Uncovering hidden dispersion patterns of radioactive cesium-rich microparticles from Fukushima Daiichi

宮崎 加奈子*; 笛田 和希*; 門脇 正尚   ; 寺田 宏明   ; 香西 直文   ; 岩田 孟 ; 堀江 憲路*; 竹原 真美*; 山崎 信哉*; Grambow, B.*; Law, G. T. W.*; Ewing, R. C.*; 宇都宮 聡*

Miyazaki, Kanako*; Fueda, Kazuki*; Kadowaki, Masanao; Terada, Hiroaki; Kozai, Naofumi; Iwata, Hajime; Horie, Kenji*; Takehara, Mami*; Yamasaki, Shinya*; Grambow, B.*; Law, G. T. W.*; Ewing, R. C.*; Utsunomiya, Satoshi*

Radioactive cesium-rich microparticles (CsMPs) released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) in 2011 pose a persistent environmental concern, yet their initial atmospheric dispersion has remained poorly constrained. To elucidate their release dynamics, we quantified CsMPs in 100 surface soil samples collected across Fukushima Prefecture in July 2011. CsMP abundance and radioactive fraction (RF) ranged from 0 to 52,300 particles kg; (dry weight) and 0-61.85% of deposited Cs, respectively. Integrating these results with WSPEEDI atmospheric simulations reveals a major formation event of CsMPs around 03:00 JST on 15th March 2011. The plume emitted at this time was heavily enriched in CsMPs, whereas plumes released after 00:00 JST on 16th March 2011 contained no detectable CsMPs, indicating that particle formation had ceased by then. The widespread distribution of CsMPs across Fukushima Prefecture is therefore attributed primarily to this single plume on 15th March 2011. Directional differences in CsMP abundance reflect temporal variations in CsMP density within the plume, with maximum estimated concentrations of 2,070 particles m; toward the southwest and 4,700 particles m; toward the northwest. Additionally, elevated CsMP densities coupled with relatively low RF in the northwest suggest supplementary deposition of water-soluble cesium between 17:00 and 23:00 JST on 15 March. These findings constrain the timing and mechanisms of CsMP formation and dispersion, providing essential insights for environmental reconstruction and future radiological risk assessment.