Significant presence of cesium-rich microparticles in an abandoned school close to the Fukushima nuclear power plant

Dust samples from floors

Utsunomiya detailed that the team were given permission to access a school building 2.8 km away from the nuclear power plant that had been abandoned in 2011.

“Fortunately, we got permission to investigate inside an elementary school. When entering the school building, we were all shocked by what we saw. Five years had passed by the time of sampling in 2016, but everything was left as it was at the moment of the 2011 earthquake. It’s as if time had stood still,” says Utsunomiya.

The team collected dust samples from floors near to the school’s entrance, on its second floor, and in the near-by school yard. Using an autoradiography method previously invented by the team, students Kazuki Fueda and Tatsuki Komiya were able to count the number of CsMPs in the school samples and assign how much of each samples Cs radioactivity came from the microparticles.

The Cs radioactivity of the dust on the school’s floors ranged from 340—4040 Bq/m² (134+137Cs activity decay-corrected to March 15, 2011. This corresponds to 125—1490 Bq/m² in 2023), with 4.5—38.9% of that radioactivity coming from the CsMPs.

The highest amount of CsMPs were found near to the school’s entrance (up to 2481 particles per m²), while significant (but lower) amounts of CsMPs were found on the second floor (up to 1273 particles per m²). This shows penetration of the tiny fallout particles deep into the building. Interestingly, the CsMP numbers and the proportion of Cs radioactivity attributed to CsMPs in nearby outdoor samples were much lower (23–63 particles/g of dust or soil and 1.14–1.61% or total Cs radioactivity, respectively).

The team state that the indoor environment recorded Cs fallout from dry deposition, and as the school had remained undisturbed since the accident, the CsMPs remained there in significant numbers. The outdoor samples meanwhile record Cs fallout from dry and wet deposition.

There, it’s likely that some CsMPs may have been washed or blown away by rain and wind, thus Cs remaining in the outside location is dominated by that derived from wet deposition. Those Cs species fell in easily dissolved (soluble) forms, which have now largely become associated with dust and soil particles (e.g., clays).

In the study, the team completed a ‘wet test’ on the indoor samples to evaluate whether common soluble Cs species, such as CsOH or CsI, were present in the samples. After wetting samples with water, no water-soluble Cs species were found in the indoor dusts.

Utsunomiya states that “the properties of the school CsMPs resemble ones reported in many previous studies; they are even similar to the ones found in Tokyo. These CsMPs were distributed across a wide area, including the Fukushima prefecture and Kanto (Tokyo) region. The CsMPs may present a threat; as shown in our work, CsMPs may accumulate locally and form hot spots, even in indoor environments.”

Professor Gareth Law of the University of Helsinki, a co-investigator on the study, added that “the potential occurrence of CsMPs in indoor environments dictates a need for detailed studies of indoor CsMPs in residential areas impacted by FDNPP fallout.” He continues that “further research on the behavior of CsMPs if inhaled, and study of health impacts, were needed.”

Useful information to help repair the damage in Ukraine

Law and Utsunomiya added that “considering the fact that microparticles seem to be a universal radioactive contaminant in severe nuclear accidents, detailed knowledge of their properties, new approaches to estimate their amounts, and studies of their behavior and impacts in environmental and biological systems will be critical if we are to understand the true impact of the nuclear disasters.”

Recent activities in Ukraine near nuclear installations, make the need for such research more pressing.

Professor Bernd Grambow, a co-investigator from the University of Nantes added, “Any decontamination strategy needs to acknowledge that in close vicinity to the power plant, large differences in contamination levels may be encountered and that both ionic and particle bound Cs needs to be analyzed and removed, with the latter being particularly dangerous to cleaning workers.”

Professor Rodney C. Ewing of Stanford University further added, “This study is latest of an extended series of papers that deal with the composition, characteristics, formation, and number of CsMPs. The groundbreaking aspect of these works is the combined application of multiple advanced techniques e.g., the team’s autoradiography methods, high-resolution electron microscopy, and isotopic analysis. The atomic-scale resolution of these studies provides a model for future studies of environmental contamination.”

Utsunomiya finally spoke about the team’s sampling trip and the history of their work on CsMPs. “I believe it is our duty to conduct rigorous scientific research on the tragic Fukushima events, to find and publicize new knowledge that will be important to society and the next generation.”

“Maybe one day time can begin again for abandoned buildings like the school, but for that to happen, significant clean-up efforts are needed, and if that is to proceed, we first need to know about the forms and extent of contamination in those buildings, such that workers and potential occupants can be protected.”