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

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. Here we quantify CsMP abundance and radioactive fraction (RF) in 100 surface soil samples collected across Fukushima Prefecture in July 2011 and integrate the results with WSPEEDI atmospheric simulations. CsMP abundance ranged from 0 to 52.3 particles g⁻¹ (dry weight), with RF values of 0-61.85%. The combined analysis identifies a major CsMP formation and release event at ∼03:00 JST on 15th March 2011, producing a plume strongly enriched in CsMPs. Plumes released after 00:00 JST on 16th March contained no detectable CsMPs, indicating that particle formation had ceased by that time. The widespread distribution of CsMPs across Fukushima is therefore attributed primarily to this single plume. Directional variations in CsMP abundance reflect temporal changes in plume composition, with peak concentrations of ∼2070 particles m⁻³ toward the southwest and ∼4700 particles m⁻³ toward the northwest. These findings constrain CsMP formation mechanisms and improve reconstruction of radiological dispersion relevant to the long-term environmental risk assessment of nuclear power plants.