The microscopic particles unleashed by the plant’s explosions are also a potential environmental and health concern
“THE ONLY CLUE”
It was initially thought all the radioactive cesium released in the Fukushima plumes was in a water-soluble form, and would disperse more or less evenly throughout the environment. But when aerosol specialist Yasuhito Igarashi, then of the University of Tsukuba, and his colleagues examined an air filter from the Meteorological Research Institute in Tsukuba, 170 kilometers southwest of Fukushima, they noticed the filter contained radioactive hotspots. Using specialized imaging techniques they detected high concentrations of radioactive cesium as well as bits of iron and zinc, packed into particles just a couple of microns in diameter (about the size of the average Escherichia coli bacterium). Subsequent studies noted these bits were encapsulated in silica, giving them a glassy texture. Particles found within a few kilometers of the power plant also contained nanosize pieces of uranium dioxide, the nuclear fuel used in the plant.
Because the cesium-rich particles were born early in the meltdown, they offer scientists an important window into the exact sequence of events in the disaster. Indirect evidence suggested the tsunami knocked out the reactors’ cooling water systems, causing the nuclear fuel to heat up. As temperatures rose, steam corroded the metal cladding on the plant’s containment vessels (which enclose the nuclear fuel), giving off hydrogen gas in the process. A spark finally caused the hydrogen to explode, breaching the reactors and releasing the radioactive plumes. The specific structure of the glassy microparticles and the ratios of elements they contain form a record of the sequence of chemical reactions that took place. For scientists, this can help flesh out the time line—and illuminate the current state of the debris in the plant’s melted reactors, which remain off-limits due to high radiation levels. The beads are “the only direct evidence of the debris remaining inside the reactor. That’s the only clue,” says Satoshi Utsunomiya of Kyushu University, who studies environmental threats from various nanoparticles.
The beads’ composition tells researchers, for example, that the cesium (along with some other fission products that vaporized in the high temperatures during the meltdown) ultimately condensed like raindrops, glomming onto bits of iron dioxide and zinc dioxide that had been generated as the containment vessel and cladding corroded. The particles’ glassy texture shows temperatures in some spots reached the extremely high levels needed to melt and vaporize silica.
Although less radioactive cesium fell on Tokyo than closer to the plant, a bigger proportion of the total was packed into the microparticles, the team’s findings suggest. However, publication of the full study describing those findings, initially slated for 2017 in Scientific Reports, was postponed after researchers with the Tokyo Metropolitan Industrial Technology Research Institute (TIRI)—which had provided an air filter sample to one of the study’s authors—objected to the study over the sample’s use by the other co-authors. A 2017 investigation by several institutions in Japan found no evidence of wrongdoing by the co-authors—and “there’s never, in any of the discussion, been concern about our scientific results,” says Ewing, the Stanford nuclear materials expert who is also a study co-author.
But for two years the study was “caught in limbo,” he says. The co-authors said they were unable to talk about the study’s findings for this article because of the journal’s restrictions on discussing studies prior to publication—but a description of the key finding appeared in a subsequent study, also published there. Then, last Friday the journal (which is owned by the same parent company as Scientific American) withdrew its offer of publication for the study, citing its inability to adjudicate the sample issue. The journal said it would reconsider publishing the work if that issue is resolved, according to correspondence shown to Scientific American. The researcher at TIRI who initially objected to the publication referred all questions toScientific Reports, which declined to comment on the study specifically, but said through a spokesperson that journal editors do not settle disputes on issues such as ownership of data and materials.
Understanding how the microparticles move and how far they spread, including to places like Tokyo, is crucial to assessing any potential environmental and health risks they may pose. Utsunomiya, who has spoken with residents concerned about the particles, is trying to figure out how long it will take for these beads to dissolve in water; their glassy casing means they are likely to break down slowly, their radioactive components leaching out like a timed-release medicine capsule, as Ewing describes it. If dissolution happens slowly enough, though, it could mean the radioactive elements decay before the particles fully dissolve. Calculations of radiation doses from similar particles also suggest there is little concern about radiation exposure from the cesium in the particles—even if inhaled deep into the lungs. But Scott is concerned about the uranium that has been found in some of the particles, as well as the potential for some to harbor plutonium—both of which are chemically toxic. Uranium-containing particles seem to be limited to a relatively small area very near the plant, however, and it is unclear if the amounts of either element would be large enough to pose a significant concern.
Researchers are also finding these particles can congregate in certain areas such as river bends or in drain downspouts, where they collect after being washed from rooftops by rain. This could potentially create hotspots. Scientists additionally want to understand how easily these particles might get re-suspended in the air; some research suggests they become naturally buried into the soil quite quickly, which would reduce the risk of their being re-suspended, Scott says. Understanding the particles’ behavior could help guide decontamination efforts, which so far have included removing the top layer of soil and pressure-washing buildings and roads in the contamination zone.
Read more at Radioactive Glass Beads May Tell the Terrible Tale of How the Fukushima Meltdown Unfolded