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

• Published in: Scientific reports 2020-02, Vol.10 (1), p.3281-3281, Article 3281
• Main Authors: Kurihara, Yuichi, Takahata, Naoto, Yokoyama, Takaomi D., Miura, Hikaru, Kon, Yoshiaki, Takagi, Tetsuichi, Higaki, Shogo, Yamaguchi, Noriko, Sano, Yuji, Takahashi, Yoshio
• Format: Article
• Language: English
• Subjects: 639/638/11/296; 704/172/4081; Cesium radioisotopes; Humanities and Social Sciences; Isotopes; Mass spectrometry; Mass spectroscopy; Microparticles; multidisciplinary; Nuclear accidents & safety; Nuclear power plants; Plumes; Radioisotopes; Reactors; Science; Science (multidisciplinary); Uranium
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/s41598-020-59933-0

Spherical radioactive caesium (Cs)-bearing microparticles (CsMPs) were emitted during the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March, 2011. The emission source (timing) and formation process of these particles remain unclear. In this study, the isotopic ratios of uranium ( 235 U and 238 U) and caesium ( 133 Cs, 134 Cs, 135 Cs, and 137 Cs) isotopes in the five spherical CsMPs (ca. 2 μm in size) sampled at 50 km west of the FDNPP were determined using secondary ion mass spectrometry and laser ablation-ICPMS, respectively. Results showed that the 235 U/ 238 U ratios of CsMPs were homogeneous (1.93 ± 0.03, N  = 4) and close to those estimated for the fuel cores in units 2 and 3, and that the Cs isotopic ratios of CsMP were identical to those of units 2 and 3. These results indicated that U and Cs in the spherical CsMPs originated exclusively from the fuel melt in the reactors. Based on a thorough review of literatures related to the detailed atmospheric releases of radionuclides, the flow of plumes from the FDNPP reactor units during the accident and the U and Cs isotopic ratio results in this study, we hereby suggest that the spherical CsMPs originate only from the fuel in unit 2 on the night of 14 March to the morning of 15 March. The variation range of the analysed 235 U/ 238 U isotopic ratios for the four spherical particles was extremely narrow. Thus, U may have been homogenised in the source through the formation of fuel melt, which ultimately evaporating and taken into CsMPs in the reactor and was released from the unit 2.