Hot Particle Science: Radionuclide Migration in Contaminated Environments

In March 2011, the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released ~520 PBq of radionuclides into the environment, contaminating more than 10⁶ km² across eastern Japan. Although radiation levels have decreased over time, long-lived ^137Cs (half-life 30.1 years) remains the dominant contributor to environmental dose. Radioactive cesium was released in two principal forms: soluble species (e.g., CsOH, CsI), which adsorbed onto aerosols and soils, and sparingly soluble cesium-rich microparticles (CsMPs)—high-activity particles (10¹⁰–10¹¹ Bq/g) with diameters of 0.1–10 μm.

Our research revealed that CsMPs are complex silicate-based particles containing Fe, Zn, and Cs, along with trace fission products and actinides. We achieved atomic-scale characterization of nanoscale nuclear fuel fragments associated with CsMPs. These included UO₂₊ₓ nanocrystals (~70 nm) and (U,Zr)O₂₊ₓ nanocrystals (~200 nm) containing intrinsic pores formed during crystallization, reflecting the extreme thermal conditions of reactor meltdown. Importantly, plutonium-bearing nanoparticles (~10 nm) were identified within CsMPs, and isotopic ratios confirmed their origin from irradiated reactor fuel.

Because CsMPs are respirable (often <PM10), they represent a unique pathway for debris transport and potential health risk. Long-distance dispersion was confirmed as far as the Kanto region, including Tsukuba and central Tokyo (170–230 km from FDNPP). To quantify environmental distribution, we developed the QCP (Quantitative CsMP) method, integrating autoradiography and gamma spectrometry to determine CsMP number density and radioactivity fraction in soils. Application of this method showed that 8.5–31.8% of total soil Cs activity near FDNPP was associated with CsMPs, with 1–318 particles per gram of soil.

These findings demonstrate that CsMPs served as critical carriers of nanoscale fuel debris and actinides during the accident. Our work established the framework for Advanced Nuclear Forensics, integrating multi-scale analytical techniques to reconstruct nuclear accident processes and improve understanding of radionuclide transport in contaminated environments.