Identification of Plutonium isotopes in radioactive particles released from the Fukushima Daiichi Nuclear Power Plant

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident resulted in the large release of radionuclides into the environment. The release process and environmental behavior of volatile fission product such as radioactive Cs was mainly investigated. On the other hand, the studies of nonvolatile fuel elements, such as Pu, have still been limited.It is known that the amount of Pu released from the FDNPP accident was much smaller compared with that of radioactive Cs [1]. However, the existence of Pu isotopes from the global fallout which was mainly derived from the atmospheric nuclear weapon tests makes it difficult to accurately assess Pu contamination from the FDNPP accident. In many environmental samples, Pu was affected not only by the FDNPP accident but also by global fallout. It is necessary to distinguish the source of Pu contaminations to assess the accurate Pu contamination from the FDNPP accident.For identification of Pu contamination source, the Pu isotopic ratios provide important information. The ratios were completely different by the kinds of nuclear weapons and nuclear accidents. The Pu isotopic ratios were measured in some environmental samples around the FDNPP. However, the ratios varied by the researches due to the contamination by global fallout. Therefore, the ratios should be determined from the environmental samples which were not influenced by the global fallout.In the FDNPP accident, radioactive particles which were insoluble in water and containing highly concentrated radioactive Cs were reported [2]. The radioactive particles were estimated to be formed in the reactor during the meltdown and directly released from the reactor. Due to its insolubility, the radioactive particles kept their elemental composition in the environment and retained the original property at the time of emission from the reactor. In other words, the radioactive particles are free from the contamination of global fallout. In this study, we identified Pu isotopes in radioactive particles found around the FDNPP. We performed radiochemical analysis and inductively coupled plasma-mass spectroscopy (ICP-MS) measurements of Pu isotopes for radioactive particles. Figure 1 shows the scanning electron microscopy (SEM) image of the radioactive particle analyzed in this study. For this particle, 239+240Pu activity was (1.70 ± 0.20) × 10-5 Bq with a 240Pu/239Pu atom ratio of 0.330 ± 0.077. The Pu isotopic ratio was higher than the ratio from global fallout (0.18) [3], but consistent with the simulation results from the ORIGEN code [4] and measurement results from some environmental samples around the FDNPP that were considered less influenced by global fallout [1]. We successfully determined the accurate Pu isotopic ratios from the FDNPP accident by analyzing radioactive particles. In this presentation, the properties of the investigated radioactive particles will be presented, and detailed experimental method will also be shown. In addition, we will discuss about Pu isotopic ratios in radioactive particles and compare the results with other environmental samples.Envira2019, 5th International conference on Environmental Radioactivity Variations of Environmental radionuclide