Release of plutonium isotopes into the environment from the Fukushima Daiichi Nuclear Power plant accident: what is known and what needs to be known

The Fukushima Daiichi Nuclear Power Plant accident (FDNPP) has caused serious contamination in the environment. The release of Pu isotopes renewed considerable public concern because they present a large risk for internal radiation exposure. In this review, we summarize and analyze published studies related to the release of Pu from the FDNPP accident based on environmental sample analyses and the ORIGEN model simulations. Our analysis emphasizes the environmental distribution of released Pu isotopes, information on Pu isotopic composition for source identification of Pu releases in the FDNPP-damaged reactors or spent fuel pools, and estimation of the amounts of Pu isotopes released from the FDNPP accident. Our analysis indicates that a trace amount of Pu isotopes (ca. 2 x10-5 % of core inventory) was released into the environment from the damaged reactors, but not from the spent fuel pools located in the reactor buildings. Regarding the possible Pu contamination in the marine environment, limited studies suggest that no extra Pu input from the FDNPP accident could be detected in the western North Pacific 30 km off the Fukushima coast. Finally, we identified knowledge gaps remained on the release of Pu into the environment and recommended issues for future studies