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内容記述 |
On March 11, 2011, a magnitude 9.0 earthquake occurred in the western North Pacific about 130 km off the northeast coast of Japan followed by a tremendous tsunami which arrived at Fukushima Daiichi Nuclear Power Plant (FDNPP) about 45 min later. The tsunami severely damaged the nuclear reactor cooling system, leading to hydrogen explosions in the reactor buildings. As a result, massive radionuclides were released into the environment. As one of the most important actinides, Pu isotopes attracted great public attention after the FDNPP accident because they present a high risk for internal radiation exposure via ingestion of contaminated agricultural crops and seafood. It has been estimated that the total amount of 137Cs released from the FDNPP accident ranged from 15-35 PBq, while for the refractory actinides, in particular, Pu isotopes, tiny amount (2x10-5% of the core inventory) was released into the environment [1,2]. This study reviewed the current knowledge of plutonium (Pu) isotopic composition (the atom or activity ratios) and activities of 238Pu, 239Pu, 240Pu, and 241Pu derived from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011. In this critical review, we documented the characteristic values of Pu atom or activity ratios (fingerprints) and presented its spatial distributions around the FDNPP site. Based on multiple Pu fingerprints (238Pu/239+240Pu activity ratio, 240Pu/239Pu atom ratio, and 241Pu/239Pu atom ratio), we clarified there received the Pu contamination from the FDNPP accident in a restricted terrestrial area, while Pu in the northwest Pacific Ocean is still predominately sourced from the Pacific Proving Grounds (PPG) fallout and the global fallout. Using a simple two end–member mixing model, the contributions of Pu from the FDNPP accident were calculated to be on average 27±34% (n=149) in soils, 55±32% (n=38) in litter samples, and 67±26% (n=129) in the air dust/black substances, respectively. In the marine environment, the PPG source contributed 45±15% (n=76) in the seawaters and 42±12% (n=48) in the sediments [3]. The spatial distribution of Pu atom or activity ratios based on existing studies suggested that: 1) the Pu contamination in the northwestern FDNPP site was mainly observed in an area within 50 km radius; 2) in the southwestern FDNPP site, the Pu contamination was mainly observed in a zone within 30 km radius. The studies of Cs-bearing radioactive particles indicated that Pu occurred as Pu oxide and the fuel fragments containing Pu were released from the reactors to the surrounding environment associated with micron-scale Cs-bearing radioactive particles [4]. It should be noted that the fractionation between Pu and other radionuclides occurred after release. Finally, these new findings about the Pu fingerprints around the FDNPP site help us to establish a reference background database for the coming environmental risk assessment and geochemical study in this region. |
