Abstract
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 (235U and 238U) and caesium (133Cs, 134Cs, 135Cs, and 137Cs) 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 235U/238U 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 235U/238U 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.
Highlights
On March 2011, the Great East Japan Earthquake triggered the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident
Adachi et al.[13] and subsequent related studies[14,15,16,17,18,19] showed that (i) Cs is present in several mass percent; (ii) silicate glass is the matrix of particles; (iii) other elements that are dissolved in the glass are mostly iron (Fe), zinc (Zn), potassium (K), rubidium (Rb), tin (Sn), chlorine (Cl) and sometimes manganese (Mn) and Pb and (iv) the particles contain trace amounts of U and fission products (FPs)
We developed a wet separation method with counting by the NaI scintillation counter, which is described in Sample preparation and Fig. S1
Summary
On March 2011, the Great East Japan Earthquake triggered the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Ono et al.[22] found different types of CsMPs from those mentioned above in the northwestern region near the FDNPP These particles were estimated to be released from unit 1 based on the comparison with the 134Cs/137Cs activity ratio of the nuclear fuel at the time of the accident (the ratios of units 1, 2 and 3 were approximately 0.94, 1.08, and 1.05, respectively) calculated with the ORIGEN2 code by Nishihara et al.[23]. These particles were sampled within 5 km of the FDNPP (Fig. S4)
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