Uranium Dioxides and Debris Fragments Released to the Environment with Cesium-Rich Microparticles from the Fukushima Daiichi Nuclear Power Plant

Abstract

Trace U was released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) during the meltdowns, but the speciation of the released components of the nuclear fuel remains unknown. We report, for the first time, the atomic-scale characteristics of nanofragments of the nuclear fuels that were released from the FDNPP into the environment. Nanofragments of an intrinsic U-phase were discovered to be closely associated with radioactive cesium-rich microparticles (CsMPs) in paddy soils collected ∼4 km from the FDNPP. The nanoscale fuel fragments were either encapsulated by or attached to CsMPs and occurred in two different forms: (i) UO nanocrystals of ∼70 nm size, which are embedded into magnetite associated with Tc and Mo on the surface and (ii) Isometric (U,Zr)O nanocrystals of ∼200 nm size, with the U/(U+Zr) molar ratio ranging from 0.14 to 0.91, with intrinsic pores (∼6 nm), indicating the entrapment of vapors or fission-product gases during crystallization. These results document the heterogeneous physical and chemical properties of debris at the nanoscale, which is a mixture of melted fuel and reactor materials, reflecting the complex thermal processes within the FDNPP reactor during meltdown. Still CsMPs are an important medium for the transport of debris fragments into the environment in a respirable form.

Isotopic ratios of uranium and caesium in spherical radioactive caesium-bearing microparticles derived from the Fukushima Dai-ichi Nuclear Power Plant

“…The 137 Cs activity per unit volume in the particles were consistent with the relationship between 137 Cs activity and particle volume described by Satou et al 16 . The 134 Cs/ 137 Cs activity ratio with decay-corrected values was approximately 1 for all particles (average = 1.07), and this result agreed with those in previous studies [13][14][15][16][17][18][19]24,[27][28][29][30] . The energy-dispersive X-ray spectrometry (EDS) results of the five particles showed the X-ray peaks of silicon (Si), oxygen (O), Cl, K, Fe, Zn, Sn, Cs and some particles (particles C and D) with aluminium (Al) peak (Fig.…”

“…The Rb X-ray peak was not identified, because its peak overlapped with the Si peak. As revealed by Kogure et al 17 , the elemental compositions differ slightly among the CsMPs but are basically similar to those in previous studies [13][14][15][16][17][18][19]24,[27][28][29][30] .…”

“…(a-e) SEM images of (a) particle A (diameter: 2.7 μm), (b) particle B (1.8 μm), (c) particle C (2.0 μm), (d) particle D (1.6 μm), and (e) particle E (2.1 μm). secondary adhesion of Al to the particle surface, because Al was scarcely detected inside CsMPs in previous studies 15,[17][18][19]27,28 . The Rb X-ray peak was not identified, because its peak overlapped with the Si peak.…”

“…S4 of Supplementary Information. Ochiai et al 28 found U dioxides (UO 2 ) and debris fragments in CsMPs sampled at the same sampling site as Furuki et al 18 . However, these CsMPs were not spherical unlike the typical CsMPs [13][14][15]17,19 .…”

μDose: A compact system for environmental radioactivity and dose rate measurement

“…However, it needs to be borne in mind that in environmental samples also other β emitters can be found, e.g. natural 87 Rb or anthropogenic 137 Cs (Ochiai et al, 2018;Buesseler et al, 2015;Porȩba et al, 2015;Faure and Powell, 1972;Evangeliou et al, 2014;Sanderson et al, 2016). This needs to be considered individually for each sample as it may introduce an error in the 40 K assessment.…”