discovered 02_2012

FOCUS// The HZDR Research Magazine WWW.Hzdr.DE 38 39 in the long lasting and cumbersome process of preparing the samples. The chemists all knew each other through the ACTINET network, which is funded by the European Community to promote nuclear safety research. “By collaborating closely within the network, we were able to choose the best instruments and laboratories for our work,“ Andreas Scheinost explains. Nevertheless, it was a huge challenge for Regina Kirsch, the PhD student working on the project, to organize and conduct the research. She has recently completed her PhD at HZDR and Grenoble University in France, and will commence working as a postdoc at the Lawrence Berkeley National Lab in California, USA, in September. High safety requirements Some four years ago, Regina Kirsch started to synthesize the rust minerals at Grenoble University. The advantage of synthetic minerals is that their properties can be controlled very tightly. With a size of only a few nanometers, these particles closely resemble those actually found in nature. In order to be able to investigate the reaction between the rust minerals and plutonium, the scientists needed access to a highly specialized lab that protects them from the radioactive and highly toxic samples. This is commonly done by using gloveboxes running with underpressure, so that particles or aerosols eventually released from the samples would always remain inside the box. A number of such gloveboxes are available at the HZDR Institute of Resource Ecology. The problem with these labs, however, is that tiny amounts of oxygen are taken in from outside, “poisoning” the glovebox atmosphere. This is why normal anoxic gloveboxes are running with a slight overpressure inside. To fulfill both requirements, the underpressure needed for radioprotection and the extremely low oxygen level, the researchers were looking for a suited glovebox in all relevant facilities in Europe, and finally found a suited one at the Karlsruhe Institute of Technology (KIT). “We were searching for this glovebox for almost a year, and where then very happy to have an oxygen level of only 1 ppm, i.e. one oxygen molecule in one million air molecules”, says Scheinost. “Our colleagues at KIT are also ideal partners as far as the very complicated plutonium chemistry goes. They prepared the plutonium stock solutions with just the right properties to conduct the subsequent experiments with the rust minerals.“ The solid components were then separated by centrifugation and, afterwards, meticulously packed into special sample holders with double confinement - another safety requirement for plutonium experiments. What is more, the materials used in the sample holders have to withstand low temperatures during the experiments, down to minus 263 degrees Celsius, and subsequent heating-up to room temperature. The design of the sample holders combining these features - a high level of security and resilience in the face of temperature changes - was ultimately provided by CEA, the French Atomic Energy Commission. Andreas Scheinost very much appreciates the expertise and long-time experience of CEA in this field. To transport the samples from KIT to the HZDR beamline in Grenoble, France, support came from colleagues from the Swiss Paul Scherrer Institute, where scientists had recently built a specialized container for transporting highly radioactive substances using liquid nitrogen as coolant. This creates an oxygen-free atmosphere, and also slows down chemical reactions, preserving sample conditions for the beamline experiments. When the samples finally arrived at the ROBL beamline facility, they were subjected to another safety procedure: the ESRF radioprotection group carefully checked them for surface contamination. In addition, the group was also responsible for ensuring that all samples leaving KIT actually arrived at ESRF, and that they go back to Germany after the experiment, which is required by international and national laws to prevent nuclear proliferation. Only after all of this was completed, were the samples cleared for the measurements at the beamline. They then revealed - under the intense light from the synchrotron beam source - how and where exactly the individual plutonium molecules were “sitting“ on the surface of the iron minerals, the extent to which they approached and interacted with each other, and whether they formed chemical bonds - and, if so, what type of bonds these were. With these experiments, the researchers have determined that plutonium either accumulates at the rust minerals surface or precipitates in the form of a rather poorly soluble mineral compound. Regardless of its fate, however, what this means is that the radionuclides are firmly bound and that rusting waste containers are able to maintain their retaining function over thousands of years. Scientists assume that, with time, the nuclides may become incorporated into the rust minerals, producing even more highly stable compounds. They may also take up plutonium isotopes that have already escaped through the rust - which, again, would have a very positive effect on security at permanent nuclear waste repositories. All of these various processes will be further investigated as part of the new ACTINET follow-up project IMMORAD, with funding made available by the German Federal Ministry for Education and Research. “Our ultimate goal is to extend our work with plutonium to other elements and to be able to make still longer-term prognoses,“ says Andreas Scheinost. He is looking forward to continuing the fruitful ACTINET partnership once the preliminary work is now completed. LITERATURE R. Kirsch et al.: “Oxidation state and local structure of plutonium reacted with magnetite, mackinawite, and chukanovite,” in Environmental Science & Technology, vol. 45 (2011), p. 7267-7274 (DOI-Link: http://dx.doi.org/dx.doi. org/10.1021/es200645a) Contact _Institute of Resource Ecology at HZDR Dr. Andreas Scheinost scheinost@esrf.fr

discovered 02_2012

Pages