2:15 PM - 2:30 PM
[MAG38-03] Temporal variations and future estimations of 90Sr and 137Cs in atmospheric depositions after the Fukushima Daiichi Nuclear Power Plant accident with long-term observations
★Invited Papers
Keywords:Environmental radionuclide, Fukushima, Atmosphere, Resuspension, Deposition, Fallout
Atmospheric nuclear tests and nuclear power plant accidents have released artificial radionuclides into the atmosphere, land surface, and ocean. We have measured the artificial radionuclides, especially 90Sr and 137Cs, in atmospheric depositions since 1957 in the Kanto areas around Tokyo, Japan. As a result, we clarified the variations in 90Sr and 137Cs, which were emitted from atmospheric nuclear tests and nuclear power plant accidents, and their environmental processes due to their diffusion, deposition, and resuspension (figure). In March 2011, the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident occurred in Japan, and significant increases in 90Sr and 137Cs were detected at our observation sites: a suburban site in the Kanto Plain (site A) and a top of the mountain in the northwestern corner of the Kanto Plain (site B), respectively. In this study, we show our long-term observation results of 90Sr and 137Cs in monthly atmospheric deposition samples collected at sites A and B and estimate the current environmental processes and decay periods of 90Sr and 137Cs with measurements of 134Cs and stable elements and isotopes (Na, Mg, Al, K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, Sr, Ba, 9Be, 133Cs, 232Th, and 238U).
During the period of plume arriving at site A from the FDNPP, 90Sr and 137Cs in monthly deposition samples increased to 2.7×103 and 3.2×106 times, respectively, higher than those before the accident. Our continual observations revealed that the peak of 137Cs due to the FDNPP accident was much higher than those from the nuclear weapon tests and the Chernobyl accident. On the other hand, the peak of 90Sr due to the FDNPP accident was lower than that from the nuclear weapon tests in the 1960s. The monthly deposition rate of 137Cs in 2018 declined to ~1/8100 and ~1/4500 of the peak levels at sites A and B, respectively, but it remained more than ~400 and ~130 times higher than those before the accident. Cesium-134 was also measured at both sites until recent observations, implying that most of the radioactive Cs were originated from the FDNPP. The current 90Sr deposition, on the other hand, has returned to the same radioactive level as that before the accident at both sites. The chemical analysis suggested that dust particles were the major carriers of 90Sr and 137Cs during the resuspension period at site A. On the other hand, at site B, 90Sr was reserved and recycled in the forest, but the source of 137Cs could not be identified. Presently, the effective half-lives for 137Cs deposition at sites A and B due to radioactive decay and other environmental factors are estimated as 4.7 and 5.9 years, respectively. These estimations suggest that approximately 42 and 48 years from 2011 are required to reduce the atmospheric 137Cs deposition to a state similar to that before the accident at sites A and B.
During the period of plume arriving at site A from the FDNPP, 90Sr and 137Cs in monthly deposition samples increased to 2.7×103 and 3.2×106 times, respectively, higher than those before the accident. Our continual observations revealed that the peak of 137Cs due to the FDNPP accident was much higher than those from the nuclear weapon tests and the Chernobyl accident. On the other hand, the peak of 90Sr due to the FDNPP accident was lower than that from the nuclear weapon tests in the 1960s. The monthly deposition rate of 137Cs in 2018 declined to ~1/8100 and ~1/4500 of the peak levels at sites A and B, respectively, but it remained more than ~400 and ~130 times higher than those before the accident. Cesium-134 was also measured at both sites until recent observations, implying that most of the radioactive Cs were originated from the FDNPP. The current 90Sr deposition, on the other hand, has returned to the same radioactive level as that before the accident at both sites. The chemical analysis suggested that dust particles were the major carriers of 90Sr and 137Cs during the resuspension period at site A. On the other hand, at site B, 90Sr was reserved and recycled in the forest, but the source of 137Cs could not be identified. Presently, the effective half-lives for 137Cs deposition at sites A and B due to radioactive decay and other environmental factors are estimated as 4.7 and 5.9 years, respectively. These estimations suggest that approximately 42 and 48 years from 2011 are required to reduce the atmospheric 137Cs deposition to a state similar to that before the accident at sites A and B.