IV Environmental Radioactivity
Distribution of Plutonium Isotopes in Marine Sediments off Japan Before and After the Fukushima Dai-ichi Nuclear Power Plant Accident: A Review
Wenting Bu, Jian Zheng, Qiuju Guo, Keiko Tagami, and Shigeo Uchida
Abstract A large amount of radionuclides was released into the atmosphere and directly discharged into the ocean as a consequence of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. The possible actinides, especially plutonium contamination in the ocean from the nuclear accident, attracted general public and scientific concern. In this review, we summarize studies of the distribution of Pu isotopes in the marine sediments off Japan before and after the FDNPP accident to assess possible Pu contamination from the nuclear accident. Our results suggested that there was no detectable additional Pu injection from the FDNPP accident in the marine environment 30 km off the FDNPP 1 year after the accident. The detectable Pu isotopes originated from global fallout and the Pacific Proving Ground close-in fallout.
Keywords FDNPP • Marine sediments • Plutonium isotopes
On March 11, 2011, the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) was hit and damaged by a magnitude 9.0 earthquake in the Northwest Pacific about 130 km off the northeast coast of Japan and the following gigantic tsunami. As a result, large amounts of radionuclides were released into the atmosphere and directly discharged into the sea. More than 70 % of the released radionuclides were deposited over the North Pacific Ocean . Studies concerning the fission products, such as 131I and radiocesium, were carried out intensively after the nuclear accident. However, information about actinides, especially the plutonium (Pu) isotopes, is limited.
Pu isotopes are of public and scientific interest because they are characterized by radiotoxicity and strong chemical toxicity. They contribute much to the longterm dose for humans because of their relatively long half-lives (238Pu, 87.7 years; 239Pu, 24,110 years; 240Pu, 6,561 years; 241Pu, 14.4 years). Investigating the characterization of Pu isotopes in marine sediments is important for purposes of environmental monitoring and radiotoxicity assessment. Moreover, Pu isotopes are considered as new geochemical tracers for the study of oceanic processes, such as sedimentation and particle scavenging, and the isotopic composition of Pu (240Pu/239Pu) has been used as an important fingerprint for environmental radioactive source identification [2–5].
Abnormal atom ratios of 240Pu/239Pu and 241Pu/239Pu, and activity ratios of 241Pu/239+240Pu and 238Pu/239+240Pu, have been reported in surface soil and litter samples in the 20to 30-km zone around the FDNPP [6, 7], suggesting the atmospheric release of Pu isotopes from the nuclear accident. For the marine environment, in previous studies [8, 9] we investigated the distribution of Pu isotopes in marine sediments collected in the Pacific Ocean, 30 km off the FDNPP site, from July 2011 to July 2012 after the nuclear accident. Sakaguchi et al.  determined Pu concentration in seawaters in the Pacific, 50 km off the FDNPP, and they observed no significant amount of extra components from the nuclear accident. However, possible long-term Pu contamination from the nuclear accident in the marine environment, especially in the Japanese near-coastal (within 30 km) marine environment, remains unknown because so far no information on Pu isotopes in the released radioactive liquid and in the FDNPP near-coastal marine environment is available.
Pu isotopes are particle reactive and can be easily incorporated into the sediments in the marine environment. Radiocesium (134Cs and 137Cs) released from the FDNPP accident was detected in sinking particles collected from the deep sea in the Western North Pacific 1 month after the accident . The sediment–water distribution coefficient of Pu is two orders of magnitude higher than that of Cs . Thus, Pu could be more easily incorporated into the sediments, and Pu distribution in the marine sediments could give direct information about Pu contamination in the marine environment.
In this review, we first summarize studies about the distribution of Pu isotopes in the marine sediments off Japan before and after the FDNPP accident. Then, we compare the results to assess possible contamination by Pu from the FDNPP accident. Perspectives about future research are presented as well.