Abstract:

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released large amounts of radionuclides into the environment. In addition to the atmospheric releases, the release of highly contaminated water from the damaged reactor buildings resulted in the direct discharges of radionuclides into the sea. It has been estimated that the total amount of 137Cs released from the FDNPP accident to the atmosphere ranged from 15-35 PBq. For the refractory actinides, although in a tiny amount, the release of Pu isotopes has been detected in various environmental samples, such as soil, litter, aerosol, black substance and river sediment. The released radioactive materials posed radiation threat to human society. Thus, source identification  of radioactive contamination and long-term environmental behavior of released radioactive materials are important issues of study after the FDNPP accident.

Since the FDNPP accident, intensive studies on the distribution of released fission products, in particular 134Cs and 137Cs, in the environment have been conducted, and the activity ratio of 134Cs/ 137Cs has been widely used as a tracer for contamination source identification. However, due to the short half-life of 134Cs (2.06 y), this tracer will become unavailable in the near future. Thus, for long-term environmental behavior studies of the released radionuclides, new tracers which can be used in a time scale of several tens-years or hundred years are highly required. In this work, we discuss new geochemical tracers, such as Pu isotope ratios (240Pu / 239Pu and 241Pu/239Pu) and radioactive Cs isotope ratios (135Cs/137Cs and 135Cs/ 133Cs) for contamination source identification and long-term environmental studies of Pu and Cs released from the FDNPP accident. To establish the feasibility of the proposed tracers, we identified the isotopic compositions of radioactive Cs and Pu isotopes released from the FDNPP accident, and estimated the released amounts of 135Cs and Pu isotopes. The potential applications of the obtained FDNPP-sourced Cs and Pu fingerprints in the study of long-term environmental behavior of the released radioactive materials will be discussed.