Chornobyl is the one nuclear accident everyone can name. It’s the site of the worst nuclear accident in history. It was so bad that it’s still radioactive today.<\/p>\n
You can picture it. The empty apartment blocks, the Ferris wheel in Pripyat, the steel arch parked over reactor No. 4. But what the pictures don’t show is the dust: specks of actual reactor fuel, thinner than a human hair, scattered through the soil 40 years after the explosion and still radioactive enough that the people who collect them go in wearing protective suits.<\/p>\n
Now a German team has taken six of those specks to France and measured each one from 2,000 different angles under an X-ray beam. And the fuel inside them, it turns out, is structurally still pretty much the same<\/em> fuel that went into the reactor.<\/p>\n The work comes from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and Leibniz University Hannover, and HZDR put the results out on October 6<\/a> alongside a paper in the Journal of Hazardous Materials. The main finding is that the crystal structure of the uranium dioxide in the six particles has stayed largely intact, which the researchers say surprised them. An intact crystal means the fission products (the radioactive leftovers from splitting uranium) tend to stay put inside the particle rather than leaching into soil and groundwater.<\/p>\n When reactor No. 4 blew apart in 1986, it didn’t just send gas and smoke over Europe. It threw out solid bits of its own fuel, and those bits are what the field calls hot particles. The six in this study measure between 0.0003 and 0.002 inches across, and they’re still highly radioactive four decades later. It’s hard to imagine, because it’s not something you can really see.<\/p>\n Tobias Weissenborn, a physicist and doctoral candidate at Hannover, splits them into three classes. The first group is still chemically and physically close to plain uranium dioxide, the ceramic that the fuel pellets were made of. The second group got hot enough during the accident that the fuel melted and bonded to the surrounding zirconium cladding, so you end up with a particle partly or fully wrapped in, or fused with, its own metal jacket. If you’ve read about the fuel debris still stuck to the pipes at Three Mile Island<\/a>, it’s the same basic idea… Just at the scale of a grain of pollen.<\/p>\n The third class is where it gets less comfortable. Chernobyl’s reactor used graphite to slow neutrons down, and that graphite burned for ten days after the explosion. In that fire, fuel oxidized into other uranium compounds, including U3<\/sub>O8<\/sub>. The problem with U3<\/sub>O8<\/sub> is that it’s mechanically weak. It crumbles into even finer dust that the wind can pick up, and that dust is the inhalation risk the release flags.<\/p>\n X-ray diffraction is a pretty old trick. You shine X-rays at a crystal, the atoms sitting in their neat rows bounce the beam back at specific angles, and the pattern of those bounces tells you which crystal structure you’re looking at. Uranium dioxide gives one pattern, U3<\/sub>O8<\/sub> gives another, and so on. Crystallographers call each of those forms a phase, and sorting out which phases are in a sample is a phase analysis.<\/p>\n So why ship them to France? Because these particles are tiny and absurdly radioactive, and a benchtop X-ray machine won’t cut it. The Hannover group isolated the six from samples taken inside the exclusion zone, fixed each one to a tungsten electrode (basically a needle), packed them in several layers of containment, and shipped them to Grenoble. That’s where HZDR runs its own station, the Rossendorf Beamline<\/a>, at the European Synchrotron Radiation Facility.<\/p>\n A synchrotron is a ring-shaped accelerator where electrons running near the speed of light throw off X-rays far brighter than anything in a hospital. The ESRF ring in the French Alps hosts a few dozen beamlines, and HZDR’s is one of 12 run by outside research groups. It was set up specifically for actinides (uranium, plutonium, and their neighbors) inside a lab rated to handle alpha emitters. Which is why a German study of Ukrainian dust ends up in France.<\/p>\n Christoph Hennig, the HZDR crystallographer who led the beam work, said the team first had to figure out how to look at the structure at all, and then settled on rotating the whole particle inside a beam focused down to about the thickness of a human hair.<\/p>\n “We measured each particle from 2,000 different angles to seamlessly capture all the reflections,” Hennig said in the HZDR release. The paper describes it as triple-axis rotation, which basically involves spinning the sample around three different axes so that no reflections are missed. The beam itself, at 0.004 inches wide, was bigger than any of the six particles.<\/p>\n Four kinds of crystal, going by the paper’s abstract: uranium dioxide, U4<\/sub>O9<\/sub> (an intermediate oxide one small step up from UO2<\/sub>), U3<\/sub>O8<\/sub>, and phases where uranium and zirconium had mixed. The UO2<\/sub> and U4<\/sub>O9<\/sub> showed up largely intact, and the authors expect those two frameworks to keep holding their fission products and actinides in place, for now at least.<\/p>\n That’s arguably the more useful result here, and not because it makes the particles sound friendlier. From the phase analysis, the team says it can estimate how quickly a given particle releases its radioactive content. HZDR bills it as the first phase analysis ever run on fragments like these, and the abstract says the diffraction data hadn’t been reported in the literature before. If you want a contamination model for the zone that’s better than a guess, that release rate is frankly what you’d want to plug in.<\/p>\n Also from the abstract<\/a>: not all of this material spent its 40 years in dirt. The paper covers particles that sat in soil and in asphalt inside the exclusion zone. The release says it’s still largely unclear why the particles weather at different rates, and I’d guess the asphalt-versus-soil question is part of what the team is chasing.<\/p>\n Not really, or at least no one involved is saying so.<\/p>\n I mean… Don’t go there.<\/p>\n Weissenborn’s own caveat is that each particle has a different structure, and the experiment examined six of them from two locations. Drawing any general conclusion about Chornobyl particles would mean many more samples from many more places, and even then, he says, you’d be stuck with averages. Averages don’t help you with the stubborn outlier that hangs on to its radionuclides for years and then lets go later.<\/p>\n So the exclusion zone isn’t opening up anytime soon, and the release says as much.<\/p>\n The release also doesn’t say which two spots the particles came from, or how the six split across the three classes, and I couldn’t find that in the abstract either. The full paper is open access if you want to dig through it.<\/p>\nSo what’s a hot particle?<\/h2>\n
Why measure a speck from 2,000 angles?<\/h2>\n
So what did the beam find?<\/h2>\n
So is the zone safer than we thought?<\/h2>\n