The need to understand the processes operating within a natural or man-made geological system have ensured that the natural decay series have a broad range of applications. Geochemical differences between members of the naturally occurring Th-232, U-238 and U-235 decay series can lead to fractionation of the different elements in natural systems. The U-series radionuclides display greater geochemical diversity than those in the Th-232 series and the higher concentrations of U-238 in nature relative to U-235 mean that the U-238 series is of greatest interest. Within the U-238 series, Ra-226 (1600 year), Th-230 (7.5 x 104 year), U-234 (2.5 x 105 year) and U-238 (4.5x 109 year) all have intermediate or long half-lives, which render them suitable for use as geochemical tracers. This can be of enormous value in researching the behaviour of radionuclides in radioactive waste repositories. This book will be of interest of wide range of scientists including chemists, environmentalists, geochemists and engineers.