Abstract
Like other countries, the UK has opted for deep geological disposal for the long-term, safe management of higher-activity radioactive waste. However, a site and a geological environment have yet to be identified to host a geological disposal facility. In considering its long-term safety functionality, it is necessary to consider natural processes, such as permafrost development, that have the potential to alter the geological environment over the time-scale of glacial-interglacial cycles. We applied a numerical model to simulate the impact of long-term climatic variability on groundwater flow and permafrost dynamics in two contrasting geological settings in Great Britain: (i) higher strength rocks (HSR) overlain by higher permeability sandstones with a high topographic gradient (GS1); (ii) a mixed sedimentary sequence of high and low permeability rocks resting on igneous HSR with a very low topographic gradient (GS2). We evaluated the sensitivity of simulated permafrost thickness to a variety of climatic and subsurface conditions. Uncertainty in the scaling of the surface temperature time-series, 10–25 °C below present day temperature, has the largest impact on maximum permafrost thickness, PFmax, compared to other variables. However, considering plausible parameter ranges for UK settings, PFmax is up to twice as sensitive to changes in thermal conductivity and geothermal heat flux than to changes in porosity. Heat advection only affects modelled PFmax for high hydraulic conductivity rocks and if permafrost is considered to be relatively permeable. Whilst local differences in permafrost thickness of tens of meters, caused by variations in heat advection, are of minor importance over glacial-interglacial cycles, heat advection can be important in the development of taliks and the maintenance of a more active groundwater flow system. We conclude that it is likely to be important to simulate the effect of heat advection on coupled permafrost and groundwater flow systems in settings containing higher permeability lithological sequences.
Highlights
The disposal of radioactive waste represents a significant challenge for countries with developed or past nuclear industries
The variation of PFmax for different lithological units is more pronounced for geological setting 2 (GS2), since there is a larger contrast in thermal conductivity between the different units (Table 2)
Permafrost forms slowly and reaches a maximum depth towards the end of the glacial cycle, when permafrost thaw occurs over a relatively short time-scale of a few where Δy is the change in the parameter for which relative sensitivity (RS) is calculated, ymin and ymax define the full range over which y is varied, and ΔPFmax is the change in PFmax over Δy
Summary
The disposal of radioactive waste represents a significant challenge for countries with developed or past nuclear industries. Most countries, including the UK, have opted for deep geological disposal for the longterm, safe and secure management of higher activity radioactive waste (DECC, 2014). Deep geological disposal involves the emplacement of wastes in an engineered facility at depths of between 200 m and 1000 m below the land surface, making use of the surrounding geological environment as one of the many barriers to ensure that no harmful quantities of radioactivity ever reach the surface environment (NDA, 2010). The geological environment provides two functions: to contain the radionuclides and to isolate the disposed waste from the surface environment. The geological environment should be relatively stable to ensure the waste is isolated from the biosphere for the long-term and its behaviour adequately predictable to enable scientifically sound evaluations of the long-term radiological safety of a disposal facility (IAEA, 2011)
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