SPring-8, the large synchrotron radiation facility

Release Date

08 Aug, 2014

Tokyo University of Science

Details of study
   Cs balls are spherical particles with a diameter of approximately 2 µm and contain radioactive Cs, as indicated by their name. It has been pointed out that Cs balls were emitted during the FDNPP accident. The research group carried out X-ray analyses of Cs balls using BL37XU X-rays focused to a width and height of 1 µm or smaller for examining the chemical composition, chemical state, and crystal structure of the Cs balls at the single-particle level nondestructively and in detail.
   First, X-ray fluorescence analysis was performed using a high-energy X-ray (37.5 keV) to identify the elements other than Cs in the Cs balls. The results indicated that the elements considered to be the fission products of U fuel, such as barium (Ba), rubidium (Rb), and molybdenum (Mo), were contained in the Cs balls. In addition, U fuel itself was also detected in some Cs balls (Fig.1). These results suggest that not only was highly volatile Cs emitted from the FDNPP into the atmosphere but also the plant was damaged sufficiently to emit the U fuel to the outside.
   Moreover, elements originating from the components of the plant structure, such as silicon (Si), iron (Fe), and zinc (Zn), were also detected in the Cs balls. X-ray absorption near-edge structure and X-ray diffraction analyses revealed that the Cs balls were formed in the high-oxidation glassy states (Fe³⁺, Mo⁶⁺, Sn⁴⁺). These results indicate a scenario in which the fuel, containment vessel, and plant structures were melted and mixed in the plant during the accident and the mixture was emitted into the atmosphere and rapidly cooled to form Cs balls in the glassy state. This scenario gives chemical support to the hypothesis that the fuel rods melted and fell to the bottom of the containment vessel during the accident.
   Thus far, the possibility that U emitted during the accident reached near the Kanto Region has been already pointed out. However, how the heavy material U was dispersed in the environment has remained unclear. The research group clarified that the glass microparticles were one of the carriers of U.
   As above, nondestructive X-ray analyses of particles with a diameter of only 2 µm provided much information on the state of the plant during the accident and the emission of radioactive materials into the atmosphere. This could be only realized by multiple X-ray analyses using a SPring-8 synchrotron radiation X-ray microbeam. The achievements of this study were published in the ACS journal Analytical Chemistry.

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