[Paper Review] Caesium fallout in Tokyo on 15th March, 2011 is dominated by highly radioactive, caesium-rich microparticles

Satoshi Utsunomiya, Genki Furuki+5 more|arXiv (Cornell University)|Jun 1, 2019

Radioactive contamination and transfer2 references21 citations

TL;DR

This study reveals that caesium-rich microparticles (CsMPs) with high radioactivity dominated the fallout in Tokyo on 15 March 2011, constituting 80–89% of total caesium radioactivity despite their small size (0.58–2.0 µm). Using high-resolution transmission electron microscopy and dissolution experiments, the authors show these CsMPs consist of Zn-Fe-oxide nanoparticles embedded in amorphous SiO2, with caesium primarily associated with the oxide phases rather than the glass matrix, resulting in extreme specific radioactivity (up to 1×10¹¹ Bq/g), posing significant long-term internal radiation risks upon inhalation.

ABSTRACT

In order to understand the chemical properties and environmental impacts of low-solubility Cs-rich microparticles (CsMPs) derived from the FDNPP, the CsMPs collected from Tokyo were investigated at the atomic scale using high-resolution transmission electron microscopy (HRTEM) and dissolution experiments were performed on the air filters. Remarkably, CsMPs 0.58-2.0 micrometer in size constituted 80%-89% of the total Cs radioactivity during the initial fallout events on 15th March, 2011. The CsMPs from Tokyo and Fukushima exhibit the same texture at the nanoscale: aggregates of Zn-Fe-oxide nanoparticles embedded in amorphous SiO2 glass. The Cs is associated with Zn-Fe-oxide nanoparticles or in the form of nanoscale inclusions of intrinsic Cs species,rather than dissolved in the SiO2 matrix. The Cs concentration in CsMPs from Tokyo (0.55-10.9 wt%) is generally less than that in particles from Fukushima (8.5-12.9 wt%).The radioactivity per unit mass of CsMPs from Tokyo is still as high as 1E11 Bq/g, which is extremely high for particles originating from nuclear accidents. Thus, inhalation of the low-solubility CsMPs would result in a high localized energy deposition by beta (0.51-12)*1E-3 Gy/h within the 100-micrometer-thick water layer on the CsMP surface) and may have longer-term effects compared with those predicted for soluble Cs-species.

Motivation & Objective

• To understand the chemical speciation and environmental behavior of low-solubility caesium-rich microparticles (CsMPs) from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident.
• To determine the contribution of CsMPs to total caesium fallout in Tokyo on 15 March 2011.
• To characterize the atomic-scale structure and caesium distribution within CsMPs collected in Tokyo.
• To assess the radiological implications of these CsMPs, particularly their potential for localized, long-term internal exposure upon inhalation.

Proposed method

• High-resolution transmission electron microscopy (HRTEM) was used to analyze the nanoscale structure of CsMPs collected from air filters in Tokyo.
• Chemical composition and elemental distribution within individual CsMPs were analyzed using electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS).
• Dissolution experiments were conducted on collected CsMPs to assess their solubility and release kinetics of caesium in aqueous environments.
• The specific radioactivity of CsMPs was measured and compared to that of soluble caesium species to evaluate internal radiation dose potential.
• The microstructure and phase composition of CsMPs from Tokyo were compared with those from Fukushima to assess consistency in formation and composition.
• Radiation dose calculations were performed based on beta energy deposition rates within a 100-µm water layer surrounding the particles.

Experimental results

Research questions

• RQ1 What proportion of total caesium radioactivity in Tokyo on 15 March 2011 was attributable to caesium-rich microparticles (CsMPs)?
• RQ2 What is the nanoscale structural and chemical composition of CsMPs collected in Tokyo, and how does it compare to those from Fukushima?
• RQ3 Where is caesium primarily located within the CsMPs—dissolved in the SiO2 matrix or associated with discrete phases?
• RQ4 What is the specific radioactivity of CsMPs from Tokyo, and how does it compare to other environmental radioisotopes?
• RQ5 What are the implications of the high radioactivity and low solubility of CsMPs for internal radiation dose following inhalation?

Key findings

• CsMPs between 0.58 and 2.0 µm in size accounted for 80–89% of the total caesium radioactivity in Tokyo on 15 March 2011, indicating their dominant role in initial fallout.
• The CsMPs from Tokyo exhibit the same nanoscale texture as those from Fukushima: aggregates of Zn-Fe-oxide nanoparticles embedded in amorphous SiO2 glass.
• Caesium is predominantly associated with Zn-Fe-oxide nanoparticles or present as nanoscale inclusions of intrinsic Cs species, not dissolved in the SiO2 matrix.
• The caesium concentration in Tokyo-derived CsMPs ranged from 0.55 to 10.9 wt%, generally lower than in particles from Fukushima (8.5–12.9 wt%).
• The specific radioactivity of CsMPs from Tokyo reached up to 1×10¹¹ Bq/g, an extremely high value for particles from a nuclear accident.
• Inhalation of these CsMPs would lead to localized beta radiation doses of (0.51–12)×10⁻³ Gy/h within a 100-µm water layer on the particle surface, suggesting prolonged and intense internal exposure compared to soluble caesium species.