Background and relevance to radioactive waste management\ud International consensus confirms that placing radioactive wastes and spent nuclear fuel deep\ud underground in a geological repository is the generally preferred option for their long-term\ud management and disposal. This strategy provides a number of advantages compared to leaving it\ud on or near the Earth’s surface. These advantages come about because, for a well chosen site, the\ud geosphere can provide:\ud • a physical barrier that can negate or buffer against the effects of surface dominated natural\ud disruptive processes such as deep weathering, glaciation, river and marine erosion or\ud flooding, asteroid/comet impact and earthquake shaking etc.\ud • long and slow groundwater return pathways from the facility to the biosphere along which\ud retardation, dilution and dispersion processes may operate to reduce radionuclide\ud concentration in the groundwater.\ud • a stable, and benign geochemical environment to maximise the longevity of the engineered\ud barriers such as the waste containers and backfill in the facility.\ud • a natural radiation shield around the wastes.\ud • a mechanically stable environment in which the facility can be constructed and will\ud afterwards be protected.\ud • an environment which reduces the likelihood of the repository being disturbed by inadvertent\ud human intrusion such as land use changes, construction projects, drilling, quarrying and\ud mining etc.\ud • protection against the effects of deliberate human activities such as vandalism, terrorism and\ud war etc.\ud However, safety considerations for storing and disposing of long-lived radioactive wastes must\ud take into account various scenarios that might affect the ability of the geosphere to provide the\ud functionality listed above. Therefore, in order to provide confidence in the ability of a repository\ud to perform within the deep geological setting at a particular site, a demonstration of geosphere\ud “stability” needs to be made. Stability is defined here to be the capacity of a geological and\ud hydrogeological system to minimise the impact of external influences on the repository\ud environment, or at least to account for them in a manner that would allow their impacts to be\ud evaluated and accounted for in any safety assessments.\ud A repository should be sited where the deep geosphere is a stable host in which the engineered\ud containment can continue to perform according to design and in which the surrounding\ud hydrogeological, geomechanical and geochemical environment will continue to operate as a\ud natural barrier to radionuclide movement towards the biosphere. However, over the long periods\ud of time during which long-lived radioactive wastes will pose a hazard, environmental change at\ud the surface has the potential to disrupt the stability of the geosphere and therefore the causes of\ud environmental change and their potential consequences need to be evaluated.\ud As noted above, environmental change can include processes such as deep weathering,\ud glaciation, river and marine erosion. It can also lead to changes in groundwater boundary\ud conditions through alternating recharge/discharge relationships. One of the key drivers for\ud environmental change is climate variability. The question then arises, how can geosphere stability be assessed with respect to changes in climate? Key issues raised in connection with\ud this are:\ud • What evidence is there that 'going underground' eliminates the extreme conditions that\ud storage on the surface would be subjected to in the long term?\ud • How can the additional stability and safety of the deep geosphere be demonstrated with\ud evidence from the natural system?\ud As a corollary to this, the capacity of repository sites deep underground in stable rock masses to\ud mitigate potential impacts of future climate change on groundwater conditions therefore needs to\ud be tested and demonstrated. To date, generic scenarios for groundwater evolution relating to\ud climate change are currently weakly constrained by data and process understanding. Hence, the\ud possibility of site-specific changes of groundwater conditions in the future can only be assessed\ud and demonstrated by studying groundwater evolution in the past. Stability of groundwater\ud conditions in the past is an indication of future stability, though both the climatic and geological\ud contexts must be taken into account in making such an assertion
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