Radionuclide migration in plutonic rocks: implications for high-level nuclear waste disposal

Geochemical studies of deeply buried intrusive rocks provide the foundation for evaluating the suitability of crystalline rocks as repositories for solidified HighLevel Nuclear Waste (HLW). This geochemical study examines the migration of natural and introduced radionuclides during interaction between groundwater and plutonic rock, to provide an understanding of the processes which may operate in the potential repository environment. Rock cores from the Coles Bay Granite (Tasmania), Kambalda Granodiorite (Western Australia) and the Roxby Downs Granite (South Australia) were selected for this study on the basis of the variation in mineralogy, fracture density and degree of alteration. Firstly, the behaviour ofU- and Th-decay series radionuclides in each rock was investigated as a natural analogue for some HLW elements to identify the nuclide migration pathways and significant sites of rock/radionuclide interaction. Secondly, Synroc doped with actinides and fission products was used as a source of radionuclides to evaluate the interactions between waste elements and intrusive rocks in a simulated water-saturated repository environment. This integrated approach has identified the mechanisms controlling radionuclide release, migration and retention. In the natural analogue studies, the application of fission-track micromapping has determined that primary uranium is distributed in the three intrusives as background U in the major rock-forming minerals, and as resistate U in the primary accessory phases. Two modes of redistribution of this uranium are evident; as secondary U in the secondary minerals formed during alteration, and as fracture U associated with the fracture-infilling minerals. The mechanisms for uranium retention are dominated by adsorption and ion exchange. The study of uranium-series disequilibrium has determined that significant radionuclide mobilisation has occurred in the recent past ¬´1.2 Ma) as a result of groundwater interaction. Disequilibrium between 23OTh, 234U and 238U in the three intrusives indicates that fractures form the dominant nuclide migration pathways and the most significant sites of rock/radionuclide interaction are the secondary and fracture-infilling minerals. Leach testing of Synroc with the three intrusives was carried out to determine the mechanisms and processes which occur during Synroc/water/granitic host rock interaction. Significant geochemical and mineralogical changes were observed in all three intrusives during leach testing, including loss of crystal structure and formation of surface reaction products. These changes are reflected in a change in the leach solution conditions and may also affect the distribution of radionuclides during leach testing. The presence of the intrusives significantly inhibited the total release of the actinides (Np, Pu and Cm) and the less soluble fission products (Zr, Ce, Nb and Ru) from Synroc, as a result of the change in solution chemistry and nuclide solubility induced by the presence of the granites. Substantial preferential uptake of all radionuclides by specific secondary and fracture infilling minerals (such as sericite, hematite, Fe- and Ti-oxides/hydroxides) in intrusives was also observed, which was controlled by rapid ion exchange, redox reactions, sorption and surface deposition of colloids and pseudocolloids. These results imply that fractures will form the main pathways for radionuclide mobilisation in the HLW disposal environment, and that the most significant sites for rock/radionuclide interaction will be the secondary and fracture-infilling minerals. Sorptive processes will dominate radionuclide retention and therefore retard migration of these elements away from the waste package into the surrounding near-field geological environment. The geochemical evolution which occurs during rock/water interaction may affect radionuclide release, migration and retention through changes in solution characteristics and sorptive capacity. These qualitative experimental observations of the chemically complex interactions which occur in the predicted repository environment may be used for quantitative predictive modelling in repository assessment.