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- How to get out of a volcano: data on Chernobyl's "lava" will help eliminate traces of accidents at nuclear power plants
How to get out of a volcano: data on Chernobyl's "lava" will help eliminate traces of accidents at nuclear power plants
For the first time, crystallographers analyzed a sample of the so—called Chernobyl lava, a substance formed in the reactor room of the destroyed fourth power unit of the NPP, and revealed its instability to external conditions and high reactivity. The data obtained can be used to model and predict the behavior of materials that occur after man-made accidents. This is important both for analyzing changes in the Chernobyl ecosystem and for planning the construction of nuclear power plants, experts told Izvestia. With the help of such data, it is also possible to develop more effective methods for the conservation of destroyed reactors and to create sustainable materials for the disposal of radioactive waste.
What is Chernobyl lava?
The research team, which included representatives of St. Petersburg University, the A.E. Fersman Mineralogical Museum of the Russian Academy of Sciences, the V.G. Khlopin Radium Institute and the A.F. Ioffe Institute of Physics and Technology, analyzed the phase composition of a microarray of the so-called Chernobyl lava melt obtained from reactor room No. 305/2 of the Chernobyl NPP. It is believed that this zone was the main source of highly radioactive melt after the accident at the station, which occurred on April 26, 1986.
Such "lava" consists of the products of the interaction of nuclear fuel (uranium oxide) with zirconium shells of fuel elements and silicate materials (serpentinite and concrete). Through the steam pipes, the melt flowed into the lower sub-reactor rooms, where it solidified, turning into a substance that looks like volcanic lava.
Such "lava" is usually divided into two main types — black and brown, the scientists explained. They differ in chemical composition, including the concentration of uranium, as well as the number of inclusions contained in them. It is still unclear whether there was a single molten hearth, which subsequently split into layers, or whether two independent hearths of different colors formed under the reactor, experts said.
St. Petersburg State University crystallographers together with colleagues also studied the yellow plaque formed on the sample during its storage at the Radium Institute in the period from 1990 to 2011. According to scientists, dense and durable-looking formations began to collapse even in laboratory conditions, which indicates their chemical instability under the influence of the environment.
— The formation of secondary minerals in the form of plaque is associated with the transition of uranium from an oxidation state of 4+ to 6+. In natural conditions, this occurs under the influence of groundwater or atmospheric precipitation, after which uranium compounds acquire the ability to migrate, which creates an environmental threat. In our case, the presence of groundwater was not even required — only atmospheric moisture was enough. This demonstrates how susceptible Chernobyl lavas are to external influences and how strict control over such objects is necessary to ensure environmental safety," Vladislav Gurzhiy, one of the authors of the study, professor of the Department of Crystallography at St. Petersburg State University, explained to Izvestia.
Vladislav Gurzhiy, Professor of the Department of Crystallography at St. Petersburg State University
The frozen masses are covered by two protective structures erected above the reactor. The main ones are the "Shelter" (the first sarcophagus built after the disaster) and the New Secure confinement (NBK, "Arch"), a massive arched structure that covered the original shelter. These facilities were created to isolate the destroyed power unit, reduce radiation emissions and protect personnel from ionizing radiation. There are also other facilities for radioactive waste management and spent nuclear fuel storage on the territory of the station today.
Is the danger of radionuclides in Chernobyl decreasing
As explained to Izvestia at St. Petersburg State University, after the accident at the Chernobyl nuclear power plant, samples of silicate highly radioactive melts were selected for their subsequent study by liquidating experts from the Kurchatov and Radium Institutes.
Over time, the danger of some radionuclides decreases due to the natural decay of the elements, but some long-lived isotopes persist, it is their release into the environment that can lead to negative environmental consequences, scientists told Izvestia.
— In order to draw conclusions about the properties and useful use of the material, it is desirable that its composition remains unchanged. However, during the melting of nuclear fuel inside the Chernobyl reactor, many components were mixed: fuel assemblies, guide channels, graphite, concrete structures, electronics and wires. As a result, the chemical diversity inside the lava can include thousands of compounds. And, despite the formation of a more or less homogeneous mixture, each sample probably includes variations," said Vladimir Mikhalchik, Associate Professor of the Department of Physics and Problems of Materials Science at the National Research Nuclear University MEPhI.
Therefore, he noted, in order to come to accurate conclusions, it is necessary to continue building up the statistical base and deepen knowledge about this phenomenon. This will contribute to the creation of effective solutions to eliminate the consequences of such accidents.
Where the research results are applicable
The results of this study revealed for the first time the composition of the "proto-lava" from the epicenter of the Chernobyl accident, Oleg Shichalin, a researcher at the Laboratory of Nuclear Technologies at the FEFU Institute of High-Tech Technologies and Advanced Materials, told Izvestia.
— The main conclusion is that the material is chemically unstable. Over the years of storage in the laboratory, a new mineral, vorlanite, has formed on its surface. This proves that lava has a low resistance to air and moisture. In practice, this means a serious long-term risk. Upon contact with water, uranium and other radionuclides can easily be leached out of lava and migrate into the environment, posing a threat of secondary pollution," the expert noted.
These data are critically important for modeling the consequences of severe accidents at nuclear power plants, for example, at Fukushima. By studying the behavior of real fuel melts, it is possible to more accurately predict their evolution for hundreds of years ahead, develop improved methods for the conservation of destroyed reactors, and create more resistant materials for the disposal of radioactive waste.
"The Chernobyl lava is a unique natural experiment, whose lessons help improve nuclear safety around the world," Oleg Shichalin emphasized.
According to the St. Petersburg State University researchers, the results obtained can be used to model and predict the behavior of highly active materials formed after man-made accidents. This is important both for analyzing future changes in the Chernobyl ecosystem and for adjusting plans for the construction of modern nuclear power plants.
The study was conducted using the infrastructure of the resource center "X-ray Diffraction Research Methods" of the St. Petersburg State University Science Park.
The results of the research, supported by a grant from the Russian Science Foundation, are published in the Journal of Nuclear Materials.
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