By sciencedaily |
Radioactive Glass Beads May Tell the Terrible Tale of How the Fukushima Meltdown Unfolded
On March 14 and 15, 2011, explosions unleashed invisible radioactive plumes from the Fukushima Daiichi Nuclear Power Plant, crippled three days earlier when the strongest recorded earthquake in Japan’s history triggered a massive tsunami. As the plumes drifted over the neighboring countryside, their contents—including radioactive cesium, a by-product of the plant’s fission reactions—fell to the ground and over the ocean.
What no one knew or expected was the fallout also contained bacteria-size glassy beads, with concentrations of radioactive cesium that were far higher than those in similar-size motes of tainted dust or dirt.
Since these particles were discovered in 2013, scientists have plucked them from soil samples and air filters throughout the contamination zone, including filters as far away as Tokyo. The beads could pose an under-recognized heath risk, researchers say, because they are tiny enough to be inhaled deep into the lungs—and their glassy makeup means they may not easily dissolve or erode. They also present an opportunity to conduct what one researcher called “nuclear forensics”: By analyzing the particles’ composition, scientists can piece together a clearer image of what happened during the white-hot violence inside the plant itself, and of the current condition of the debris in the three reactors that experienced meltdowns. This could help inform the strategy for cleaning up the ruins of the plant.
Researchers say a picture of the unusual beads is coming into focus against a backdrop of the Japanese public’s general nuclear wariness, and the government’s desire to put the Fukushima incident behind it—particularly with Tokyo poised to host the 2020 Olympics. “I think, unfortunately, the reaction to this discovery [of the beads] has been not very welcomed in Japan,” says Rod Ewing, a mineralogist and nuclear materials expert who co-directs the Center for International Security and Cooperation at Stanford University.
“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 fuel rods (which enclose the nuclear fuel), giving off hydrogen gas in the process. A spark finally caused the hydrogen to explode, breaching the reactor building 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.
Utsunomiya thinks that happened when the containment vessels finally failed and dropped down onto the reactor’s concrete pedestal: Some of the silica in the concrete vaporized into silicon dioxide, which condensed around the particles. Larger, millimeter-scale particles found closer to the plant seemed to get their silica from the insulation in the reactor coolant system, according to an analysis done by Tom Scott, a professor of nuclear materials at the University of Bristol, and his colleagues.*
It is possible each reactor unit underwent slightly different meltdown processes, yielding particles with characteristic makeups. Pinpointing what went wrong in each reactor helps tell those working to clean up the plant what they might be dealing with in the radioactive plant wreckage, as well as “better understand the mechanisms and the contributing factors” involved in the disaster, Scott says—“because if you understand that, you can prevent it happening ever again.”
A Sharper Picture Emerges
Scott’s group works with scientists from the Japan Atomic Energy Agency (which is tasked with spearheading the research to support the Fukushima cleanup), and is also using details it gleans from the beads to refine maps of contamination and radiation risk in the area around the plant. Although the microparticles were apparently not distributed as widely as other forms of radioactive cesium—some of which were blown around the globe—they have turned up throughout the most contaminated areas. Utsunomiya and his colleagues have found as many as 318 particles in a single gram of soil near the plant. They also discovered the particles blew farther than anyone initially anticipated. Some were found in an air filter in Tokyo, about 240 kilometers from Fukushima.
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 to Scientific 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.
As studies of these particles continue to accumulate, researchers are gradually gaining an improved understanding of them—“much better than we had a year or two ago,” Scott says. “But there are still unanswered questions.”
*Editor's Note (3/14/19): The asterisked paragraphs were edited after posting to clarify references to the locations of the cladding and insulation inside the reactors.