Release of Plutonium Isotopes into the Environment from the

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Release of plutonium isotopes into the environment from the Fukushima Daiichi nuclear power plant accident: what is known and what needs to be known Jian Zheng, Keiko Tagami, and Shigeo Uchida Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es402212v • Publication Date (Web): 31 Jul 2013 Downloaded from http://pubs.acs.org on July 31, 2013

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Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Environmental Science & Technology

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

Jian Zheng*, Keiko Tagami, and Shigeo Uchida Research Center of Radiation Protection, National Institute of Radiological Sciences Anagawa 4-9-1, Inage, Chiba 263-8555, Japan

________________________________________________________________________________ *Corresponding author, Tel: 0081-43-206-4634, Fax: 0081-43-255-0721, email: [email protected] 1


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ABSTRACT 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 ×10-5 % 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.

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INTRODUCTION The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has caused serious contamination in the environment from the atmospheric fallout and the direct discharge of highly contaminated liquid wastes.1-13 In addition to the massive releases of highly volatile fission products, such as 127mTe, 132Te, 131I, 133Xe, 134Cs, 136Cs, and 137Cs,14-17 the possible release of non-volatile Pu isotopes has attracted considerable public attention because Pu isotopes present a large risk for internal radiation exposure via ingestions of contaminated agricultural crops, particularly for 241Pu (a β-emitter, T1/2 = 14.4 years). As

241

Pu decays, the ingrowth of

241

Am (an α and γ-emitter, T1/2 = 432.7 years)

will present a new radiation risk. Plutonium is generally produced in reactor fuel as a mixture of isotopes. The predominant isotope, 239Pu (T1/2 = 24100 years) is produced by neutron capture in 238U. If a fuel element containing

239

Pu is left in a reactor for any length of time further

neutron capture can occur, yielding isotopes with higher mass such as 240Pu (T1/2 = 6560 years), 241Pu and 242Pu (T1/2 = 3.76×105 years). In addition, small quantities of two other isotopes,

236

Pu (T1/2 = 2.87 years) and

238

Pu (T1/2 = 87.74 years), are produced during

the irradiation of the fuel in normal operating conditions.18 The isotopes 240

Pu, and 242Pu all decay by emission of αparticles, while

3


241

238

Pu,

239

Pu,

Pu undergoes β decay

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to produce α- and γ-emitting

241

Am. The FDNPP consisted of six boiling water

reactor units (Units 1 to 6). Among the four damaged reactor units (Units 1 to 4), three reactors (Units 1 to 3) were in operation at the time of the earthquake which triggered the nuclear crisis. Unit 4 had been shut down since November 30, 2010, and all of its fuel had been removed and stored in its spent fuel pool. Most of the fuel in the FDNPP reactors was UO2, but Unit 3 reactor had 32 mixed-oxide fuel assemblies containing ∼ 6% Pu, corresponding to ∼4% of the core loading.19 The reactor cores of Units 1 to 3 contained about 256 metric tons of nuclear fuel. The spent fuel pools located in the four damaged reactor buildings contained an additional 461 tons of nuclear fuel; among them, the spent fuel pool in the Unit 4 reactor building was the largest one on site, containing 1331 spent nuclear fuel assemblies, which is 1.4 times the amount of fuel loaded in Units 1 and 3.20-21 Therefore, two possible sources for the plutonium released to the environment from the FDNPP accident can be considered: release from the damaged reactor cores of Units 1 to 3, and the release from the spent fuel pools. However, to date, information on the release of Pu isotopes from the FDNPP accident to the environment is very limited. Several studies, listed in Table 1, on the determination of Pu isotopes in environmental samples, such as soil, litter, and aerosols, have confirmed the release of

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Pu isotopes from the FDNPP accident.22-27 Recently, core inventory calculation results of Pu isotopes within the FDNPP reactors using the ORIGEN model simulation have been reported.20, 28-29 These studies suggested that the damaged reactors were the major contributor of Pu to the environment. However, the release of Pu from the spent fuel pools remains unknown. In this work, we summarize and analyze the published studies related to the release of Pu isotopes from the FDNPP accident. Our analysis focuses on: (1) the environmental distribution of Pu isotopes derived from the FDNPP accident; (2) the Pu isotopic composition for source identification; (3) the sources for Pu release in the damaged reactor units or spent fuel pools; and (4) the amounts of Pu isotopes released from the FDNPP accident. We conclude with recommendations for future research directions.

ENVIRONMENTAL DISTRIBUTION OF PLUTONIUM ISOTOPES DERIVED FROM THE FDNPP ACCIDENT According to the air dose monitoring data from the Japanese MEXT (Ministry of Education, Culture, Sports, Science and Technology) and the atmospheric dispersion simulation by SPEEDI, a high concentration plume of released radionuclides moved towards the northwest from the power plant during the daytime on 15 March 2011;

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subsequently, large amounts of radionuclides were deposited on the ground by precipitation.14 Investigation conducted by the MEXT using alpha counting technique, which cannot resolve

240

Pu (5.168 MeV (73.5%), 5.123 MeV (26.4 %)) from

239

Pu

(5.157 MeV (73.3 %), 5.144 MeV (15.1 %), 5.106 MeV (11.5 %)) and is usually reported as the sum of 239Pu and 240Pu activity (i.e.,

239+240

Pu), on the distribution of Pu

isotopes in surface soils in Fukushima Prefecture found no significant increase of 239+240

Pu activity in soil samples; however,

238

Pu/239+240Pu activity ratios (0.33-2.2)

higher than that of global fallout (0.026) were detected at five sites, indicating a possible distribution of Pu related to the accident northwest of the FDNPP.22 Similarly, using the alpha counting technique, a field survey was made soon after the accident in some heavily contaminated areas outside the 20 km exclusion zone, as well as in Okuma Town adjacent to the plant.23-26 The anomaly of the 238Pu/239+240Pu activity ratios (0.059-2.60) indicated the presence of trace amounts of Pu isotopes originating from the accident in soils from Iitate Village and Okuma Town. These studies, however, did not provide more information on Pu isotopic composition in the environmental samples. In particular,

241

Pu, the principle isotope contributing to the dose due to external

exposure from radioactivity deposition after the accident, was not considered in those investigations.

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The release of

241

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Pu from the FDNPP accident was confirmed for the first time by

Zheng et al.27 They analyzed Pu activity (239+240Pu and composition (240Pu/239Pu and

241

241

Pu) and its isotopic

Pu/239Pu atom ratios) using a sector-field ICP-MS

analytical method 32 in surface soil and litter samples collected in Fukushima Prefecture in the 20-30 km zones (sampling dates, April and May, 2011) and soil samples collected in Mito City, Kamagaya City and Chiba City (Sampling dates, April–August, 2011). For the samples collected in Fukushima Prefecture, activities of 239+240Pu ranged from 0.019 to 1.4 mBq/g, which were within the typical global fallout

239+240

Pu activity range of

0.15 to 4.31 mBq/g observed in Japanese soils before the FDNPP accident.33 However, high activities of 241Pu ranging from 4.52 to 34.8 mBq/g were detected in samples of the J-Village surface soil (0-2 cm) and in litter at sites S2 and S3 (Fig. 1a).

241

Pu was

released into the environment during atmospheric nuclear weapons testings in the last century. Because of its short half-life of 14.4 years, the activity of

241

Pu in Japanese

soils before the FDNPP accident was quite low (ca. 1.2 for 241Pu/239+240Pu activity ratio, 241

Pu decay corrected to March 15, 2011). Therefore, the finding of high 241Pu activities

in these samples suggested an additional Pu input. The 240Pu/239Pu and 241Pu/239Pu atom ratios found in these samples ranged from 0.303 to 0.330 and from 0.103 to 0.135, respectively. They were significantly higher than those of global fallout (0.180±0.007,

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1σ for 240Pu/239Pu atom ratio, and 0.00194±0.00014, 1σ for 241Pu/239Pu atom ratio)34 and the atmospheric fallout deposition in Japan from 1963 to 1979 (0.1922±0.0044, 1σ for 240

Pu/239Pu atom ratio, and 0.00287±0.00056, 1σ for 241Pu/239Pu atom ratio),35 indicating

new Pu input from the FDNPP accident. We noted that in the surface soil (0-1cm) under the litter layer at sites S3 and S2, no 241Pu was detected and 240Pu/239Pu atom ratios were 0.144 and 0.177, respectively, close to the global fallout value of 0.180. This phenomenon indicated that the released Pu deposited in the litter layer, had not reached the underlying surface soil by May 2011 when the samples were collected or only a trace amount of the FDNPP-sourced Pu deposited in the litter layer reached the underlying surface soil, which is not sufficient to alter the Pu isotopic composition. No 241

Pu could be detected in surface soils in Mito, Kamagaya and Chiba Cities, located

100-200 km away from the FDNPP; in addition, atom ratios of 240Pu/239Pu in these samples showed the typical global fallout value, indicating that the Fukushima-sourced contribution to the total Pu activity in these areas was negligible. Determination of 238Pu, 239+240

Pu activities in soils collected in central Tokyo also showed no Pu contamination

from the FDNPP accident.36 Schwantes et al.20 analyzed 238Pu/239+240Pu activity ratios observed in surface soils as they change with distance from the FDNPP soon after venting Units 1 and 3, based on

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the MEXT surface soil contamination investigation data.22 Using a simple model fitting, they found that

238

Pu/239+240Pu activity ratios decreased with distance from the source,

suggesting most of plutonium contamination was localized within a few tens of kilometers from the site. However, long-distance transport of Fukushima-source Pu was suggested by Lujaniene et al.27-29 They observed an elevated

238

Pu/239+240Pu activity

ratio (1.2) and 240Pu/239Pu atom ratio (0.244±0.018) in an integrated air sample collected from 23 March to 15 April, 2011 (n = 30) in Vilnius, Lithuania, and suggested 43%-59% Fukushima-derived 239+240Pu to the total

239+240

Pu activity. Obviously, further

study is needed to clarify the proposed long-distance transport of Fukushima-source Pu, as the possibility of contributions of re-suspended Pu from Siberian nuclear plants and the Chernobyl accident could not be excluded. In Korea, an attempt was made to analyze Pu isotopes in rainwater samples after the FDNPP accident, no Pu isotopes were detected above the minimum detectable activity (

Release of Plutonium Isotopes into the Environment from the

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