| Popis: |
The safety case for a proposed geological disposal facility (GDF) for radioactive wastes relies upon a series of engineered and natural barrier systems to limit the migration of harmful radionuclides into the geosphere over geological timescales. Natural minerals, dominantly phyllosilicates, are expected to be the most reactive components of both the host rock and the clay-based backfill surrounding the highly radioactive waste canisters for as long as 100,000 years. Upon eventual canister degradation, α-emitting radionuclides will leach into the backfill material (and eventually beyond) and the constituent mineral systems will accumulate radiation damage upon radionuclide uptake and/or surface precipitation. The following study is an assessment of the structural and chemical effects caused by α-particle bombardment of silicate minerals, as proxies for the radiation stability of natural materials present in the near and far field of a GDF.Microscopy and spectroscopy studies from naturally occurring radiation damage accumulated in silicates over geological timescales (forming distinct ‘radiohaloes’) have shown that both α-particle and α-recoil bombardment results in altered unit cell dimensions caused by the accumulation of point (Frenkel) defects. In the example of highly damaged biotite, structural breakdown through the reorientation of discrete lattice crystallites was observed; the variability of the interlayer spacing within these regions reveal the potential for damaged mica to adopt the structure of phyllosilicate breakdown products over geological time.Controlled α-particle irradiation using the Dalton Cumbrian Facility’s 5 MV tandem pelletron ion accelerator, combined with microfocus spectroscopy analysis has revealed the mechanisms of high fluence α-radiation damage across 2:1 phyllosilicate minerals (biotite and chlorite); reducing the layered structures into a series of loosely connected domains of alternating lattice expansion and collapse. Radiation induced Fe redox changes have been revealed, with Fe reduction apparent at relatively low α-particle doses, giving way to Fe oxidation at high doses. A ‘redox gradient’, based on α-particle energy deposition through a silicate structure has therefore been proposed. In addition, the increase in ‘edge’ sites generated by structural deformation has been shown to be favourable for the adsorption of the Se(IV) oxyanion to the mica surface.Comprising a body of additional work, a core sample has been extracted from a spent nuclear fuel pond wall at the decommissioned Hunterston A nuclear power station and the radioactive contamination on the painted core surface has been analysed by microfocus spectroscopy. The contaminant radiostrontium has been shown to be associated with the Ti rich pigment in the surface paint, resulting in a ‘patchy’ accumulation of radioactivity at the core surface. In addition, inert Cs reactivity experiments using the underlying concrete have shown that Cs is preferentially uptaken by phyllosilicates within the altered mafic clasts used in the concrete aggregate. |
