TALENT EDUCATION

The research results of Wang Yanwu’s team of Soochow University were published in Nature


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At present, the crisis of energy shortage and carbon emissions are causing countries around the world to re-examine the importance of nuclear power development. However, as a prerequisite for the sustainable development of nuclear energy, how to safely and efficiently dispose of the highly radioactive nuclear waste generated by the nuclear fuel cycle is still an unsolved worldwide problem.

Recently, the team of Professor Wang Yanwu of the State Key Laboratory of Radiation Medicine and Radiation Protection of Soochow University, together with researchers in the field of radiochemistry such as Tsinghua University, Colorado University of Mines in the United States, Jülich Research Center in Germany, and ShanghaiTech University, designed an inorganic absent polyacid cluster that can accurately match the coordination configuration of hexavalent americium from the coordination chemistry of hexavalent americium. The polyacid cluster formed a water-soluble nanoscale complex through strong complexation with hexavalent americium ions, thereby taking the lead in realizing the ultra-long-term stabilization of hexavalent americium in aqueous solution. Based on this, the research team developed a novel ultrafiltration separation method based on the size difference of lanthanum actinium species (Figure 1), which can potentially be applied to a series of important tasks such as spent fuel reprocessing, radioactive contamination control, radioisotope separation and purification, radiochemical diagnostic analysis in China. The research results were published in the journal Nature on April 20, 2023 under the title “Ultrafiltration separation of Am(VI)-polyoxometalate from lanthanides”, link: https://www.nature.com/articles/s41586-023-05840-z.

Figure 1 Schematic diagram of the separation of new lanthanum and actinium

Studies have shown that the most environmentally harmful to nuclear waste after uranium-plutonium separation is the secondary actinide americium, which has multiple long-half-life radioactive isotopes (such as americium-241 and americium-243). Americium is a by-product of nuclear power generation processes and a major source of long-term radiotoxicity of nuclear waste. The separation transmutation technology of americium is currently the most recognized disposal method and research hotspot, and its core idea is to efficiently separate americium and turn it into a low-toxicity, short-lived nuclide through neutron transmutation. If this technology can be realized, it will greatly reduce the negative effects of nuclear energy development on human society, so it is of great significance to the sustainable development of global nuclear power.

The traditional view is that the chemical properties of americium are very similar to trivalent lanthanides, and lanthanides as neutron poisons significantly affect the transmutation efficiency of americium. Therefore, the trivalent lanthanum actinide separation is not only one of the most challenging scientific problems in nuclear waste disposal at present, but also a major technical bottleneck to be overcome to solve the long-term radiotoxicity problem of nuclear waste. If the trivalent americium can be oxidized to hexavalent, and the separation can be achieved by using the difference in coordination configuration between hexavalent americium and trivalent lanthanide, it is expected to fundamentally solve the problem of lanthanum and actinium separation. However, hexavalent americium belongs to the unconventional valence state of americium, which can only exist for a few seconds in the traditional extraction and separation process, and is easily reduced to trivalent, which again causes separation difficulties. Previously, there was no internationally feasible strategy to stabilize hexavalent americium and thus achieve effective lanthanum actinide separation.

It is reported that the radiochemical joint research team first used spectroscopic technology to study the mechanism of action of hexavalent actinides (uranium, neptunium, plutonium, americium) and tcan lanthanide europium and the polyacid cluster (Figure 2). The results showed that in acidic solution, the polyacid cluster could undergo strong complexation with hexavalent americium due to the presence of precisely matched absent binding sites, and there was almost no interaction with temergent europium. In addition, the polyacid cluster is a protective group with “chemical inertness” and “steric hindrance effect” for hexavalent americium, which can stabilize hexavalent americium in acidic solution for more than 24 hours, thus providing a prerequisite for the subsequent separation process. In order to visualize the interaction between hexavalent actinides and polyacid clusters, the research team prepared a series of single crystal compounds of hexavalent actinide polyacid nanocomposites, and obtained the first stable single crystal structure of hexavalent americium compounds in the world. X-ray single crystal diffraction analysis showed that hexavalent americium was completely encapsulated in a pre-designed absence binding site, which further confirmed the accurate identification effect of absent polyacid clusters on hexavalent americium.

Fig.2 Chemical study of polyacids and actinide complexed solutions

Using high-resolution electron microscopy, the research team resolved the single-particle structure of the hexavalent actinide polyacid nanocomposite, which was highly consistent with its single-crystal structure (Figure 3). Finally, a novel lanthanide-actinium ultrafiltration separation method was developed by taking advantage of the size difference between americium polyacid nanocomplexes and hydrated lanthanide ions, combined with commercial ultrafiltration technology, and obtained a single-step separation factor of up to 780 binary lanthanum actinium and a single-step americium recovery rate of 91% (Figure 4). This is also the best separation effect between hexavalent americium and trivalent lanthanide that has been reported internationally so far. This method has a series of advantages such as high efficiency, safety, environmental friendliness, fast and low energy consumption, and has a good application prospect.

Fig. 3 Solid chemical study of actinide polyacids

Fig. 4 Comparison of lanthanum actinide separation process and results of new ultrafiltration technology

Associate researcher Zhang Hailong, State Key Laboratory of Radiation Medicine and Radiation Protection of Soochow University, master’s student Li Yu and postdoctoral fellow Li Kai are co-first authors of the paper, and Professor Wang Yanwu and Professor Wang Yaxing of Soochow University, Associate Professor Xu Chao of Tsinghua University and Professor Thomas E. Albrecht-Schönzart of Colorado University of Mines are co-corresponding authors. Academician Chai Zhifang of Soochow University gave important guidance on the theory of radiochemical separation, the team of Associate Professor Hu Yanshi of Tsinghua University provided theoretical calculation support for this work, the team of researcher Cao Kecheng of ShanghaiTech University carried out electron microscopy data acquisition and analysis, Evgeny V. Alekseev, researcher of the Ulich Research Center of the Helmholtz National Research Center Federation in Germany, and Dr. Zhang Zhenyi of Beijing Bruker Company made contributions to crystal structure analysis. The work was funded by the National Natural Science Foundation of China, the Department of Science and Technology of Jiangsu Province, and the State Key Laboratory of Radiation Medicine and Radiation Protection of Soochow University.

Figure 5 Radiochemistry research team of Soochow University
 
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