Identification, extraction, and analysis of Cs-bearing microparticles from Fukushima

Abstract

Following several explosions at the Fukushima Daiichi Nuclear Power Plant (FDNPP), after the severe earthquake and tsunami on March 11, 2011, large quantities of radioactive material were dispersed into the environment. This included the short-lived and volatile isotopes typically expected in such an accident, such as 131I and 133Xe, but also 134Cs and 137Cs, which have much longer half-lives.A special type of contamination are the so-called caesium microparticles (CsMPs). As already described in several publications, these contaminations are rich in cesium, especially in the two isotopes 134Cs and 137Cs, and typically have a rounded or even spherical shape. The origin of these microparticles has not yet been fully clarified, but it is assumed that they couldhave formed in the high-efficiency particulate air (HEPA) and pre-HEPA filters of the reactor buildings during the explosions. The fibers of these filters could have melted together with the released radioactive cesium, which would explain their round shape. They were also found to be largely made of silicon, which could be explained by the mineral fibers of the filters. In addition, some microparticles were found to contain large amounts of surface carbon, possibly originating from activated carbon filters. However, their exact origins have not yet been confirmed.Given their small size (up to a few hundred micrometers at most), they could travel great distances in the Japanese landscape from their origin in the nuclear power plant and even contaminate sealed buildings indoors. Depending on the characteristics of each microparticle, such as size, shape, activity and the ratio of 134Cs/137Cs, a system of categorization was established and further developed in some publications to identify the origins of these particles. Given the characteristic properties of the particles, attempts were made to determine the exact source of the particles in the FDNPP. Such environmental forensic methods allow the extent of the contamination to be further investigated and the background to the accident to be deciphered. In this thesis, samples collected years after the reactor accident at FDNPP were examined. One sample is a piece of a fleece jacket collected in 2015 from a locked house within the evacuation zone northwest of FDNPP. The other samples are several passive air samplers designed to collect dust and other aerosols without power. They were specifically designed to capture radioactive particles from the air, which can be resuspended even years after the accident. These were placed near FDNPP for several months and collected in 2023. In this paper, the results of the identified isotopes, the activity and distribution of the radionuclides on these samples are presented on the basis of autoradiographic images and low-level gamma spectrometry. In addition, in collaboration with the University Service Facility for Transmission Electron Microscopy (USTEM) at TU Wien, an attempt was made to find and extract candidates for individual Cs microparticles in the samples.