Radioactive Nano- and Microparticles Released from Fukushima Daiichi: Technical Challenges of Multiple Analytic Techniques

Abstract

A total activity of ∼1019 Bq, including ∼1016 Bq of 137Cs, was released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) in 2011, among which 137Cs (30.1 years half-life) will remain in the environment for decades either in the form of: (i) Cs bound to clays, or (ii) highly radioactive Cs-rich microparticles (CsMPs). CsMPs are nano- to microscale particles that were dispersed as far away as ∼230 km, thus the characterization of CsMPs has been extremely challenging. This chapter summarizes the technical challenges in the application of state-of-the-art analytical techniques including atomic-resolution electron microscopy, secondary ion mass spectrometry, and synchrotron-based micro X-ray analysis. CsMPs consist of a glassy silicate matrix and contain Cs (<0.55–30 wt%), Fe, Zn, as well as other trace elements. The 134Cs/137Cs activity ratios of individual CsMPs are ∼1, confirming their FDNPP origin. The nanoscale texture of CsMPs indicates that intrinsic Cs phase(s) and other fission fragment nanoparticles formed in the reactors during meltdown. Nanoscale fragments of fuel debris, incorporated into the CsMP matrix, reveal a variety of physico-chemical properties including euhedral, uraninite crystals. 235U/238U isotopic ratios within the CsMPs range from ∼0.019 to ∼0.030 reflect the variation in the burn-up of the nuclear fuel in different reactors. Trace Pu occurs associated with U(iv) oxide nanoparticles, which are incorporated into the CsMPs. Thus, CsMPs are not only an important medium with localized Cs; microparticulates also provide a mechanism for the transport of debris fragments of incorporated U and Pu into the environment in a respirable form.