Measurement of uranium series nuclides in rock bordering a fracture

Alpha spectrometry was used to measure the 234U/238U and 2'30Th/234U activity ratios in crushed, thinly-cut sections of rock running parallel to a major fracture, as well as in the fracture-filling material. In addition, gamma spectrometry was used to measure the level of 226Ra in each sample, enabling the 226Ra/230Th activity ratios to be estimated. The results show the highest concentration of uranium to be in the fracture, while the influence of groundwater on the surrounding rock could be seen by sharp variations in activity ratios within a few centimetres of the fracture. The use of a mild leaching reagent showed 234U to be located in more potentially mobile sites than 238U. The experiment provides a basis for further study of the role of fractures in crystalline rock in promoting the transport of uranium series nuclides. Fractures are considered to be a potential source of mobilisation of radionuclides from a deep radioactive waste repository, so data of the sort described here are considered useful for validation of radionuclide transport models