Modelling permafrost thickness in Great Britain over glacial cycles.

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.

Potential migration of buoyant LNAPL from intermediate level waste (ILW) emplaced in a geological disposal facility (GDF) for U.K. radioactive waste.

A safety case for the disposal of Intermediate Level (radioactive) Waste (ILW) in a deep geological disposal facility (GDF) requires consideration of the potential for waste-derived light non-aqueous phase liquid (LNAPL) to migrate under positive buoyancy from disposed waste packages. Were entrainment of waste-derived radionuclides in LNAPL to occur, such migration could result in a shorter overall travel time to environmental or human receptors than radionuclide migration solely associated with the movement of groundwater. This paper provides a contribution to the assessment of this issue through multiphase-flow numerical modelling underpinned by a review of the UK's ILW inventory and literature to define the nature of the associated ILW LNAPL source term. Examination has been at the waste package-local GDF environment scale to determine whether proposed disposal of ILW would lead to significant likelihood of LNAPL migration, both from waste packages and from a GDF vault into the local host rock. Our review and numerical modelling support the proposition that the release of a discrete free phase LNAPL from ILW would not present a significant challenge to the safety case even with conservative approximations. 'As-disposed' LNAPL emplaced with the waste is not expected to pose a significant issue. 'Secondary LNAPL' generated in situ within the disposed ILW, arising from the decomposition of plastics, in particular PVC (polyvinyl chloride), could form the predominant LNAPL source term. Released high molecular weight phthalate plasticizers are judged to be the primary LNAPL potentially generated. These are expected to have low buoyancy-based mobility due to their very low density contrast with water and high viscosity. Due to the inherent uncertainties, significant conservatisms were adopted within the numerical modelling approach, including: the simulation of a deliberately high organic material--PVC content wastestream (2D03) within an annular grouted waste package vulnerable to LNAPL release; upper bound inventory estimates of LNAPLs; incorporating the lack of any hydraulic resistance of the package vent; the lack of any degradation of dissolved LNAPL; and, significantly, the small threshold displacement pressure assumed at which LNAPL is able to enter initially water-saturated pores. Initial scoping calculations on the latter suggested that the rate at which LNAPL is able to migrate from a waste package is likely to be very small and insignificant for likely representative displacement pressure data: this represents a key result. Adopting a conservative displacement pressure, however, allowed the effect of other features and processes in the system to be assessed. High LNAPL viscosity together with low density contrast with water reduces LNAPL migration potential. Migration to the host rock is less likely if waste package vent fluxes are small, solubility limits are high and path lengths through the backfill are short. The capacity of the system to dissolve all of the free LNAPL will, however, depend on groundwater availability. Even with the conservatisms invoked, the overall conclusion of model simulations of intact and compromised (cracked or corroded) waste packages, for a range of realistic ILW LNAPL scenarios, is that it is unlikely that significant LNAPL would be able to migrate from the waste packages and even more unlikely it would be sufficiently persistent to reach the host rock immediately beyond the GDF.

Illustrative assessment of human health issues arising from the potential release of chemotoxic substances from a generic geological disposal facility for radioactive waste.

Many countries have a programme for developing an underground geological disposal facility for radioactive waste. A case study is provided herein on the illustrative assessment of human health issues arising from the potential release of chemotoxic and radioactive substances from a generic geological disposal facility (GDF) for radioactive waste. The illustrative assessment uses a source-pathway-receptor methodology and considers a number of human exposure pathways. Estimated exposures are compared with authoritative toxicological assessment criteria. The possibility of additive and synergistic effects resulting from exposures to mixtures of chemical contaminants or a combination of radiotoxic and chemotoxic substances is considered. The case study provides an illustration of how to assess human health issues arising from chemotoxic species released from a GDF for radioactive waste and highlights potential difficulties associated with a lack of data being available with which to assess synergistic effects. It also highlights how such difficulties can be addressed.