Isotopic signature and nano-texture of cesium-rich micro-particles: Release of uranium and fission products from the Fukushima Daiichi Nuclear Power Plant

Junpei Imoto,Kenji Horie,Toshihiko Ohnuki,Shinya Yamasaki,Rodney C Ewing,Gareth T W Law,Genki Furuki,Satoshi Utsunomiya,Asumi Ochiai,Bernd Grambow,Mami Takehara,Mizuki Suetake,Kenji Nanba,Ryohei Ikehara

doi:10.1038/s41598-017-05910-z

Abstract

Highly radioactive cesium-rich microparticles (CsMPs) released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) provide nano-scale chemical fingerprints of the 2011 tragedy. U, Cs, Ba, Rb, K, and Ca isotopic ratios were determined on three CsMPs (3.79–780 Bq) collected within ~10 km from the FDNPP to determine the CsMPs’ origin and mechanism of formation. Apart from crystalline Fe-pollucite, CsFeSi2O6 · nH2O, CsMPs are comprised mainly of Zn–Fe-oxide nanoparticles in a SiO2 glass matrix (up to ~30 wt% of Cs and ~1 wt% of U mainly associated with Zn–Fe-oxide). The 235U/238U values in two CsMPs: 0.030 (±0.005) and 0.029 (±0.003), are consistent with that of enriched nuclear fuel. The values are higher than the average burnup estimated by the ORIGEN code and lower than non-irradiated fuel, suggesting non-uniform volatilization of U from melted fuels with different levels of burnup, followed by sorption onto Zn–Fe-oxides. The nano-scale texture and isotopic analyses provide a partial record of the chemical reactions that occurred in the fuel during meltdown. Also, the CsMPs were an important medium of transport for the released radionuclides in a respirable form.

Highlights

Radionuclides with ~520 PBq initial total activity were released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) as a result of the nuclear disaster that occurred after the Tohoku earthquake on March 11, 20111
Four Cs-rich microparticles (CsMPs) were found at three localities, which are hereafter labeled as OTZ3, OTZ10, KOI2, and OMR1 (Fig. 1)
The 136Ba/138Ba ratio may indicate the contribution of a low-yield fission product; the amount of 136Cs can be remarkable in some cases depending on the amount of volatilized Ba

Summary

Introduction

Radionuclides with ~520 PBq initial total activity were released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) as a result of the nuclear disaster that occurred after the Tohoku earthquake on March 11, 20111. A possible cause of the heterogeneity is the formation of Cs-rich microparticles (CsMPs), with a high Cs radioactivity per unit mass, found at a range of distances from the FDNPP12 This is another important route of Cs migration in the environments. The occurrence of U derived from inside the reactors is potentially useful for understanding the reactions that nuclear fuels experienced during meltdown and even the status of the melted nuclear fuels in the damaged reactors[17]. The CsMPs are a very unique form of condensed matter that was created during the meltdown events inside the reactors at the FDNPP, very different from general concept of Cs release as a soluble Cs species, such as CsI and CsOH. Isotopic analysis of the other nuclides, stable and radiogenic, allow us to identify their sources, natural or fissionogenic

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