Safety research with laser light

His working clothes are as yellow as a sunflower. When Dr. Thorsten Stumpf enters the controlled area of the Institute for Nuclear Waste Disposal at the Research Center in Karlsruhe, he pulls over his overalls and slips into his special shoes and then enters a world in which strict security measures apply. Stumpf is studying how radioactive waste products from nuclear power plants react with their environment in the long-term when they are finally disposed in salt deposits, or in granite and other geological formations deep beneath the earth surface. Of course, good estimates on this have long been available, but Stumpf wants to have a deeper understanding. Because the chemistry of radioactive metals from the group of actinides, which include plutonium, americium and also curium, has so far remained largely unexplored. This is why most transport calculations are based on so-called distribution coefficients that are determined experimentally from radionuclide concentrations in the solid and fluid phase. However, these values can only be extrapolated with a certain degree of uncertainty over longer time periods, such as millions of years. What happens at molecular level and how radioactive atoms react in detail with their environment in the final deposit site are questions that still remain to be explained. "Only when I understand what reactions take place there and I know the stability of the reaction products, only then will I be able to draw truly reliable conclusions. Because the laws of nature will also apply in 100,000 years' time," says Stumpf.

It's dark and cool in the lab. Laser systems are at work here with which the researchers are able to follow the reactions very closely. Because some actinides have a special characteristic: they are fluorescent. The laser makes the samples light up and the emitted light contains detailed information, both on the reaction progress over time and on the energy levels, and, in particular, on the structure of the compound that was created. "Fluorescent spectroscopy is a fantastic method for obtaining structural information and it even allows us to work with samples that contain just a few molecules of radioactive material," explains the radiochemist. With conventional spectroscopic methods, the concentrations would have to be much higher, namely by a factor of hundreds of millions. The lifetime of fluorescence tells the researchers how many water molecules surround an actinide atom. Only when the actinide has been fully incorporated into the host lattice of the surrounding material will the hydration shell have disappeared and the lifetime of the fluorescence have become very long. And not only the various compounds, but also the exact geometrical positions into which the radioactive atom was integrated can be deduced from the spectra. Stumpf and his team mainly work with curium, an actinide element whose special fluorescent characteristics allow the most detailed conclusions. For example, curium atoms can be used as atomic probes to study chemical reactions, even at ultra-small concentrations. The radiotoxicity of the nuclear wastes produced by nuclear power generation will be dominated over long time periods by plutonium and americium. However, americium and curium are chemically very similar: both elements are trivalent and even the ionic radius is identical. Plutonium, too, is also very probably "reduced" in the deep geological formations of the final deposit site, so that it then chemically resembles curium as well. Curium is also convenient to work with, because it hardly radiates on account of its half-life of 340,000 years. So what do the experiments show? Curium can indeed be firmly incorporated into minerals. Firmly embedded in the crystal structure there, the radioactive actinides can be kept far away from the ecosphere for very long periods of time.