The fascinating bottom of the periodic table

This year is the International Year of the Periodic Table. Remember the bottom of the table from chemistry class? It might sound like fundamental science, but it is far from that. Blended in this group are radioactive elements with others used in industry. And the research opens doors to new environmental applications.

Lanthanides and Actinides are at the bottom of the Periodic Table.

The 30 elements that make up the so-called Lanthanides and Actinides have in common the fact that they are filled with 4f/5f valence shell, but they are poorly understood. Safety restrictions and the scarcity of instruments to study these elements are the main cause for this lack of knowledge. The ESRF is one of the very few places where experiments can be carried out. Kristina Kvashnina, from the Helmoltz-Zentrum Dresden-Rossendorf (HZDR) but based at the Rossendorf Beamline (BM20) of ESRF, has been awarded an ERC starting grant to pursue her studies in the domain of actinides and lanthanides. She is now leading the TOP (Towards the bottom of the periodic table) group.

kristina kvashnina on the rossendorf beamline.
"We have the Rossendorf beamline, as well as other beamlines at the ESRF and we have the know-how to study these elements in depth, to learn more about them. The benefits of our research, which is fundamental, can have direct impact both in the disposal of radioactive materials and in the development of new applications in chemical synthesis, electronics, nanotechnology, biology and medicine." KRISTINA KVASHNINA

From glowing stars to the Black Forest

Kvashnina's TOP group aims to study all the elements at the bottom of the table, but so far has been focused on uranium, thorium, plutonium, cerium, europium and praseodymium.

Europium is used to give fluorescence to materials, such as Euro notes, where invisible fluorescent ink is applied to fight against counterfeiting, or in glowing stars stickers for children. Cerium has been used as a sensor in catalytic converters in automotive applications, controlling the air-exhaust ratio to reduce NOx and carbon monoxide. Plutonium and uranium are used in nuclear power plants to generate energy, but the radioactive wastes of the plants is still a huge environmental problem. Uranium as a low radioactive natural element is widely present in rocks, for example in the Black Forest (Germany). Uranium is also the main component of highly radioactive spent nuclear fuel.

How do we deal with radioactive waste?

Nuclear plants use uranium and plutonium to produce electricity through nuclear fission reaction. After being used, the spent fuel needs to be reprocessed or disposed of in deep geological repositories in rocks. What happens to the radioactive elements in the environment?

"We need to know very well the physical and chemical properties of these elements to find the best way to get rid of them", explains Kvashnina. The best approach, according to Kvashnina, would be to recycle the fissile uranium and plutonium and re-use it in the reactor, but so far, this is technologically difficult. In many countries, like Sweden or Finland, radioactive wastes get stored in containers deep underground. How can we be sure that the wastes will be stable, won't react with the material of the container and won't dissolve ions in groundwater?

A nuclear power station.

The challenge of the elements' behaviour

Knowing more about the elements and how the atoms interact would be a solution to repository for radioactive wastes. "We are trying to understand what happens with these elements at the nanoscale. For example, thanks to our research we have found why cerium is much more effective as a catalyst at the nanoscale than as large particle", says Kvashnina.

For plutonium and uranium, the main elements used in nuclear fuels, the story is more complicated. The TOP team has found that plutonium can form soluble and very low soluble compounds depending on what substances it is in contacted with. If plutonium is stored in a container but there is water nearby, it will react and create colloidal nanoparticles. "It is extremely dangerous as they may migrate far away", states Kvashnina. Uranium will also react differently depending on the conditions around it.

"All in all, we in our research we have found out that the bottom of the periodic table is like a zoo: you need to study the elements one by one, which we didn't think we'd had to when we started".

The role of the ESRF

The ESRF is one of the few places where experiments on actinides and lanthanides can take place. Neutron sources can do the job but they need sizeable samples, which are very difficult to get due to their dangerous nature. As for synchrotrons, the ESRF is still their best bet for research. "The knowledge and experience of the safety group, coupled with the very coherent beam and the smaller beamsize, make this an ideal place", explains Kvashnina. "The new machine EBS will give us a much better resolution than before, so we are very excited for the future".

The Rossendorf Beamline has been designed with special traits, such as extra thick shielding, to allow the study of radioactive materials. X-ray spectroscopy is the only technique that can show how electrons behave in these systems. the techniques of XRD, SAXS/WAXS and PDF, also allow the study of these systems at beamlines at ESRF, such as ID26, ID15, ID20, BM14, BM26, all of them used by the top group.

Looking into the future

How far away we are from cleaning radioactive waste or creating optimal repositories? The answer is not straight forward: "I don't think we have a realistic way of disposing of radioactive wastes in a 100% safe way yet. Chemists carry out trial and error tests in the lab, but they need to work with physicists to fundamentally understand the elements."

"Radioactive waste is a reality, so we need to find a way of keeping it safe. For the future generations."
The TOP team is made up of chemists and physicists on purpose, so that they combine the knowledge to get a full picture of the behaviour of the elements. From left to right: Stephen Bauters, Kristina Kvashnina, Ivan Pidchenko, Lucia Amidani, Jurij GalaNzew and evGENY GERBER.


We thank Helmoltz-Zentrum Dresden-Rossendorf for their video footage.

  • PHOTOS: Moulyneux, Greg Webb / IAEA, Pierre Jayet, Etienne Bouy, the TOP team.
  • TEXT: Montserrat Capellas Espuny.

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