Pu levels in the environment after the Fukushima accident

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 (Zheng et al., 2012; 2013).For Pu isotopes, it is difficult to estimate the contamination level caused by the FDNPP accident if only Pu activity measurement is conducted due to the fact that only tiny amount of core Pu was released. As shown in Figure 1, a comparison of 239+240Pu activity in surface soils (0-5 cm) before and after the FDNPP accident indicated there was no increase of 239+240Pu activity after the accident. Experience obtained from studies on Pu contamination in the Chernobyl accident and other global and/or regional Pu contamination events indicated that accurate determination of Pu isotopic composition (in particular 240Pu/239Pu and 241Pu/239Pu atom ratios) is essential for reliable source identification. This study will focus on the distribution of Pu isotopes in the terrestrial environment and the marine environment following the Fukushima nuclear accident and identification of the contamination sources of Pu isotopes, and using a two end-member mixing model to estimate the contamination levels of Pu in the environment after the Fukushima accident. Based on the contamination levels of Pu isotopes, the potential radiation risk to humans was estimated. In addition, recent studies on the distribution of Pu isotopes in the radioactive Cs-bearing particles will be discussed to understand the release process of Pu in the Fukushima nuclear accident.the 6th International Conference on Environmental Radioactivity, ENVIRA 2021: Fukushima Accident – 10 years of Environmental Investigations, and New Challenges in Environmental Radioactivity Studie