Radionuclide and rare earth element geochemistry of the Criffel pluton, southwest Scotland : an analogue study for nuclear waste disposal

This thesis describes a study utilising natural decay series radionuclides and rare earth elements in the granitic rocks of the Criffel pluton, southwest Scotland, in an investigation of processes affecting radionuclide transport and retardation in the environment. The work was performed in the context of a natural analogue study of relevance to radioactive waste disposal on the basis that many of the processes which govern the long term performance of a deep geological repository for radioactive waste also take place and can be characterised in nature. Such an analogue study can play an important role in validating the importance of far-field radionuclide retardation mechanisms in the case of radioactive waste disposal in crystalline bedrock where fracture flow will be the dominant mode of radionuclide transport upon failure of the near field barriers. Natural decay series analyses were performed on the whole rock samples obtained from different zones of the pluton, giving results consistent with the expected geochemical behaviour of these two elements in igneous rock. Thus, uranium was observed to be highly susceptible to oxidation-induced dissolution and is consequently relatively mobile, whereas thorium exhibited an extremely low solubility and is effectively immobile. The study shows that the Criffel pluton has experienced removal of between 20 and 30 percent of the uranium from the uppermost section of the granite, with a removal probability for uranium dissolution of 1.9 x 10-[5] y-1. There is no significant 'whole rock' re-deposition of uranium within the pluton. However, it is evident that matrix diffusion, sorption/scavenging by fracture lining minerals and redox front trapping processes are potentiallly very important for uranium retardation in the pluton. All samples from the pluton displayed 234U/238U ratios about unity, whereas 230Th/234U ratios were greater than unity. This suggests that the pluton has experienced a recent and rapid removal of uranium from the uppermost section of the granite (i.e. since the end of the last glaciation), whereas thorium has remained relatively immobile. The activity ratio data of a representative granite core in the pluton indicate that the average rate of downwards movement of the weathering front is about 12 m in 10[6] y. This observation suggests that the postulated maximum rate of far-field movement of a repository-related redox front of about 50 m in 10[6] y is a realistic value for use in models. Concentrations and activity ratios of natural decay series radionuclides were studied in two rock sections that traversed redox fronts and intersecting water-bearing fractures in Craignair quarry in the pluton. Investigation of natural redox fronts in the quarry revealed that both redox sensitive (U) and non-redox sensitive (Ra and REE) elements are subjected to dissolution and re-deposition processes in the vicinity of the front. Uranium concentrations are depleted in the oxidised rock, whereas slight enrichments are observed in the reduced rock close to the redox front. There is also some uranium re-deposition on the oxidised side of the front which would not be expected from a simple thermodynamic viewpoint. The 226Ra/230Th ratios displayed disequilibrium, reaching a value above unity on the oxidised side of the front but dropping to less than unity in the reduced rock. This situation indicates that radium is readily removed from the reduced rock but is re-deposited in the oxidised rock. Taken together, the radium, REE and uranium distributions indicate that both advection and diffusion processes control the distribution of radionuclides about the front. Fission track studies revealed that uranium is associated with fracture- infilling minerals, suggesting uptake by iron-manganese oxyhydroxides, clays and carbonate minerals during fracture flow. Also, uranium mobility apparent in rocks adjacent to fractures implied a depth of matrix diffusion of at least 7 cm. It was also observed that uranium concentrations in fracture-lining minerals increased with depth, suggesting that uptake of uranium by such minerals is not a permanent process.