Forty years of rain, frost, earth and asphalt were not enough to unravel six tiny ones fragments of nuclear fuel missing during the Chernobyl disaster. By analyzing them with powerful X-rays, a group of German researchers discovered that their crystalline structure has remained largely intactpreserving radioactive substances within it that could otherwise be dispersed into the environment.
The discovery comes from a study published in Journal of Hazardous Materials and also illustrated by Leibniz University Hannoverwho conducted the research together with the Helmholtz-Zentrum Dresden-Rossendorf. It is an encouraging result for understanding how the residues of the 1986 nuclear accident behave. With one clarification, however: scientists examined only six particlestoo few to establish what is happening to the entire contaminated area.
Smaller than a hair, radioactive for forty years
When reactor number 4 at the Chernobyl plant exploded on April 26, 1986, countless fragments of nuclear fuel also ended up in the environment. Some were so small that they could be carried along with the dust.
They are called hot particleshot particles, although the name does not refer to their temperature. I am highly radioactive fragments measuring between 8 and 50 micrometersso generally less than the thickness of a human hair. And they continue to emit radiation four decades after the accident.
They aren’t all the same either. Some retain a similar composition to the original reactor fuel, others have fused with the zirconium that coated the nuclear rods. Still others suffered the effects of the graphite fire, which burned for ten days after the explosion, transforming some of the fuel into various uranium compounds.
These differences make it difficult to predict how long the particles will continue to release radioactive substances.
The researchers observed each particle from 2,000 angles
To study them, the researchers selected six fragments from two locations in the exclusion zone: Pripyat, the city evacuated after the disaster, and Kopachi, a village located a few kilometers from the plant.
Two particles came from asphalt samples collected in Pripyat, the other four from agricultural soil samples from Kopachi.
The fragments were isolated and transferred toEuropean Synchrotron Radiation Facility in Grenoble, France, where researchers used a particle accelerator capable of producing extremely intense X-rays.
Each tiny fragment was rotated within the beam and observed from approx 2,000 different angles. In this way, scientists were able to reconstruct how the atoms were arranged and verify which materials had resisted the passage of time. The result surprised them: some structures of uranium oxide, one of the main components of nuclear fuel, were still largely intact.
And there was another interesting detail. Two particles taken from the soil of the same locality presented very different conditions: one showed strong oxidation, while the other remained almost unchanged. The authors suggest that at least part of these differences may be traced back to the extreme conditions of the accident, rather than being due solely to exposure to weathering in subsequent decades.
In practice, some fragments may have retained characteristics acquired during the explosion and fire of the reactor for forty years.
Radioactive substances are still trapped in the fragments
The discovery is of particular interest to those who study soil and water contamination. When a radioactive material deteriorates, some of the substances it contains can be released and reach the surrounding environment. If, however, its structure remains stable, it can continue to retain them.
This is what the analyzes conducted on the six Chernobyl fragments suggest: some crystalline structures can still retain fission products and other radioactive elements. This could mean a slower release of certain substances into the soil and water.
Leibniz University Hannover considers the result useful for improving future environmental and health risk assessments, but the researchers themselves urge caution. Each particle has its own characteristics. Even by collecting a much larger number of samples and obtaining reliable average values, the possibility of finding particularly resistant fragments, capable of releasing radionuclides much later than the others, would remain. To understand the behavior of radioactive waste, further analyzes are therefore needed, conducted on different particles and coming from multiple locations.
Chernobyl remains dangerous, even if the particles are stable
The chemical stability observed in the study does not make the fragments harmless. The particles continue to be radioactive and some forms of uranium oxide can become mechanically unstable, breaking into even smaller, wind-borne powders. Inhalation of these materials represents a serious health risk. The Chernobyl exclusion zone therefore remains subject to restrictions and the discovery offers no reason to consider its reopening imminent.
Furthermore, the six fragments analyzed represent only a very small part of the materials dispersed during the disaster. Their conditions do not allow us to measure the overall radioactivity of the area, nor to establish how much contaminating material has already reached the soil or aquifers.
Researchers are already carrying out new experiments on other radioactive waste, in particular on materials containing transuranic elements, i.e. elements heavier than uranium. They will have to understand how long these structures can resist and what conditions favor their degradation. Meanwhile, forty years after the explosion, some fragments still retain the structure they had in 1986 almost intact. And they continue to be radioactive.
