Microscopic nuclear fuel particles from Chernobyl still intact after 40 years

Forty years after explosions scattered radioactive debris from Chernobyl’s reactor across the surrounding landscape, some microscopic fragments still preserve the crystal structure of their original nuclear fuel.

A study of six particles from Ukraine found that the material remained more chemically stable than researchers had expected.

Scientists at Germany’s Leibniz University Hannover and Helmholtz-Zentrum Dresden-Rossendorf (HZDR) examined the fragments using synchrotron X-ray diffraction in Grenoble, France.

Their findings help explain how these remnants retain radioactive substances, although the small sample cannot establish conditions across the contaminated region.

Microscopic remnants of the disaster

The radioactive “hot particles” described in the research measure just 8 to 50 micrometers. Despite their size, they remain highly radioactive four decades after the 1986 accident.

Researchers distinguished three broad classes. Some particles remain chemically and physically similar to uranium dioxide, the reactor’s nuclear fuel. Others are partly or fully enclosed in zirconium, or fused with the metal that originally surrounded the fuel.

Those combined particles formed when intense reactor temperatures melted the fuel and bonded it to its protective zirconium layer.

A third class emerged as the reactor’s graphite moderator burned. The moderator normally slowed neutrons to sustain the chain reaction, but during the disaster, its fire lasted ten days.

The fuel oxidized into uranium compounds, including triuranium octoxide. This mechanically unstable material readily forms microscopic particles that wind can carry away, creating an inhalation hazard.

Hot particle from the 1986 reactor accident: the nuclear fuel particle is partially fused with zirconium oxide. (Image credit: Leibniz University Hannover)

X-rays reveal enduring fuel structures

Understanding why these fragments weather at different rates requires examining their internal structures.

The Hannover researchers isolated particles from Ukrainian soil samples and attached them to tungsten electrodes. Multiple confinement layers secured the radioactive samples for transport to Grenoble.

At the Rossendorf Beamline, HZDR crystallographer Dr. Christoph Hennig and colleagues rotated the particles inside a focused X-ray beam.

“Ultimately, we fully rotated the particles in an X-ray beam focused down to 100 micrometers, or roughly the thickness of a human hair. We measured each particle from 2,000 different angles to seamlessly capture all the reflections,” Hennig said.

The measurements identified different oxide phases within the samples. Crucially, the nuclear fuel’s crystal structure remained largely intact in the particles examined.

That stability helps keep fission products inside the fragments, potentially limiting their release into surrounding soil and water. The phase analysis also enabled researchers to estimate how quickly the particles release radioactive substances.

Six particles cannot define regional risks

The findings come with a significant limitation: the researchers examined only six particles collected from two locations.

“However, every single particle has a different structure, and our experiment only studied six such particles from two different locations,” warned Tobias Weissenborn, a physicist and doctoral candidate at Leibniz University Hannover.

Establishing broader patterns requires many more particles from additional locations. Even then, averages cannot capture every fragment’s behavior or establish universal health risks across the region.

Some more persistent particles can retain radionuclides longer and release them later, complicating assessments of contamination over time.

The results therefore provide no basis for lifting restrictions across the Chernobyl Exclusion Zone. Weissenborn and Hennig are already conducting follow-up experiments on highly radioactive transuranic phases within the disaster’s remnants.

Source: https://interestingengineering.com/science/chernobyl-nuclear-fuel-particles-remain-stable