Diffusion of neptunium, plutonium and americium in granitic rock

The migration of the actinides neptunium, plutonium and americium in crystalline rock has been studied. Rock samples were taken from drilling cores in granitic and granodioritic bedrock, and small (2x2x2 cm) rock tablets from the drilling cores were exposed to groundwater solutions, containing one of the studied elements, for up to three years. The concentration of the element versus depth in the rock tablet was measured radiometrically. The sorption on the rock surfaces and the migration into the rock was studied by an autoradiographic technique. The migration of neptunium, plutonium and americium is slow (in the mm-range after 2-3 years contact time). The apparent diffusivities are in the range (0.4-4)x10-15 m2/s. The migration of the actinides into the rock is largely confined to microfissures in the rock matrix. (orig.)