EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmospheric Atmospheric Chemistry Chemistry and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques Discussions Open Access Biogeosciences, 10, 2497–2511, 2013 Open Access www.biogeosciences.net/10/2497/2013/ Biogeosciences doi:10.5194/bg-10-2497-2013 Biogeosciences Discussions © Author(s) 2013. CC Attribution 3.0 License. Open Access Open Access Climate Climate of the Past of the Past Discussions Vertical distributions of plutonium isotopes in marine sediment Open Access Open Access cores off the Fukushima coast after the FukushimaEarth Dai-ichi System Earth System Dynamics Dynamics Nuclear Power Plant accident Discussions W. T. Bu1,2, J. Zheng2, T. Aono2, K. Tagami2, S. Uchida2, J. Zhang3, M. C. Honda4, Q. J. Guo1, and M. Yamada5 Open Access 1State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University,Geoscientific Geoscientific Open Access Beijing 100871, China Instrumentation Instrumentation 2 Research Center of Radiation Protection, National Institute of Radiological Sciences, AnagawaMethods 4-9-1, Inage, and Methods and Chiba 263-8555, Japan 3Department of Environmental Biology and Chemistry, University of Toyama, 3190 Gofuku,Data Toyama Systems 930-8555, Japan Data Systems 4Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 2–15 Natsushima, Discussions Open Access Open Access Yokosuka 237-0061, Japan Geoscientific 5Department of Radiation Chemistry, Institute of Radiation Emergency Medicine, HirosakiGeoscientific University, 66-1 Hon-cho, Hirosaki, Aomori 036-8564, Japan Model Development Model Development Discussions Correspondence to: J. Zheng ([email protected]) Open Access Received: 21 December 2012 – Published in Biogeosciences Discuss.: 10 January 2013 Open Access Revised: 22 March 2013 – Accepted: 22 March 2013 – Published: 15 April 2013 Hydrology and Hydrology and Earth System Earth System Abstract. The Fukushima Dai-ichi Nuclear Power Plant global fallout and Pacific ProvingSciences Ground close-in fallout. Sciences (FDNPP) accident led to the release of large amounts of Discussions Open Access radionuclides into the atmosphere as well as direct dis- Open Access charges into the sea. In contrast to the intensive studies on Ocean Science the distribution of the released high volatility fission prod- Ocean Science ucts, such as 131I, 134Cs and 137Cs, similar studies of the 1 Introduction Discussions actinides, especially the Pu isotopes, are limited. To ob- tain the vertical distribution of Pu isotopes in marine sed- On 11 March 2011, a magnitude 9.0 earthquake, centered in Open Access iments and to better assess the possible contamination of the northwest Pacific about 130 km off the northeastOpen Access coast of Pu from the FDNPP accident in the marine environment, Japan, and the ensuing gigantic tsunami caused severe dam- Solid Earth we determined the activities of 239+240Pu and 241Pu as well age to the Fukushima Dai-ichiSolid Nuclear Earth Power Plant (FD- Discussions as the atom ratios of 240Pu/239Pu and 241Pu/239Pu in sedi- NPP). The cooling systems of some of the nuclear reactor ment core samples collected in the western North Pacific off units failed, resulting in hydrogen explosions in the reac- Fukushima from July 2011 to July 2012. We also measured tor buildings and venting of gases, which caused large re- Open Access Open Access surface sediment samples collected from seven Japanese es- leases of radionuclides into the atmosphere. For example, tuaries before the FNDPP accident to establish the compre- the total released amount of 137Cs to the atmosphere from The Cryosphere the FDNPP reactorsThe has been Cryosphere estimated to be in the range hensive background baseline data. The observed results of Discussions both the Pu activities and the Pu atom ratios for the sedi- of 9.9–36.6 PBq (Chino et al., 2011; Morino et al., 2011; ments in the western North Pacific were comparable to the Stohl et al., 2012). More than 70 % of the released radionu- baseline data, suggesting that the FDNPP accident did not clides were deposited over the North Pacific (Yoshida and cause detectable Pu contamination to the studied regions Kanda, 2012). In addition, highly contaminated water with prior to the sampling time. The Pu isotopes in the western large amounts of radionuclides, originating from desperate North Pacific 30 km off the Fukushima coast originated from attempts to prevent reactor cores meltdowns by injecting wa- ter into the reactor units, was directly leaked or discharged Published by Copernicus Publications on behalf of the European Geosciences Union. 2498 W. T. Bu et al.: Vertical distributions of plutonium isotopes in marine sediment cores into the North Pacific Ocean (Buesseler et al., 2011; Inoue et (Buesseler, 2012). The radiocesium (134Cs and 137Cs) orig- al., 2012; Tsumune et al., 2012). inating from the accident was detected in the sinking parti- Intensive studies on the high volatility fission products re- cles collected from the deep sea (water depth 4810 m) in the leased into the ocean, such as 131I, 134Cs and 137Cs, were western North Pacific, one month after the accident, suggest- carried out after the FDNPP accident, and data on the con- ing the quick transportation of radionuclides to the deep sea centration and distribution of these products were immedi- via atmospheric deposition (Honda et al., 2013). Otasaka and ately collected (Aoyama et al., 2013; Buesseler et al., 2011; Kobayashi (2012) observed that dissolved radiocesium from Honda et al., 2012; Inoue et al., 2012). However, similar stud- the FDNPP accident was advected southward where it de- ies focusing on the possible released actinides, especially Pu posited onto sediments along the Ibaraki coast early in the isotopes, are limited. Pu isotopes are characterized by high accident. Pu isotopes are more particle reactive than Cs and chemical toxicity, radiotoxicity and long half-lives, and they the sediment-water distribution coefficient (Kd value) of Pu have attracted much scientific and public concern. Investi- is two orders of magnitude higher than that of Cs (IAEA, gating the impact of the FDNPP accident on the distribution 2004). In mid-April 1986, by a coincidence of timing, IAEA- of long half-live radionuclides in the environment is impor- MEL scientists moored automated time series sediment traps tant for the long-term dose assessments (Yoshida and Kanda, in the Ligurian Sea, and they found that in one month after 2012). In addition, as Pu isotopes are produced by the initial the accident more than 50 % of the total 239+240Pu inven- neutron capture reaction in 238U in the nuclear reactors and tory originating from Chernobyl and deposited in that re- directly used as the component of MOX fuel in the FDNPP gion had transited through 200 m depth, while only 0.2 % unit 3 reactor (Burns et al., 2012), accurate determination of of the corresponding 137Cs deposition did so (Povinec et Pu isotopes in the environmental materials may provide im- al., 1996). In the North Pacific, the Chernobyl-derived ra- portant information to understand the reactors damages. diocesium was detected in the sinking particles collected at Abnormal atom ratios of 240Pu/239Pu (> 0.3) and activ- a depth of 780 m two months after the Chernobyl accident ity ratios of 241Pu/239+240Pu (> 100) were reported in the (Kusakabe et al., 1988). The resident time of Pu isotopes in surface soil and litter samples in the 20–30 km zone around the North Pacific was much shorter than that of radiocesium the FDNPP, providing evidence for the atmospheric release (Fowler et al., 1983; Honda et al., 2013). Thus Pu isotopes of Pu into the terrestrial environment (Zheng et al., 2012a). released from the FNDPP accident could be more quickly Imanaka et al. (2012) and Yamamoto et al. (2012) also found incorporated into sediments by the scavenging process than 238Pu/239+240Pu activities ratios, higher than the global fall- Cs, and the determination of Pu isotopes in the sediments out value, for soil samples collected in Iitate Village and should give reliable information about Pu contamination in Okuma Town, at distances of 25–45 km from the plant. In the marine environment. the marine environment, we investigated the distribution of To compare the distributions of Pu isotopes in the marine Pu isotopes in surface marine sediments in the Pacific 30 km sediments before and after the FNDPP accident, the back- off the FDNPP that were collected several months after the ground dataset needs to be established. Due to the potential accident (Zheng et al., 2012b), and observed no significant applications of Pu isotopes as a chemical tracer for oceanic variation in 239+240Pu activity and 240Pu/239Pu atom ratio processes and as a source identifier for radioactive contami- compared with previously reported values in marine sedi- nation, over the past decades, the distribution of Pu isotopes ments in the western North Pacific. Meanwhile, Sakaguchi in the western North Pacific and its marginal seas have been et al. (2012) found that Pu concentration in seawater sam- studied intensively (Buesseler, 1997; Dong et al., 2010; Hong pled from the Pacific Ocean 50 km from the FDNPP site et al., 1999; Ito et al., 2007; Kim et al., 2003; Lee et al., 1998, showed no extra component from the accident. However, the 2003, 2004, 2005; Lindahl et al., 2011; Liu et al., 2011a; possible long-term Pu contamination in the marine environ- Moon et al., 2003; Nagaya and Nakamura, 1992; Oikawa et ment remains unknown as no information is currently avail- al., 2011; Otosaka et al., 2006; Pettersson et al., 1999; Wang able on the Pu isotopes in the released radioactive liquids and Yamada, 2005; Zheng and Yamada, 2005, 2006b, c).