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WWW.HZDR.DE discovered 02.15 TITLE They are especially interested in the interactions between minerals and radionuclides. Minerals can form a natural barrier preventing them from spreading. What happens on the molecular and atomic level when a radionuclide solution comes in contact with a mineral surface is very complicated, however, and until now the details of what happens have remained largely unknown. Radionuclides generally tend to attach themselves to the mineral surface. Here they could be more or less safely bound as "naked" ions directly attached to the surface and/or covered by a layer of water molecules. Radionuclides may also be incorporated into the crystal lattice of the host mineral. Many actinides also form a larger compound, a so-called colloid, that is attached to the surface, but also displays a very different behavior. Mineral and actinide in a tube Moritz Schmidt and his colleagues are taking a very close look at these processes in the lab. Under what conditions are bonds formed, what is the coordination sphere for the radionuclide, which concentrations are reached? As they explore this, HZDR researchers take a look at different host minerals such as calcite, a calcium carbonate mineral. Since, as Moritz Schmidt explains it: "Calcite is quite common. It is also a product of decomposing concrete, which is used to build repositories." Another geological host formation chemists are researching is muscovite, a mineral containing silicate that is also very commonly found and is especially prevalent in granite. In order to take a closer look at the interactions between a mineral sample and the radionuclides, researchers have to recreate the conditions present underground in the lab. Schmidt: "We seal the mineral and our actinide in a little plastic tube. It is then shaken constantly and samples are taken regularly." Cutting-edge infrastructure is at their disposal for the analysis of the samples. One of these devices is so- called time-resolved laser fluorescence spectroscopy, which experts refer to as TRLFS for short. Samples are excited with laser light possessing a frequency that is changed at very small intervals. These samples will then begin to emit characteristic fluorescence light. The measured curves recorded from the ultra cold samples provide information about aspects such as how an ion is coordinated, how many water molecules form its shell, how much attaches to the mineral surface, and how strong its bond with the mineral is. BEAM TIME: HZDR researchers are currently using the "Advanced Photon Source" at the Argonne National Laboratory in Chicago, but are also constructing their own measuring station in Grenoble.