Four decades after the 1986 nuclear disaster, researchers from Leibniz University Hannover and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) examined six tiny, highly radioactive particles, measuring just 8 to 50 micrometers, that were released from the destroyed reactor in Ukraine.
An analysis of the findings, published in the Journal of Hazardous Materials, revealed that these "hot particles" are far more stable than previously estimated. The findings could help scientists more accurately assess the health risks posed by radioactive particles of this kind.
A radioactive 'hot particle' from the 1986 Chernobyl disaster
(Photo: Laura Stalmann)
Even now, despite the passage of time, entry into contaminated areas is permitted only while wearing protective suits.
"There are three classes of these particles," explained Tobias Weissenborn, a physicist and doctoral student at Leibniz University Hannover. "First, there are particles that are still very similar chemically and physically to the nuclear fuel uranium dioxide. Then there are particles that are partially or fully encased in, or completely fused with, their zirconium layer."
The latter type of particle formed when temperatures inside the reactor rose sharply, causing the nuclear fuel to melt and adhere to its protective cladding, made of the highly durable metal zirconium.
"The third type of particle was created when the graphite-moderator caught fire," Weissenborn said.
A moderator slows fast-moving neutrons to sustain the nuclear chain reaction. During the disaster, however, the graphite ignited and burned for 10 days.
"In the process, the fuel oxidised into various uranium oxides” – such as triuranium octoxide, or U3O8. This mechanically unstable compound rapidly forms microscopic particles that are easily borne away by wind. Inhaling such dust particles poses severe health risks."
It remains unclear why these particles weather and break down at different rates in the environment. As they deteriorate, however, they form a variety of uranium oxides. Analyzing these compounds requires highly complex X-ray diffraction experiments at synchrotron facilities, which generate intense beams of radiation for scientific research.
Ukrainian troops training in Chernobyl in 2022
(Photo: Reuters)
An unfinished reactor at the Chernobyl nuclear power plant
(Photo: Francisco Seco/AP)
Pripyat, a ghost town in Ukraine that was completely evacuated on April 27, 1986, one day after the Chernobyl disaster, because of exposure to high radiation levels
(Photo: Efrem Lukatsky/AP)
The Chernobyl nuclear power plant in 1986, the year of the disaster that released large quantities of radioactive material into the atmosphere
(Photo: Tass/AP)
Dr. Christoph Hennig, a researcher and crystallographer at the Helmholtz-Zentrum Dresden-Rossendorf, has now conducted the first successful structural analysis of these particles using a beamline at the renowned particle accelerator facility in Grenoble, France.
Working with Weissenborn, he carried out a phase analysis of six particles originating from Chernobyl. Among other things, this analysis allows researchers to estimate the rate at which the particles release radioactive materials.
"We used various methods to isolate the particles from soil samples collected in Ukraine and attach them to tungsten electrodes," Weissenborn explained.
A sign warning of radioactive contamination in Chernobyl
(Photo: Efrem Lukatsky/AP)
The Chernobyl nuclear power plant as seen in a Landsat 5 satellite image taken days after the disaster
(Photo: EOSAT/AP)
Gas masks left behind following the 1986 evacuation of Chernobyl
(Photo: Kateryna Korepanova)
The samples, secured within several layers of protective shielding, were sent to Grenoble, where a team led by Hennig examined them using diffraction analysis, a technique that studies how X-rays are scattered by the internal structure of a material.
"First, we had to figure out how to examine the particle structure," Hennig explained. "Ultimately, we fully rotated the particles in an X-ray beam focused down to 100 micrometres, or roughly the thickness of a human hair. We measured each particle from 2,000 different angles to seamlessly capture all the reflections."
Using this method, the researchers were able to experimentally identify different oxide phases within the samples.
To their surprise, they discovered that the crystalline structure of the nuclear fuel in the particles examined has remained largely unchanged to this day, suggesting that remnants of the disaster are far more chemically stable than previously believed.
The particles trap radioactive fission products within them, a finding that could be encouraging for the soil and water sources around Chernobyl.
"However, every single particle has a different structure and our experiment only studied six such particles from two different locations," Weissenborn cautioned.
Drawing broader conclusions about the stability of Chernobyl's radioactive particles will therefore require collecting samples from many more locations and examining a substantially larger number of particles.
"And even if we obtained some averages at some point, we still wouldn’t be able to make universal statements about health risks in the region," Weissenborn warned.
"Because even if the particles decay in a largely uniform pattern, there will always be outliers, more persistent particles, that will release radionuclides at a later point in time." Therefore, restrictions on entry to the Chernobyl disaster zone are unlikely to be lifted anytime soon.