Scientists identify three types of radioactive Chernobyl particles

Scientists have taken a closer look at these tiny fragments and made surprising discoveries. Some particles have preserved much of their original internal structure despite spending four decades exposed to the environment.

“There are three classes of these particles,” explains Tobias Weissenborn, a physicist and PhD candidate at Leibniz University Hannover. “First, there are particles that are chemically and physically very similar to the nuclear fuel uranium dioxide. Second, there are particles that are partially or completely encased in, or completely fused to, a layer of zirconium.”

The second category formed when temperatures inside the reactor reached extreme levels. As the nuclear fuel melted, it bonded with zirconium, a durable metal used in the fuel’s protective outer layer.

“The third type of particles was produced when the graphite moderator ignited,” Weissenborn added.

Under normal operating conditions, graphite moderators slow down fast neutrons and help sustain nuclear chain reactions. During the Chernobyl disaster, however, the graphite caught fire and continued burning for 10 days.

During the fire, the nuclear fuel oxidized into several uranium oxides, including U₃O₈.

Unlike more stable materials, this uranium oxide is mechanically fragile and can break down into microscopic pieces. Wind can carry these radioactive particles through the environment, creating serious health risks if they are inhaled.

Scientists examine Chernobyl’s radioactive dust in unprecedented detail

Despite decades of research, scientists still do not fully understand why radioactive particles from Chernobyl break down at different rates. As the materials weather, they can form several types of uranium oxide, making their long-term behavior difficult to predict.

Investigating these changes requires highly precise equipment. Researchers used synchrotron X-ray diffraction, a technique that reveals the arrangement of atoms within crystalline materials.

Dr. Christoph Hennig, a crystallographer at the Dresden-Rossendorf-Helmholtz-Zentrum (HZDR), was the first to successfully analyze the structure of this type of radioactive fragment at the Rossendorf beamline in Grenoble.

Hennig worked with Weissenborn to analyze six particles recovered from the Chernobyl area. By identifying the different crystalline phases within the particles, the researchers gained information that could help determine how quickly radioactive material leaks into the surrounding environment.

“My colleagues in Hannover and I used different methods to isolate particles from Ukrainian soil samples and deposit them on tungsten electrodes,” Weissenborn explains.

The samples were so radioactive that they had to be sealed in several layers of containment before being transported to Grenoble. Hennig’s team then used X-ray diffraction to examine the particles’ internal structures by measuring how X-rays scattered as they passed through the material.

“First of all, we had to find a way to study the structure of the particles,” explains Christoph Hennig. “In the end, we completely rotated the particles in the X-ray beam, focusing them to 100 micrometers, about the thickness of a human hair. We measured each particle from 2,000 different angles to seamlessly capture all the reflections.”

The measurements enabled scientists to distinguish between different uranium oxide phases within the radioactive debris, revealing details that had previously been inaccessible.

Unexpected discovery: Chernobyl fuel retained its crystal structure

The results revealed something unexpected. Although the six particles had spent 40 years in the environment, the nuclear fuel inside them retained much of its original crystal structure.

This suggests that at least some radioactive debris from the Chernobyl disaster is more chemically stable than researchers previously believed.

The stability of these particles could affect how radioactive contamination moves through the environment. The findings suggest that some particles may retain fission products within their structures instead of releasing radioactive material rapidly. If this behavior occurs more widely, it could help limit the transfer of radioactive contaminants into nearby soil and water.

However, the researchers stress that their findings should not be interpreted as evidence that Chernobyl’s radioactive contamination is becoming harmless.

“But all particles have different structures,” Weissenborn cautions. “And in our experiment, we studied only six such particles from two different locations.”

The small number of samples means the findings cannot establish how radioactive particles behave throughout the entire contaminated area. To better understand their stability, scientists will need to analyze many more particles collected from a wider range of locations.

Why Chernobyl’s radioactive legacy remains unpredictable

Even with decades of research, predicting the long-term dangers of Chernobyl’s radioactive debris remains difficult. Individual particles can differ in composition, structure and resistance to environmental weathering. These differences can affect when radioactive material is released.

“And even if we do get some average value at some point, we still won’t be able to make universal statements about the health risks in this region,” Weissenborn cautions. “Even if the particles decay in a roughly uniform pattern, there are always outliers, more persistent particles, that release radionuclides at later points in time.”

These persistent particles can continue emitting radioactive material long after other debris has decomposed, complicating efforts to assess future contamination and potential health risks.

As a result, the new findings do not suggest that restrictions on the Chernobyl exclusion zone will be lifted anytime soon.

Research into the disaster’s lasting effects continues. Weissenborn and Hennig are already conducting additional experiments focused on the highly radioactive transuranic phase of the remaining debris. Their work could provide a clearer picture of how Chernobyl’s nuclear material changes over time and how long its environmental effects may last.