Migration behaviors of U, Th, and Eu through a fractured granite under aerobic and anaerobic conditions

Full text of publication follows: Migration experiments have been carried out for some nuclides in a fractured granite block of 1 m scale in order to understand their retarding properties according to geochemical conditions. The nuclides used in these tests were Cl-, THO, Sr, Cs, U, Th, and Eu. At first stage, the experiment was performed with a synthetic groundwater (SGW) at neutral pH under aerobic environment. And the later stage under anaerobic environment, it was explored by varying the values of pH and Eh of the SGW with reducing agents and by purging inert gas in a closed system. The rock block has a single natural fracture having some secondary minerals at the fracture surface. The aperture distribution in the fracture was characterized by hydraulic tests with a variable aperture channels. Transport processes of nuclides were simulated with a particle tracking method. By comparing the simulated results to the experimental curves, it was checked not only the effects of matrix diffusion and geochemical conditions, but also the validity of static sorption data from the viewpoint of retardation. A large portion of uranium was sorbed little on the fracture surface and transported with the non-sorbing tracer under aerobic condition. Cesium and europium were sorbed strongly on the rock surface and did not eluted. A very small portion of thorium moved faster than expected from the batch data, and it seemed that some portion of thorium moved in the form of chemical complex, colloids, without sorption. Generally the retardation factors obtained from the migration tests showed lower values than those obtained from the static sorption data due to sorption kinetics and contact time. (authors)