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discovered_01_2016

WWW.HZDR.DE 18 19 TITLE // THE HZDR RESEARCH MAGAZINE This knowledge is taken into consideration when designing possible repositories. HZDR researchers head the field. Gerhard Geipel and his Biogeochemistry Division study various types – species – of elements, taking account of the type of bonding in which the element occurs and the different stages of oxidization. "Standard methods of analysis usually only verify the overall values of elements," says Geipel. "But in the case of heavy metals like mercury or radioactive elements it is necessary to differentiate the various species – because they have very different properties and behave differently, be it in the metabolism of people and animals or in other biological and geological systems. Thus, the various species also exhibit quite different toxicities." Tracing the path of radionuclides Geipel and his colleagues want to use species analysis to help predict the behavior of radionuclides in the biosphere. To this end, the scientists are investigating how uranium and other radionuclides behave in a variety of near-nature model systems and thereby delivering important data for repository research. This is highly relevant because Germany is currently searching for a location for a repository to store radioactive waste from nuclear power plants. Here the disused fuel rods, which are essentially composed of uranium, are supposed to be isolated from the biosphere for a very long period of time. Clay, salt stocks or crystalline rocks are potential geological formations suitable for the purpose as they are considered suitable barriers. But how does the waste react when water penetrates into the repository, the containers corrode and radionuclides are released? The conditions in the rocks differ enormously, starting with possible ligands with which the nuclide can form a chemical bond, via the pH values through to pressure, temperature and, finally, bacteria and fungi that may be present. This complex chemical-physical-biological environment determines which species occur. And this, in turn, dictates whether the radionuclides are easy to transport or tend to be immobile. The data being collected is not only important for assessing potential depository sites but also for research on former uranium mining areas, such as examining pithead stocks in Saxony to discover how their radioactive components proliferate. The measurements allow scientists to draw up risk assessments for the systems studied. Finally, the investigations provide a pool of data which can be used to better manage nuclear accidents and decontamination work of the type being done in Hanford. How plants absorb uranium For more than 20 years, researchers at HZDR have been developing and refining the technology required for conducting species analysis on uranium and other actinides. The most important procedure is laser-induced fluorescence spectroscopy. The institute has a whole arsenal of laser systems for determining the bonds actinides make. Geipel, a chemist, explains the principle of fluorescence spectroscopy: "The energy in the laser light excites the atoms in the sample, causing the electrons in the atomic shell to be raised to a more energy-rich level. When the electrons drop back to their original level, they themselves emit light." This fluorescence is analyzed with a spectrograph and a special, connected camera system. On the basis of the wavelength and the duration of the light emitted, the researchers can determine which species is present and in what concentration. "The procedure is very good, for example, for determining the various species of tetravalent and hexavalent uranium," says Geipel. Every ion responds to a different excitation wavelength to which the laser can be precisely tuned. "By measuring the fluorescence we can determine in detail how the uranium is bonded." As well as earth and water samples, organic material is also analyzed in Dresden. The researchers came across a plant, for example, that thrives on the pithead stocks in the Johanngeorgenstadt area and contains a high level of uranium. By exposing the plant to fluorescence spectroscopy Geipel and his colleagues discovered how the plant absorbs radionuclides and stores them in its cells. In the bio- and geospheres, radionuclides often combine with inorganic substance groups like carbonate, phosphate and arsenate, although organic substances such as ligands in the form of components of humus, solvent residues or medical waste are also viable. In order to study these metal-organic complexes the scientists employ so-called femtosecond fluorescence spectroscopy which utilizes an ultrashort laser pulse to excite fluorescence in organic ligands. The radionuclide extinguishes the emission. "By observing the decrease in fluorescence we can calculate the concentration of nuclide and determine the stability constants of the metal- organic complex," Gerhard Geipel explains. To do so, a special camera is used. "The fluorescence disappears very quickly, after as little as ten nanoseconds," the chemist continues. "Therefore we use a camera with an extremely short exposure time. It originated in automotive research where it is used to capture the combustion process in gasoline engines."

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