Forty years after the Chernobyl (now Chornobyl) nuclear disaster, microscopic radioactive particles are still scattered across the surrounding soil, and the area remains radioactive enough that entering it requires protective clothing. The explosions that destroyed the reactor flung enormous quantities of debris into the environment, and particles measuring just 8 to 50 micrometers are still highly radioactive. Scientists have now examined these tiny leftovers in unprecedented detail, and they found something surprising: some of them have kept much of their original internal structure despite four decades of weathering.
"There are three classes of these particles," explains Tobias Weissenborn, a physicist and doctoral candidate at Leibniz University Hannover. "First, there are particles that are chemically and physically still very similar to the nuclear fuel uranium dioxide. Then, there are particles that are partially or fully encased in, or completely fused with, their zirconium layer." That second category was born in the extreme heat inside the reactor, where the melting fuel combined with zirconium, the highly resistant metal used in the fuel's protective outer layer. The third type, Weissenborn adds, "was created when the graphite-moderator caught fire." Under normal conditions the graphite moderator slows fast neutrons and helps sustain the chain reaction; during the disaster it ignited and burned for ten days. In the process, the fuel oxidized into various uranium oxides such as U3O8 - a form that is mechanically fragile, breaks easily into microscopic fragments, and can then be carried by the wind, where inhaling it poses serious health hazards.
Despite decades of research, scientists still don't fully understand why Chornobyl's particles break down at different speeds, and as the material weathers it can produce several forms of uranium oxide, making long-term behavior hard to predict. Cracking that requires equipment of unusual precision, including synchrotron X-ray diffraction, which reveals how atoms are arranged inside crystalline materials. Dr. Christoph Hennig, a crystallographer at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), has now completed the first successful structural analysis of radioactive fragments of this kind at the Rossendorf Beamline in Grenoble.
Working with Weissenborn, Hennig analyzed six particles recovered from the Chornobyl region, identifying their different crystalline phases to learn how quickly radioactive substances might escape into the environment. "The colleagues from Hanover and I used various methods to isolate the particles from Ukrainian soil samples and attach them to tungsten electrodes," Weissenborn says. Because the samples were highly radioactive, they had to be sealed inside multiple layers of containment before the trip to Grenoble, where Hennig's team used X-ray diffraction to probe their internal structures by measuring how X-rays scattered as they passed through the material. "First, we had to figure out how to examine the particle structure," Hennig explains. "Ultimately, we fully rotated the particles in an X-ray beam focused down 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." That let the scientists distinguish different uranium oxide phases within the fragments, revealing details that had previously been inaccessible.
The results were unexpected: after four decades in the environment, the nuclear fuel inside all six particles had retained much of its original crystal structure. That suggests at least some of Chornobyl's debris is more chemically stable than researchers believed - stable enough, potentially, to keep fission products locked inside rather than releasing them quickly, which could help limit the movement of radioactive contaminants into nearby soil and water. The researchers are quick to note this is not evidence that Chornobyl's contamination is becoming harmless. "However, every single particle has a different structure," Weissenborn warns, "and our experiment only studied six such particles from two different locations." Six particles from two sites cannot establish how debris behaves across an entire contaminated region; that would take many more particles from a much broader range of locations.
Even with far more data, predicting the long-term dangers would remain difficult, since individual particles differ in composition, structure, and resistance to weathering, which affects when they release radioactive material. "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 cautions. "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." Those stubborn outliers could keep leaking long after their neighbors have broken down, complicating any future assessment of contamination and health risk - which is why the findings do not suggest the Chornobyl Exclusion Zone's restrictions can be lifted anytime soon.
Research into the disaster's enduring consequences continues. Weissenborn and Hennig are already running additional experiments on highly radioactive transuranic phases in the remaining debris, work that could sharpen the picture of how Chornobyl's nuclear materials change over time and how long their environmental effects may last.