Chernobyl 'Hot Particles' Remain Unexpectedly Stable 40 Years After the Meltdown, German Study Finds
X-ray diffraction of six radioactive fragments shows their nuclear-fuel crystal structure is largely intact, suggesting they release radionuclides more slowly than assumed.
Four decades after the 1986 Chernobyl disaster, researchers have found that tiny radioactive fragments thrown out of the destroyed reactor are far more stable than expected. Scientists at Leibniz University Hannover and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) examined six of these "hot particles," and the results could allow more precise assessments of the health risks they pose. The paper appears in the Journal of Hazardous Materials.
The explosions at the Ukrainian plant hurled massive amounts of debris into the air. Radioactive dust particles measuring 8 to 50 micrometers still contaminate the soil around the site, and people can enter the affected areas only in protective suits. Physicist Tobias Weissenborn of Leibniz University Hannover described three classes of particle. Some are chemically and physically still similar to the original uranium dioxide fuel. Others are partly or fully encased in, or fused with, their zirconium cladding, which formed when temperatures spiked and melted the fuel into the metal. The third type formed when the reactor's graphite moderator caught fire and burned for 10 days, oxidizing the fuel into uranium oxides such as U3O8.
U3O8 is mechanically unstable and rapidly forms microscopic particles that wind can carry, and inhaling such dust poses severe health risks. Why the particles weather at different rates remains largely unclear. Studying them required a complex synchrotron X-ray diffraction experiment. HZDR crystallographer Christoph Hennig carried out the first successful structural phase analysis of its kind at the Rossendorf Beamline in Grenoble.
The team isolated the particles from Ukrainian soil samples and attached them to tungsten electrodes. After shipping them under several layers of confinement, Hennig's group rotated each particle fully in an X-ray beam focused 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," he said.
The surprise was that the crystal structure of the nuclear fuel has remained largely intact to this day, meaning the accident's remnants are chemically more stable than originally assumed. The particles keep fission products locked inside, which may be good news for soil and water around Chernobyl. The researchers are careful about what that means. "Every single particle has a different structure," Weissenborn said, and the experiment covered only six particles from two locations. Broader conclusions would require many more samples.
Even with averages, he cautioned, no universal statement about health risks would be possible, because outliers and more persistent particles will release radionuclides later. The finding therefore does not mean restrictions in the Chernobyl Exclusion Zone can be lifted soon. Weissenborn and Hennig are now running follow-up experiments on the highly radioactive transuranic phases in the disaster's remnants.
