the actinides.

(iii) Buffer materials. Buffer materials are directly emplaced around the waste package. The materials used are chosen to provide beneficial functions, for example, to control the chemical or flow conditions in the repository during GDF evolution.

(iv) Backfill. Backfill is used to pack the GDF excavation tunnels, shafts, and drifts. The materials used must have the mechanical strength to support the GDF structure and are chosen to complement waste package conditioning and to allow further conditioning of the GDF to retard radionuclide mobility by pH, redox and/or flow control.

(v) Sealing systems. Highly impermeable sealing materials are required to control GDF groundwater ingress during construction and after waste emplacement.

(vi) Geology. The GDF host geology provides the final barrier for the repository. The geological barrier is likely to provide a number of beneficial functions, for example, it could support low groundwater flow or contain minerals and surfaces that sorb radionuclides from solution.

Once a GDF is closed, natural hydrochemical and biogeochemical processes will act to degrade the engineered structure and the multiple engineered barriers are expected to contain the waste for several thousand years.1 After this engi­neered containment period, it is expected that groundwaters will have pene­trated the backfill and canisters, and ultimately will have entered the waste packages, dissolving some fraction of the radionuclides. The partially degraded barriers will consist of a range of evolved mineral phases including iron oxides and aged cement phases, and for a cementitious repository, the pH is predicted to evolve from hyperalkaline to a more mildly alkaline state. Overall, the evolved system is likely to limit the mobilisation of radionuclides from the wasteform for several thousand to several tens of thousands of years. None­theless, with geological time radionuclides will transport to the surrounding geological environment, which will have been affected by the alkaline fluids from the GDF. In such instances, the radionuclides are likely to dilute and disperse, and sorption reactions with the surrounding rock and associated minerals and surfaces are intended to limit radionuclide transport into the biosphere to “acceptable levels’’. Clearly, a key challenge in successful GDF implementation is the associated communication of these uncertainties to the relevant stakeholder and public audiences. It is clear that high quality, inde­pendent, peer reviewed science is essential to allow full scrutiny of the safety case for geological disposal. It is also clear that communicating these scientific findings, coupled to explanation of the proposed GDF concept as it develops, in a clear and transparent way is likely to be pivotal in developing the ‘‘trust’’ between all players that is needed for movement toward GDF construction.