Plutonium in the atmosphere: A global perspective

Journal of Environmental Radioactivity 175-176 (2017) 39e51

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Plutonium in the atmosphere: A global perspective P. Thakur a, *, H. Khaing a, S. Salminen-Paatero b a b

Carlsbad Environmental Monitoring & Research Center, 1400 University Drive, Carlsbad, NM 88220, USA Department of Chemistry - Radiochemistry, P.O. Box 55, 00014, University of Helsinki, Finland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 February 2017 Received in revised form 6 April 2017 Accepted 9 April 2017

A number of potential source terms have contributed plutonium isotopes to the atmosphere. The atmospheric nuclear weapon tests conducted between 1945 and 1980 and the re-entry of the burned SNAP-9A satellite in 1964, respectively. It is generally believed that current levels of plutonium in the stratosphere are negligible and compared with the levels generally found at surface-level air. In this study, the time trend analysis and long-term behavior of plutonium isotopes (239þ240Pu and 238Pu) in the atmosphere were assessed using historical data collected by various national and international monitoring networks since 1960s. An analysis of historical data indicates that 239þ240Pu concentration post1984 is still frequently detectable, whereas 238Pu is detected infrequently. Furthermore, the seasonal and time-trend variation of plutonium concentration in surface air followed the stratospheric trends until the early 1980s. After the last Chinese test of 1980, the plutonium concentrations in surface air dropped to the current levels, suggesting that the observed concentrations post-1984 have not been under stratospheric control, but rather reflect the environmental processes such as resuspension. Recent plutonium atmospheric air concentrations data show that besides resuspension, other environmental processes such as global dust storms and biomass burning/wildfire also play an important role in redistributing plutonium in the atmosphere. © 2017 Published by Elsevier Ltd.

Contents 1. 2. 3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Data source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.1. Time trend analysis of 239þ240Pu in the stratosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.2. Time trend analysis of 239þ240Pu in the surface air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.3. Time trend analysis of 238Pu in the stratosphere and surface air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 3.4. Plutonium isotopic ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.5. Factors controlling plutonium concentrations in the air today . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3.5.1. Resuspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3.5.2. Global dust events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.5.3. Wildfire and biomass burning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.5.4. Large-scale volcanic eruptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.5.5. Sea-spray effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

1. Introduction * Corresponding author. E-mail address: [email protected] (P. Thakur). http://dx.doi.org/10.1016/j.jenvrad.2017.04.008 0265-931X/© 2017 Published by Elsevier Ltd.

Plutonium is not naturally present in measurable quantities in

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the ambient air. With few exceptions, the atmospheric nuclear tests have become by far the major source of plutonium in the environment. According to Beck and Bennett 2002, a total of 543 nuclear weapons tests were conducted worldwide in the atmosphere between 1945 and 1980. Besides atmospheric nuclear tests, satellite accidents and nuclear power plant accidents such as Chernobyl and Fukushima also released plutonium into the environment. Additionally, local and regional contaminations of plutonium in the environment have been resulted from nuclear fuels reprocessing facilities. The first injection of plutonium into the atmosphere occurred in July 1945 with the detonation of the first plutonium device at the “Trinity Site” near Alamogordo in New Mexico, USA. Since then, approximately 6.52 PBq (1 PBq ¼ 1  1015 Bq) of 239Pu, 5.35 PBq of 240Pu, and 142 PBq of 241Pu were released into the atmosphere from nuclear weapons testing (UNSCEAR, 2000a). The high altitude destruction of the SNAP-9A satellite power source over the South Pacific in 1964 contributed about 0.6 PBq of 238Pu to the global inventory (Krey, 1968). The Chernobyl accident in April 1986, which released about 6.1 PBq of plutonium isotopes, increased the concentration of 239þ240Pu in surface air during 1986e1987, especially in Europe and contributed slightly to the plutonium-global inventory (UNSCEAR, 2000b). The missile fire at the U.S McGuire Air Force Base in New Jersey in 1960 (Lee and Clark, 2005), the B-52 accidents in Palomares, Spain in 1966 (Espinosa et al., 1999) and the US aircraft carrying four hydrogen bombs crashed on the sea ice near Thule, Greenland in 1968 (Lind et al., 2005) resulted in the dispersion of weapon grade plutonium into the environment. Initial surveys of Palomares, showed that 2.26 km2 of urban areas and farmlands were contaminated with more than 1.2 MBq/m2 of alpha emitting radionuclides. The site was remediated by soil removal. The Thule accidents released 10 TBq of plutonium on snow surfaces and an additional 1 TBq was estimated to be taken into ice. It has been estimated that about 90% of the released plutonium was removed in clean-up operations. Sources of environmental plutonium can be categorized as a global source (stratospheric or upper atmosphere) that distributed plutonium around the world, and local or regional sources (tropospheric or lower atmosphere) that distributed plutonium on a much smaller spatial scale. In general, most of fallout debris produced by high-yield atmospheric test (>500 kt TNT) injected into the stratosphere (global fallout), while those of the low-yield atmospheric test (<100 kt TNT) injected into the troposphere and deposited downwind from the nuclear test sites (local or regional fallout). Whether a test has stratospheric input or not is determined by the explosive yield, the height at which the detonation occurred, and the meteorological conditions existing at the time of detonation. Overall, about 85% of the plutonium injected into the atmosphere has been deposited globally and 15% locally or regionally (UNSCEAR, 2000a). Stratosphere thus serves as a main reservoir of bomb-derived plutonium in the environment. Once released into the atmosphere, the radioactive debris tends to attach to sub-micrometer aerosols, which determines the mechanism governing their atmospheric transport. In the troposphere, the wet (rainfall or snowfall) and dry deposition remove most radioactive debris fairly quickly within a few weeks to months. However, in the stratosphere radioactive debris descends much more slowly because of the thermal stratification and separation from the troposphere by the tropopause (boundary between the stratosphere and the troposphere). The dominant process for the removal of these particles from the stratosphere is to move these particles first into the troposphere where they can be brought down by rainfall or snowfall. Since the processes responsible for stratospheric-tropospheric exchange depend on the time of the year, the occurrence of stratospheric fallout at the earth surface is

seasonally modulated. The maximum stratospheric-tropospheric exchange occurs in spring (MarcheJune). Consequently, plutonium concentrations varied seasonally, being highest in spring and lowest in winter due to the springtime enhanced transport of radionuclides from the stratosphere to troposphere. A typical residence time for aerosols in the troposphere is reported to be around 70 days, and that at stratosphere is about 15e18 months (Warneke et al., 2002). However, after the large-scale atmospheric nuclear tests ban treaty in 1963, only small amount of new bomb-derived plutonium was injected into the stratosphere from atmospheric nuclear weapons testing by France and China during the 1970s and 1980s. It is therefore generally accepted that the current level of plutonium in the stratosphere is negligible and that most of plutonium in the air today is associated with resuspended soil, which is contaminated from nuclear weapons testing fallout. At present, almost all plutonium released into the atmosphere can be found in the surface soil or oceans. Deposited plutonium particle from the fallout can resuspended into the air with eroded soil particles. These aerosol particles can be trapped on a filter in an air monitoring station or subjected to wash-down from the atmosphere with precipitation (rainfall, snowfall). Air samples can thus give information about activity levels both in the air and soil of a particular area, and allow evaluation of seasonal variations of plutonium in the air. In this short review, the past and present levels of plutonium in the surface air were assessed using historical data from several monitoring stations around the world as shown in Fig. 1 to research the distribution and long-term behavior of plutonium in the atmosphere. Furthermore, this review also provides information about the key environmental processes (such as soil resuspension or biomass burning etc.,) that govern the transport and redistribution of plutonium in the surface air today. Although these environmental processes have been known for decades, and the previous review of plutonium sources and its atmospheric behavior has been provided by Hirose and Povinec, 2015, there still remains great uncertainty in our understanding of plutonium behavior and its redistribution in the atmosphere. In this article, we attempt to bridge this gap by providing long -term behavior and the factors controlling the fate and transport of plutonium in the atmosphere in a broader context.

2. Data source We collected measured data of atmospheric plutonium concentrations from a variety of sources; therefore it is difficult to describe individual methods utilized by various laboratories. In general, the aerosol samples were collected using high or low volume aerosol samplers on conventional filters, mainly glass fiber or polypropylene fiber. After sampling, radiochemical analyses were performed and the activity concentrations of 238Pu and 239þ240 Pu were measured by alpha -spectrometry. The minimum detectable activity concentration (MDC) of 239þ240Pu and 238Pu vary for different laboratories, but typically is about 1 mBq/m3. The stratospheric plutonium concentrations data were taken from the U.S. Environmental Measurement Laboratory's (EML) “High-Altitude Sampling Program” (Project HASP). The surface air plutonium concentrations during the period 1965e1980 were taken from EML's Surface Air Sampling database. The WIPP's (Waste Isolation Pilot Plant) onsite surface air data were taken from www.cemrc.org. The surface air concentrations data in the vicinity of the Los Alamos National Laboratory, New Mexico, USA and Hanford site, Washington State, USA were taken from their environmental site reports published annually. Some data presented in this article were obtained directly from published reports.

P. Thakur et al. / Journal of Environmental Radioactivity 175-176 (2017) 39e51

41

Fig. 1. Sampling locations for air monitoring measurement of this study.

3. Results and discussion 3.1. Time trend analysis of

239þ240

Pu in the stratosphere

A time-series of 239þ240Pu concentration in the northern stratosphere is shown in Fig. 2. The stratospheric 239þ240Pu concentrations data were taken from the Environmental Measurements Laboratory (EML, USA), which conducted high-altitude sampling program (20e40 km altitude) from the early 1960s to the early 1980s. The lower stratosphere aerosol data (10.1e14.2 km altitude) reported by Alvarado et al., 2014 over Switzerland from 1973 to 1986 and from 2004 to 2010 is also included. The stratospheric 239þ240Pu concentration showed a maximum in 1963

because of large-scale atmospheric nuclear weapons tests occurred mainly between 1961 and 1963. There were several peaks of the stratospheric 239þ240Pu in 1970's due to the Chinese nuclear tests during 1970e1976. After 1976, the stratospheric 239þ240Pu concentrations decreased with an apparent stratospheric residence time of 1.3 ± 0.3 years (Reiter, 1975). There are no stratospheric 239þ240 Pu data available for the period between 1987 and 2003. Alvarado et al., 2014 added some new stratospheric plutonium isotopes (239þ240Pu, 241Pu and 238Pu) and 137Cs data measured during the period 2007e2011. They found two to four orders of magnitude higher levels of plutonium and 137Cs in the stratosphere than that in the surface air and suggested a longer stratospheric mean residence time of 2.5e5 years for these particles. The study also reported that fine aerosol particles (<0.02 mm in diameter) containing radionuclides could stay longer in the stratosphere for timescales of the order of several decades, and therefore radionuclides injected into the stratosphere mainly during the early 1960s might still be present there during the 2000s. Using the 2010 Eyjafjallajokull eruption as an example, these authors further noted that volcanic eruptions could potentially cause redistribution of these particles from the upper stratosphere to the lower troposphere. However, Hirose and Povinec, 2015 evaluated the longterm measurements data of plutonium isotopes in the stratosphere and troposphere and concluded that the plutonium concentrations in the stratosphere and the troposphere decrease with an apparent residence time of 1.5 ± 0.5 years and that the dominant processes affecting plutonium concentrations in the upper troposphere are global dust events and biomass burning (Hirose and Povinec, 2015). 3.2. Time trend analysis of

Fig. 2. Temporal variations of 239þ240Pu concentrations (mBq/m3) in stratosphere and surface air in the Northern Hemisphere.

239þ240

Pu in the surface air

The concentrations of plutonium in surface air were not systematically monitored during 1959e1964 at the time of the

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heaviest contributions from global fallout. Because of 90Sr global fallout pattern is similar to that of plutonium isotopes, with an average 239þ240Pu to 90Sr global activity ratio of about 0.025 (decay corrected to 2000) based on the normalized production rate for 90Sr and plutonium isotopes in nuclear explosions (UNSCEAR, 2000a). The 239þ240Pu to 90Sr ratio method (Bennett, 1978) is generally utilized to make good estimates of the plutonium concentrations. Bennett, 1978 estimated the fallout concentrations of plutonium and americium in surface air in the middle latitudes of the northern hemisphere by using information about the weapons testing in combination with an atmospheric transport model. Fig. 3 shows the predicted air concentrations of plutonium from global fallout. The depletion of the stratospheric reservoir led to decreasing concentrations followed by a leveling-off due to tests by the People's Republic of China. The Chernobyl accident in April 1986 increased the concentration of 239þ240Pu in surface air during 1986e1987, especially in Europe and contributed slightly to the plutonium global inventory (UNSCEAR, 2000b). However, following a peak in 1986, the concentrations of 239þ240Pu are shown to decrease continuously €lgye, 2008; Lujaniene  et al., 2012a). (Ho The 239þ240Pu concentrations in surface air have been measured in several monitoring sites around the world. The Environmental Measurements Laboratory (EML, USA) data, representing surface air sampling from 1965 to the early 1980s for the Canada and the US are shown in Fig. 2. Also, included in Fig. 2 is the temporal variation of 239þ240Pu concentrations in surface air for the US-Department of Energy's (US-DOE) monitoring sites near Hanford and Los Alamos. Additionally, 239þ240Pu concentrations in surface air in the vicinity of the US-DOE's WIPP site, which is the first deep underground geologic nuclear waste repository for the permanent disposal of transuranic (TRU) wastes generated from U.S. defense program are also shown in Fig. 2 and provide a longer time and location background context. The data show decreasing trend in 239þ240Pu concentrations in surface air since the monitoring begun in 1960s and that the 239þ240Pu concentrations since 1984 remained fairly constant. During the era of above ground nuclear weapons testing, the 239þ240 Pu concentrations in surface air have been shown to vary widely as shown in Table 1. These data suggest that the post-fallout 239þ240 Pu concentrations in ambient air remain largely between 0.001 and 0.01 mBq/m3 in the mid-latitude region of the Northern Hemisphere, except for some specific events such as a local € lgye, 2008), the Chernobyl accident contamination event (Ho

Fig. 3. Predicted surface air concentrations of from nuclear weapons testing.

Pu (mBq/m3) from global fallout

239þ240

 et al., 2012a), and dust resuspension events (Hirose (Lujaniene  et al., 2012a). The 239þ240Pu concentrations et al., 2003; Lujaniene in surface air do not show a decreasing post-fallout trend (after 1984). This suggests that the 239þ240Pu concentrations in ambient air since the early 1980s have not been under stratospheric control. During most years study, the peaks in 239þ240Pu concentrations generally occur in the March to June time-frame as shown in Fig. 4. While the current atmospheric plutonium data and those collected during the era of above ground nuclear weapons testing both show springtime peaks, the causes for the cycles are likely quite different. Studies conducted prior to the end of the atmospheric weapons testing (Perkins and Thomas, 1980) showed that seasonal cycle of plutonium concentrations (highest in spring and lowest in winter is associated with enhanced transport of radioactive aerosols from the stratosphere to the troposphere (see Fig. 4a). However, after the cessation of nuclear weapons testing in 1980 and comparatively small additional input from the Chernobyl, the most plutonium in the air today is associated with re-suspension soil, which is contaminated from weapons fallout (see Fig. 4b). Resuspension is considered to be the dominate mechanism in maintaining the small, but detectable residual concentrations of 239þ240Pu in surface air. The seasonal cycle of 239þ240Pu concentrations was confirmed by many monitoring data collected in the northern hemisphere. Arnold and Wershofen, 2000, have shown that 239þ240Pu concentrations in ground-level air from Germany decreased steadily after the cessation of atmospheric weapons testing. The observed seasonality in 239þ240Pu concentration is attributed to the resuspension of contaminated soil dust plus the local precipitation to some extent. In related studies Wershofen and Arnold, 2005 showed that 239þ240Pu peaked shortly after the Chernobyl accident and then begun declining. The study also showed seasonal cycle in 239þ240Pu activities and the author attributed the observed seasonality to the re-suspension of contaminated soil. Seasonal cycles in 239þ240Pu and 238Pu concentrations are also observed in €, Finland in 1963 (Salminen and Paatero, surface air of Sodankyla 2009). The authors attributed this seasonality to enhance transport of radionuclides from the stratosphere to the troposphere. The springtime enhanced activity of 239þ240Pu was also reported in the deposition samples (dry and wet deposition) collected from Tsukuba, Japan and Daejeon, South Korea (Hirose et al., 2003, 2004). The authors attributed this seasonal variation to the continental dusts storms originating in the months of March to May, the so called “Yellow Sand Event” or “Kosa” from Chinese deserts and arid regions. The trajectory analysis of dust storms suggests that 239þ240 Pu deposition in Korea and Japan during spring is originating from plutonium-bearing surface soil particles from the East Asian arid areas. A significant part of the 239þ240Pu deposition in spring in both Japan and Korea is attributable to dry deposition. We also compared the US data with other international data to assess the worldwide concentrations of plutonium in surface air. A time series of 239þ240Pu concentrations in surface air in Europe are shown in Fig. 5. Except for the brief increase during 1986e1987 due to the Chernobyl accident, the 239þ240Pu surface air data showed a decreasing trend. In many monitoring stations around the Europe, the 239þ240Pu measured during AprileMay, 1986 were more than thousand times higher than the background concentration levels for these areas. For example, in surface air concentrations of 239þ240 Pu were in the range of 10e28 mBq/m3 in Prague; 49e140 3 mBq/m in Dukovany, located about 200 km south-east of Prague € lgye and Filgas, 1987). In neighboring state of Austria, it varied (Ho in the range of 1.2e89 mBq/m3 in Vienna; 6.0e52.3 mBq/m3 in Linz; and 1.2e21.8 mBq/m3 in Salzburg (Irlweck et al., 1993). The surface €rvi, Finland on April 28, 1986 air samples collected from Nurmija were reported to contain 32 mBq/m3 of 239þ240Pu (Jaakkola et al.,

P. Thakur et al. / Journal of Environmental Radioactivity 175-176 (2017) 39e51 Table 1 Activity concentration of

43

Pu (mBq/m3) in surface air in different locations.

239þ240

Pu (mBq/m3)

239þ240

Reference

Location

Year

Activity

Norway

1957e1958, 1961e1962 1963 1961e1965 1962e1964 1965e1967 1963e1985 1963e1985 1964e1976 1970e1981 1963e1987 1966e1980 1980e1985 1986e1989 a 1998e2016 1990e2000 a 1987e1998 c 1990e2003 a 1986 1997e2006 a 1995e2011 b 2001e2002 b 2005 1990e2007 1994e1997 2001e2002 1979e1982 2001 2000e2001 1978

1.3e782

Wendel et al., 2013

0.4e95 2.0e64.7 10e20 0.019e8.69 0.006e4.99 0.012e86.10 0.09e11.80 0.37e359.6 0.082e7.88 2.2e441.8 0.011e0.87 <0.0025e0.0095 <0.0005e0.082 0.003e0.09 0.00053e0.0081 0.00039e0.0045 10e28 0.00053e0.217 0.0009e0.3 0.0017e0.0188 0.002e0.013 0.0007e0.017 0.0012e0.002 <0.004e0.98 0.026e0.79 0.005e0.13 0.014e2.5 0.001e0.27

Salminen and Paatero, 2009 Bortoli and Gaglione, 1969 Cambray et al., 1985 EML Database EML Database EML Database EML Database EML Database EML Database Iranzo et al., 1987 EML Database Playford et al., 1993 www.cemrc.org Fritz and Patton, 2002 Rosner and Winkler, 2001 Wershofen and Arnold, 2005 € lgye, 2008 Ho € lgye, 2008 Ho  et al., 2012a Lujaniene Chamizo et al., 2010 Chamizo et al., 2010 Kierepko et al., 2016 Geering et al., 1998 Choi et al., 2006 Hirose and Sugimura, 1984 Lehto et al., 2006 Lehto et al., 2006 Shinn et al., 1997

Sodankyl€ a, Finland Ispra, Italy Chilton, United Kingdom New York city, USA Moosonee, Canada Chacaltaya, Bolivia Antofagasta, Chile Rocky Flats, Colorado, USA Thule, Greenland Palomares, Spain Beaverton, Oregon, USA Milford Haven, United Kingdom WIPP Site, USA Hanford Site, USA Neuherberg, Germany Braunschweig, Germany Prague, Czech Republic Prague, Czech Republic Vilnius, Lithuania Seville, Spain Madrid, Spain Krakow, Poland Fribourg, Switzerland Anmyeondo, South Korea Tokyo, Japan Astana, Kazakstan Kurchatov, Kazakstan Bikini Island a b c

Quarterly composite. Monthly composite. Annual means.

1986). In Vinca, near Belgrade, Serbia the maximum 239þ240Pu concentrations detected were 10.6 mBq/m3 (Manic-Kudra et al., 1995). The current concentration of 239þ240Pu in surface air is about ~1000 times lower than that measured during the early 1960s and 1970s. Seasonal variation in 239þ240Pu concentrations can also be observed in Europe surface air data. The other internationally reported post-fallout data for plutonium were not as readily comparable because of differences in the temporal intervals, but are discussed here for qualitative comparison. The 239þ240Pu concentrations in surface air of Tokyo and Tsukuba, Japan during the period from 1979 to end of 1986 were reported by Hirose and Sugimura (1984, 1990). A typical seasonal variation with high values in spring and low in winter was also evident in their data. The 239þ240Pu concentration showed a maximum (0.72 mBq/m3) in 1981 because of the 26th Chinese nuclear test in 1980. Following a peak in 1981, the 239þ240Pu concentrations showed a decreasing trend. For example, annual average 239þ240Pu concentrations in surface air at Tsukuba, Japan were 0.27 mBq/m3 in 1981; 0.10 mBq/ m3 in 1982; 0.044 mBq/m3 in 1983; 0.029 mBq/m3 in 1984; 0.012 mBq/m3 in 1985. In 1986, the Chernobyl accident caused a brief increase in 239þ240Pu concentration to about 0.12 mBq/m3. By using 239þ240 Pu to 90Sr ratio method, these authors have calculated an average concentration of 239þ240Pu in surface air during the earlier fallout years in Tokyo (Fig. 6). The calculated 239þ240Pu concentrations in surface air in Japan during the period 1954e1981 showed a similar temporal distribution pattern as those observed in other locations. As expected, the highest 239þ240Pu concentration of 14.8 mBq/m3 occurred in 1963, and then decreased steadily to 0.074 mBq/ m3 by 1980, followed by a slight increase to about 0.26 mBq/m3 during 1981 due to the Chinese nuclear tests. Yamato, (1981) reported surface air concentrations of 239þ240Pu during the period

1974e1979 at Ibaraki, Japan. The seasonal variation in their data was also evident with highest concentrations generally occurring in the spring and lowest in the winter. The authors attributed this to the Chinese nuclear tests and to the spring peak phenomena. The 239þ240 Pu concentrations measured were in the range of 0.041e1.9 mBq/m3, which is consistent with the range reported by other researchers in Japan (Kasai et al., 1984). Similar to surface air, the 239þ240 Pu concentrations in deposition also follow seasonal pattern indicative of stratospheric origin during the period 1957e1980. After the last Chinese test in 1980, a sharp increase in 239þ240Pu deposition was observed in the spring of 1981. From 1981 to 1984, the annual 239þ240Pu deposition decreased according to the stratospheric half-residence time of about one year and reached the background level by 1985, suggesting termination of stratospheric fallout. After the brief increase during 1986e1987 as a result of Chernobyl accident, the observed 239þ240Pu depositions post-1988 were about an order of magnitude lower than the level measured during 1960s and did not reflect stratospheric control, but rather reflect processes such as resuspension. Studies have shown that the 239þ240 Pu in post-1988 deposition samples came primarily from the resuspension of plutonium bearing surface soil originated from the East Asian arid areas. The southern hemisphere, on the other hand, is less contaminated than the northern hemisphere because only 10% (about 208 tests) of all nuclear tests were conducted in the southern hemisphere. The southern hemisphere received a mix of tropospheric and stratospheric fallouts from USA/UK, Pacific Proving Ground (PPG) tests and from French tests at Mururoa and Fangataufa Atolls (Krey et al., 1976). The PPG tests were typically surface tests of varying yields and types that generated large amounts of tropospheric fallout (Muramatsu et al., 2001). A few atmospheric nuclear weapons testing were also conducted by UK in Australia. Therefore,

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239þ240

Fig. 6. Mean concentrations of

Fig. 4. Springtime peak in 1963 and (b) 1998e2014.

239þ240

Pu (mBq/m3) in surface air in Japan.

Pu surface air concentrations measured during (a)

Fig. 7. Temporal variations of Southern Hemisphere.

Pu concentrations (mBq/m3) in surface air in the

239þ240

efforts in 1987 due to resuspension. Following the peak in 1987, the 239þ240Pu concentrations were decreased by a factor of about two approximately every two years. However, seasonal variations in 239þ240Pu concentration were also reported in the southern hemispheric data (Bourlat et al., 1994). 3.3. Time trend analysis of 238Pu in the stratosphere and surface air

Fig. 5. Temporal variations of Europe.

Pu concentrations (mBq/m3) in surface air in the

239þ240

in the southern hemisphere we have minimal fallout inventory from a variety of weapons tests. The southern hemisphere data were from the French monitoring Program at Mururoa Island (1986e1991) in the South Pacific and from the EML database (1965e1985) as shown in Fig. 7. Bourlat et al., 1994 reported 239þ240 Pu concentrations post-environmental remediation of a French nuclear test during the period 1986e1991. The average 239þ240 Pu concentrations were shown to increase by a factor of about six (from 0.04 mBq/m3 to 0.24 mBq/m3) during the clean-up

Unlike 239þ240Pu, the major source of 238Pu in the stratosphere was from the burn-up of the SNAP-9A satellite over the South Pacific Ocean in 1964. This accident introduced about 0.63 PBq of 238 Pu into the atmosphere at height of 50 km with particles size ranging from 5 to 58 mm (Harley, 1980). The released activity was twice as high as the total activity dispersed by nuclear weapons testing. The stratospheric inventory of bomb-derived 238Pu was about 2% of the total 238Pu inventory in 1966 (Krey, 1968). Studies showed that most of the SNAP-9A derived 238Pu had been deposited (~76%) over the southern hemisphere, with only 24% over the northern hemisphere. By contrast, most of the weapons 238Pu had been deposited (~80%) on the northern hemisphere. Fig. 8 shows the stratospheric inventory of 238Pu from 1964 to 1971 taken from US-DOE, EML database. The data shows peak in stratospheric concentrations of 238Pu occurred in 1966 and then, gradually

P. Thakur et al. / Journal of Environmental Radioactivity 175-176 (2017) 39e51

decreased with a residence time of 1.2 years, and by mid-1970, most of the SNAP derived 238Pu had been deposited on the earth's surface. For comparison purpose and to show long-term trend of 238Pu in the atmosphere, the stratospheric concentrations of 238Pu (10.1e14.2 km altitude) measured by Alvarado et al., 2014 during the periods 1973e1986 and 2004e2011 are also included in Fig. 8. Long-term time series data of 238Pu (1965e1980) in the stratosphere suggest that the 238Pu concentrations decrease with an apparent residence time of 1.7 ± 0.4 years (Hirose and Povinec, 2015). A longer stratospheric residence time of 238Pu from the SNAP-9A burn-up in comparison to 239þ240Pu released from nuclear weapons tests has also been observed in Antarctic ice cores (Koide et al., 1979). Furthermore, the 238Pu stratospheric concentrations showed an increasing trend with altitude. The higher levels of 238Pu were generally occurred in the upper stratosphere (20e40 km altitude), while levels were usually low in the lower stratosphere (10.1e14.2 km altitude). This disparity indicates that the temporal variations of 238Pu in the atmosphere were controlled primarily by 238Pu originating from the burn-up of SNAP-9A satellite with minor contributions from the Chinese and French nuclear weapons tests in 1970s-1980s. The stratospheric 238Pu concentrations measured by Alvarado et al., 2014 during the period 2007e2011 were similar to those measured during 1970s and 1980s. The authors attributed these high 238Pu levels in the atmosphere to the SNAP-9A burn-up and to the atmospheric nuclear weapons testing. Given the fact that most of the SNAP-9A-derived 238Pu had been deposited on the earth surface by the end of 1970s, the belief that there could still be significant amounts of SNAP-9A-derived 238Pu present in the stratosphere during 2000s appear very unlikely. Furthermore, the time series of 238Pu clearly showed a decreasing trend until the mid-1980s (Fig. 8). It might be concluded, on the basis of data summarized in Fig. 8 that elevated levels of 238Pu measured in the lower stratosphere during 2000s might have come from some other sources and not from the SNAP-9A burn-up or nuclear weapons testing. Similar changes in surface air concentrations of 238Pu were also observed. However, the detection of 238Pu in surface air was not as frequent as that of 239þ240Pu because of the normally low levels of 238 Pu in the environment. This radionuclide is not primarily from nuclear weapons fallout, but from the burn-up of the nuclearpowered satellite SNAP-9A. Surface air measurements at ISPRA, Italy first detected the SNAP derived 238Pu in the northern hemisphere in early 1966 (Bortoli and Gaglione, 1969). The SNAP derived

Fig. 8. Temporal variations of 238Pu concentrations (mBq/m3) in stratosphere and surface air in the Northern Hemisphere.

45

238 Pu also became evident in surface air at Richland, Washington in spring of 1966 (Hardy et al., 1973). Shortly thereafter, it was detected in monthly fallout samples in New York and ISPRA, Italy (Bortoli and Gaglione, 1969). The peak in surface air concentration and deposition rate at ISPRA and other locations occurred during the spring of 1967. The concentrations of 238Pu from the SNAP-9A had been measured in monthly surface air samples from northern to southern hemisphere (Hardy, 1976). At most locations, the peak in 238Pu from the SNAP-9A was observed during 1967e1968, except for Antarctica, where maxima occurred in 1966 (Perkins and Thomas, 1980). The SNAP-9A derived 238Pu reached maximum concentrations in surface air 2e3 years later, depending upon latitude, and by 1971 was largely depleted from the atmosphere (Perkins and Thomas, 1980). The SNAP-9A derived 238Pu also caused a sharp increase in activity of this isotope deposited in ice layers deposited in 1965-66 on both the Antarctic and Greenland ice caps (Perkins and Thomas, 1980). Since the mid to late 1970s, the concentrations of 238Pu in surface air have been very low (few mBq/m3 or less) and in some places concentrations have fallen below conventional analytical detection levels and are therefore considered to be in the analytical noise range. This is presumably because 238Pu is not primarily from weapons fallout. The average concentrations of 238Pu in surface air in the northern hemisphere were about 0.00024 mBq/m3 (range from 0 to 2.1mBq/m3) in Krakow, Poland during 1990e2007 (Kierepko et al., 2016); 0.002 mBq/m3 (range from 0.059 to 87.1mBq/ m3) in Bialystok, Poland during 1991e2007 (Kierepko et al., 2016); 0.0005 mBq/m3 (range from 0 to 0.011 mBq/m3) in Prague, Czech € lgye, 2008); 0.00019 mBq/m3 Republic during 1997e2006 (Ho (range from 0.000021 to 0.00082 mBq/m3) in Braunschweig, Germany during 1990e2003 (Wershofen and Arnold, 2005); 0.03 mBq/ m3 (range from 0.002 to 0.095mBq/m3) in quarterly composite samples at the Hanford site during 1990e2000 (Fritz and Patton, 2002) 0.00021e0.0015 mBq/m3 in Fribourge, Switzerland during 1994e1997 (Geering et al., 1998). However, unusually high concentrations of 238Pu have also been reported in surface air samples. Similarly, a concentration range of 0.005e0.018 mBq/m3 was reported in weekly composite samples from Astana located about 500 km west of Kurchatov in 2001 (Lehto et al., 2006).

3.4. Plutonium isotopic ratios The origin of plutonium in the aerosols can be identified by measuring the activity ratios of 238Pu/239þ240Pu, 241Pu/239þ240Pu, and mass ratios 240Pu/239Pu in the environmental samples, which vary according to the sources. Isotopic ratios have thus been historically used as a fingerprint to identify the origin of plutonium in surface air from different known source events that released plutonium into the environment. The plutonium isotopic ratios originating from various sources are summarized in Table 2. The 240Pu/239Pu atom ratio produced in a nuclear test is a function of the design and yield of the device being tested. The fallout produced by high yield tests is characterized by higher 240 Pu/239Pu atom ratios than the fallout produced by lower yield nuclear tests. The main stratospheric events that dominate the ratios during 1952e1962 were the US and UK tests on Bikini, Eniwetok, and Christmas Island, and the Soviet tests in the Arctic. During these decades, plutonium ratios were highly variable for a number of reasons such as continued contributions from nuclear weapons, low mixing rate of stratospheric air mass and/or uneven fallout deposition. For example, the early 1950s fallout in the Antarctic exhibited a 240Pu/239Pu of >0.024 (Warneke et al., 2002). The 240 Pu/239Pu then dropped to 0.21e0.26 in the mid 1950s and to 0.16 in the late 1950s. In the early 1960s the 240Pu/239Pu was between 0.17 and 0.19. The small increase in the ratio during the late 1970s

46

P. Thakur et al. / Journal of Environmental Radioactivity 175-176 (2017) 39e51

was seen in northern hemisphere due to the Chinese tests. However, the largest source of stratospheric fallout, which peaked in 1962, is characterized by a 238Pu/239þ240Pu activity ratio of 0.024, a 241 Pu/239þ240Pu activity ratio of 13e15 (reference date 1963), and a 240 Pu/239Pu atom ratio of 0.18 (Kelley et al., 1999; Koide et al., 1985; Krey et al., 1976) based upon atmospheric aerosol sampling, soil samples and ice core data (Krey et al., 1976; Koide et al., 1985). The second source is tropospheric fallout with a lower 240Pu/239Pu ratio of 0.035 and is proposed to be originated from surface-based low yield testing at the Nevada Test Site (Hicks and Barr, 1984). Another important signature is that from the Chernobyl accident (especially in Europe) with a typical 238Pu/239þ240Pu activity ratio of ~0.45, and  et al., 2012a). the 240Pu/239Pu atom ratio of ~0.40 (Lujaniene The 238Pu/239þ240Pu activity ratio in surface air averaged about 0.024 in 1964. The activity ratio remained almost the same in 1965,

Table 2 238 Pu/239þ240Pu and

241

Pu/239þ240Pu activity ratios, and

Plutonium source

238

Global fallout, Northern hemisphere

but, by the spring of 1966, it had increased, suggesting the presence of 238Pu from the SNAP-9A (Fig. 9a). This accident changed the 238 Pu/239þ240Pu ratio from about 0.024 to 0.034 in the northern hemisphere and to 0.20 in the southern hemisphere (Hardy et al., 1973). By 1982, the 238Pu/239þ240Pu ratio in global fallout had declined to 0.024 and did not differ significantly between the hemispheres. At the present time, the typical ratios of 238 Pu/239þ240Pu that originated from the weapons grade plutonium and those that originated from the global nuclear weapons test fallout are so similar that it is difficult to unambiguously identify the source of plutonium based on this ratio alone. The 240Pu/239Pu atom ratio is a better indicator to distinguish fallout plutonium from the weapons grade plutonium because the 240Pu/239Pu atom ratio in global fallout (~0.18) differs significantly from the weapons grade plutonium ratio of less than 0.07.

240

Pu/239Pu mass ratio from different sources.

239þ240

241 Pu/239þ240Pu activity ratio

240

Pu/239Pu mass ratio

Remarks

0.024e0.029

13e15

Global fallout, Southern hemisphere

0.20

e

0.176 ± 0.014 0.180 ± 0.014 0.185 ± 0.047

Weapons grade plutonium

0.014

Trinity Test site, Soil and air BOMARC Missile Site soil

e

0.75e7.5 (1945e1974) e

Data for global fallout are cited from Krey et al., 1976; Kelley et al., 1999. Data for global fallout are cited from Kelley et al., 1999 Irlweck and Hrnecek, 1999

0.024 (0.017e0.033)

1.0 (ref. 0.79e1.29)

0.02 (0.043e0.263) 0.0576 (0.0559e0.0594)

Semipalatinsk Test Site

0.015

1.52 ± 0.04 (ref. 8/29/1949)

0.036 ± 0.001 (0.025e0.072)

Atmospheric fallout 1963e1979 in Japan

0.037

0.1922 ± 0.0044

Nagasaki Atomic bomb

0.074 ± 0.001

NTS only fallout soil Pacific Proving Ground

0.034e0.04 0.001e0.014

1.86 (ref. 1/1/2000) 1.51 ± 0.1 (ref. 8/9/1945) 4.3e7.5 (ref. 1956e1957) 27 (1945e54)

0.30e0.36

Lop Nor test site soil Thule, Greenland Bomb debris, hot particles

e

e

0.059e0.224

0.0161 ± 0.0005

0.0551 ± 0.0008

Palomares, Spain Soil, marine sediments

0.015e0.03 0.018 ± 0.003

0.87 ± 0.12 (ref. 1/10/2001) 3.3e8.4 (ref. 1/21/1968) 8.2 ± 0.8 (1/16/1966)

0.061 ± 0.006 0.056 ± 0.003

Chernobyl NPP accident

0.5

83(5/1/1986)

0.408 ± 0.003

Chernobyl fallout, Finland Air filter Chernobyl fallout, South Sweden Aerosol samples from Salzburg, Austria in April -May, 1986 Fallout from 21Ist Chinese test Fallout from 26th Chinese test Fukushima NPP accident

0.50 ± 0.13

98 ± 4

e

Accidental crash of aircraft carrying B-52 bomb at Palomares, Spain in 1966, cited from Mitchell et al., 1997 and Irlweck and Hrnecek, 1999. Data for Chernobyl accident are cited from Muramatsu et al., 2000. Salminen-Paatero et al., 2014

0.57 ± 0.07 0.49 ± 0.05

85 ± 20 64.4 ± 12.3

e e

Irlweck and Wicke, 1998

0.03 ± 0.007 0.02 ± 0.006 1.77

11 ± 1 5.5 ± 0.8 118.1

e e 0.395

Fukushima NPP accident

1.92

123.7

0.447

Fukushima Fallout, Japan Soil, marine sediments, litter Fukushima Fallout, aerosol

1.07e2.89

e

1.2

e

0.323e0.33 0.188e0.255 0.244 ± 0.018

Pu/

Pu activity ratio

e

0.028e0.037 0.054e0.063

First nuclear test at White Sand, Alamogordo, NM, USA cited from Douglas, 1978 A missile caught fire at the US McGuire Air Base in New Jersey, resulting in dispersal of plutonium into the environment. Lee and Clark, 2005 Former Soviet Union's Nuclear Test Site, Kazakh republic, cited from Yamamoto et al., 2002 and Irlweck and Hrnecek, 1999. Data for atmospheric fallout in Japan (1963 e1979) are cited from Zheng et al. (2013) Data for Nagasaki A-bomb are cited from Yamamoto et al., 1983 US-Nuclear test site cited from Hardy, 1976 and Irlweck and Hrnecek, 1999. Data for the Pacific Proving Ground are cited from Muramatsu et al., 2001 Chinese nuclear test site cited from Bu et al., 2015 Accidental crash of aircraft carrying B-52 bombs at Thule in 1968, cited from Eriksson et al., 2008 and Irlweck and Hrnecek, 1999.

Hirose et al., 2001 Hirose et al., 2001 ORIGEN-ARP model calculation based on mean inventories of plutonium isotopes in rector1-3 units cited from Zheng et al., 2013. ORIGEN-ARP simulations of the average inventories in rectors unit 1 and unit 3 cited from Zheng et al., 2013. ICP-MS analysis of plutonium in litter, soil, Marine sediments cited from Zheng et al., 2013. Alpha spec. analysis of composite aerosol samples collected over Lithuania between March 23 and April 15, 2011 cited from Lujaniene et al., 2012b.

P. Thakur et al. / Journal of Environmental Radioactivity 175-176 (2017) 39e51

Since mid 1980s, the 240Pu/239Pu atom ratios measured in surface air samples has been close to that of the surface soil (e.g., 0.18), indicating that resuspension of global fallout deposited plutonium from the earth surface. However, some higher and occasionally some lower 238Pu/239þ240Pu activity and 240Pu/239Pu atom ratios have been reported in aerosol samples, especially in Europe, indicating the presence of sources other than the global fallout (Fig. 9b,) (Zheng et al., 2012). For example, Arnold and Wershofen, 2000 reported 238Pu/239þ240Pu activity ratio of 0.41 and 0.46 in surface air samples collected from Braunschweig, Germany. Similar increase in 238Pu/239þ240Pu activity ratio (0.014e0.5) and 240Pu/239Pu atom ratio (0.10e0.44) were also found in aerosol samples collected from Krakow and Bialystok, Poland (Kierepko et al., 2016), Prague, €lgye, 2008), Nurmija €rvi, Finland (SalminenCzech Republic (Ho  et al., Paatero et al., 2012) and Vilnius, Lithuania (Lujaniene 2012a). These higher ratios were attributed to resuspension of the Chernobyl-derived plutonium. Studies have shown longdistance transport of Chernobyl-derived plutonium from the Chernobyl exclusion zone over to the Europe, especially during high wind season. Contamination around Chernobyl also brought up the “hot particles” to these areas, resulting in sporadic increase in 238Pu, 239þ240Pu and 241Pu concentrations. Additionally, speciation and particle size distribution of aerosol and soil samples contaminated from Chernobyl derived plutonium indicated that these particles originate primarily from resuspension. An exponential decreased in 240Pu/239Pu atom ratio from 0.30 to 0.19 during

47

the period 1995e2003 was likely due to the gradual decrease in Chernobyl originated particles in the environment as a result of deposition and mixing with globally derived plutonium (Lujaniene et al., 2009). These authors have reported a typical residence time of about 1.6 ± 0.4 years (Lujaniene et al., 2009) for the Chernobyl derived plutonium in the environment, which is twice as long as the mean tropospheric residence time (71 days) of fallout plutonium. The Fukushima accident is also a source of plutonium in the environment. However, the plutonium released from the Fukushima accident was very low and localized within a few tens of kilometers. A few studies from Japan found signature of Fukushima derived plutonium in soil samples at several sites within 20e30 km from the Fukushima nuclear power plant (Zheng et al., 2013). Using a simple two-term mixing model, these authors estimated that the contribution of Fukushima-derived 239þ240Pu in the J-Village soil was 87% and the remaining 13% of 239þ240Pu was of global fallout origin (Zheng et al., 2012). Fukushima derived plutonium were characterized by higher 238Pu/239þ240Pu ratio of 1.2, 241Pu/239þ240Pu ratio of 102 and 240Pu/239Pu atom ratio of 0.30e33 (Zheng et al., 2013). No detections of Fukushima plutonium in the environmental samples collected outside Japan were identified in the  et al., 2012a,b reported literature searched. However, Lujaniene observance of Fukushima derived plutonium in the composite filter samples collected during March 23- April 15, 2011 from Vilnius, Lithuania. The 239þ240Pu concentration of 44.5 ± 2.5 nBq/m3, which is higher than the values previously measured in the air samples (12.0 ± 0.6 nBq/m3 in March 2006 and 29.2 ± 1.5 nBq/m3 in May, 2006) was observed. They observed an elevated 238Pu/239þ240Pu activity ratio of 1.2 and 240Pu/239Pu atom ratio of 0.244 ± 0.018 in composite aerosol samples collected from Vilnius, Lithuania during March 23-April 15, 2011 (n ¼ 30). The high activity and atom ratio of plutonium suggested about 43e59% contribution of Fukushima 239þ240 Pu to the total 239þ240Pu activity. However, further research is required to unambiguously determine the source of plutonium in the aerosol samples, because the contributions of re-suspended plutonium from the global fallout and the Chernobyl accident remain a possibility. Besides these two activity and atom ratios mentioned above, the 241 Pu/239þ240Pu activity ratio is also a good indicator for identifying plutonium sources in the environment, because of short half-life (14.4 years) of 241Pu and clearly distinguishable ratio values between different plutonium sources. The 241Pu/239þ240Pu ratios in global fallout samples were reported to be around 13e15. The 241 Pu/239þ240Pu activity ratios of weapons grade plutonium from different nuclear weapon devices tested between 1945 and 1963 were shown to vary from 0.75 to 7.5 (Irlweck and Hrnecek, 1999). Alvarado et al., 2014 reported 241Pu/239þ240Pu activity ratios of 12e14 in stratospheric aerosols during 1970s, which agree with the 241 Pu/239þ240Pu ratio of 11 measured in deposition samples during 1977 (Hirose et al., 2001)-(assuming 241Pu/239þ240Pu ratio in global fallout ~13). Almost constant 241Pu/239þ240Pu ratio during 1970s suggests that most of the plutonium in these samples came primarily from the Chinese atmospheric nuclear tests conducted in 1970s. Lower 241Pu/239þ240Pu (1.5e3.2) activity ratio measured in the troposphere (altitude of 3e13 km) during the 2000s can probably be attributed to the aged plutonium derived from the global fallout. 3.5. Factors controlling plutonium concentrations in the air today

Fig. 9. (a) The 238Pu/239þ240Pu activity ratio and (b) spheric and surface air in the northern hemisphere.

240

Pu/239Pu mass ratio in strato-

3.5.1. Resuspension After the cessation of nuclear weapons testing, and considering the fact that the residence time of plutonium in the atmosphere is on the order of 1.5 years, the contribution of so called “global fallout

48

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plutonium” is no longer a dominant factor for the plutonium concentration in air; instead most plutonium in the air today is associated with resuspended soil (Arimoto et al., 2002) which is contaminated from weapons fallout or distributed by global dust events as discussed later. Given the length of time since the cessation of nuclear weapon testing, the most plutonium deposited on the land surface is expected to be fully adsorbed on the soil matrix. This is the basis of the hypothesis that surface air plutonium is associated with resuspended soil particles. Literature has consistently defined resuspension to be the predominant mechanism for maintaining the small residual plutonium in surface air. The resuspension of soil particles typically exhibits a threshold with respect to wind velocity. Thresholds typically range from 6 to 13 ms-1 at 0.3 m (Arimoto et al., 2002) and are related to the force required to cause the saltation of large soil particles. Arimoto et al., 2002 have shown that in semi-arid conditions like at the WIPP, the plutonium bearing aerosols tend to increase in environmental air samples with winds of 4 ms-1 with a maximum at wind speeds of 7 ms-1 and stay constant at wind speeds above 7 ms-1. These aerosol particles are then readily trapped on a filter in an air monitoring station leading to their frequent detections during high wind events. Airborne plutonium measurements in the vicinity of contaminated sites showed a definite contribution from resuspension. For example, air concentrations near Rocky Flats, Nevada test site, Marshall Island in particular showed elevated levels (Shinn et al., 1997; Harley, 1980), and similar levels were measured in Palomares, Spain (Iranzo et al., 1987), areas near Semipalatinsk test site (Lehto et al., 2006). Studies have also been performed on a contaminated area near the Savannah River Plant in the United States to show the effect of operation of farm machinery on the levels of airborne dust. Plutonium concentrations in the area were shown to increase during machinery operation (Harley, 1980). 3.5.2. Global dust events The Saharan dust (African dust) and Kosa (Asian dust) are the two largest aeolian dust sources on earth. During the outbreak of Saharan and Asian dust events, millions of tons of dust aerosol are injected into the atmosphere. Current estimates of global dust emissions from these two events range between 1000 and 3000 Tg/ y (tera gram per year) of which Saharan dust and Asian dust contribute about 50e70% and 10e25% of the total, respectively. It has been reported that during the dust events aerosols can reach as high as 10 km (Masson et al., 2010), and thus can influence radionuclides redistribution in the atmosphere. Since wind-blown dust arising from surface soil is a significant carrier of anthropogenic radionuclides, the Saharan and Asian dusts are shown to cause scattered increase in anthropogenic radionuclide concentrations in the atmosphere. Pham et al., 2017 reported that the 239þ240Pu levels in Saharan dust deposits ranged from 0.68 to 0.98 Bq/kg, which is significantly higher than the level usually found in Saharan soil (0.096 Bq/kg). The primary source of anthropogenic radionuclides in the dust is surface soil contaminated from global fallout and possibly from regional fallout such as Chernobyl or Fukushima. The activities and atom ratios of dust samples collected during different Saharan dust events confirmed their global fallout origin and/or mixing with local re-suspended soil particles during transport. Several studies have reported elevated concentrations of 137Cs and 239þ240Pu in surface air and in deposition samples during the Saharan air mass intrusions in Monaco and southern France (Pham, 2005; Masson et al., 2010). According to these authors, the Saharan dust has following characteristic isotope ratios: 238Pu/239þ240Pu activity ratio of 0.028e0.049; 241Pu/239þ240Pu activity ratio of 241 3.0e3.11; Am/239þ240Pu activity ratio of 0.38e0.44; 239þ240 Pu/137Cs activity ratio of 0.025e0.029; and 240Pu/239Pu atom

ratio of 0.194 (Pham, 2005; Masson et al., 2010). Similarly, higher than normal concentrations of 137Cs and 90Sr were also recorded in the monthly deposition samples collected during 2000s in Japan (Igarashi et al., 2011; Hirose et al., 2003). The deposition data showed a typical seasonal variation with highest levels occurring in spring and lowest in winter. The authors attributed this seasonal variation in radionuclide concentrations to the continental dust event originating from the East Asian arid regions. Hirose et al., 2003, 2004 also reported an elevated 239þ240Pu concentrations in the monthly deposition samples collected from South Korea and Japan during spring. The average 240Pu/239Pu atom ratios ~0.187 in Korea and ~0.19 in Japan suggest their global fallout origin. The backward trajectory analysis of storm confirmed that the plutonium in the dust originates from the East Asian continent deserts and arid areas. 3.5.3. Wildfire and biomass burning Wildfire and biomass burning is another important pathway for transport of radionuclides in the environment. Following the Chernobyl and Fukushima accidents, substantial radioactive contamination of farmland and forests occurred in many countries in the northern hemisphere. The Gomel and Mogilev regions of Belarus, the Kiev region of Ukraine, and the Bryansk region of the Russian Federation constitutes the largest area in the world with the highest contamination by radionuclides and is located in a fire-prone forest environment in the center of Europe. According to a study by NRU 2011, it is estimated that 2e8 PBq of 137Cs and 1e4 PBq of 90Sr still remain in the contaminated areas of Ukraine and Belarus, whereas the rest (238þ240Pu and traces of 241Am) are in the order of TBq. When forests laden with radionuclides burn, they emit radioactive cesium, strontium, and plutonium in respirable fine particles (Hao et al., 2009). Additionally, the radionuclides resuspended by wildfires can be transported over intercontinental distances. An elevated level of 137Cs (253 mBq/m3) in air over Vilnius, Lithuania was attributed to transport of Chernobyl-derived cesium from the Chernobyl exclusion zone of Ukraine following the wildfire event of 1992 (Lujaniene et al., 2006). The recent wildfire events of April and August 2015 in the area have been estimated to have released about 12.5 TBq (1012 Bq) of radioactivity mostly 137Cs, 90Sr, 239þ240Pu, 238 Pu, and 241Am into the atmosphere (Evangeliou et al., 2016). Based on the release estimates, this wildfire event can be classified as a “level 3” nuclear event on the INES (International Nuclear Event Scale). 3.5.4. Large-scale volcanic eruptions The strong volcanic eruptions could potentially cause redistribution of radionuclides in the atmosphere. Alvarado et al., 2014 presented an interesting result of investigations of plutonium isotopes and 137Cs in the stratospheric and tropospheric aerosols €kull volcano plume. These authors during the passage of Eyjafjallajo reported about three order of magnitudes higher 239þ240Pu (8e24 mBq/m3) and 137Cs (~1mBq/m3) in aerosol samples collected from the lower troposphere (altitude 1e3 km) during the eruption of €kull volcano in 2010. The authors argue that radionuEyjafjallajo clides attached to the fine aerosol particles (<0.02 mm in diameter) could have longer residence time in the stratosphere, and therefore radionuclides injected to the stratosphere mainly during the early 1960s might still be present there during the 2000s. Using the 2010 Eyjafjallajokull eruption as an example, these authors further noted that volcanic eruptions could potentially cause redistribution of these particles from the upper stratosphere to the lower troposphere. As for how the volcanic debris has redistributed anthropogenic radionuclides in the atmosphere, the authors proposed that steam-lifted ash and sulfur dioxide particles injected into the

P. Thakur et al. / Journal of Environmental Radioactivity 175-176 (2017) 39e51

atmosphere during the volcanic eruption picked up radionuclides from the stratosphere and brought them down into the troposphere resulting in enhance transport of radionuclides to the troposphere. Alvarado et al., 2014 also observed increased 239þ240 Pu and 137Cs levels during 2007 and 2008 in the lower stratosphere (altitude 10.7e12.5 km). However, surface air samples for the same period showed no increase. Furthermore, it has been reported that about 15e20 megatons of volcanic ash and gases (i.e., SO2) were injected into the atmosphere when one of the 20th century greatest volcano Mt. Pinatubo erupted in June 1991. However, due to lack of stratospheric/tropospheric data for this period, it is difficult to unambiguously establish the role of volcanic debris in redistribution of anthropogenic radionuclides in the atmosphere. Using estimates of the stratospheric fallout mean residence time of 1.5 ± 0.5 years (Hirose and Povinec, 2015) and the mean tropospheric residence time of plutonium aerosols of 71 days (Holloway and Hayes, 1982), it is likely that most of the stratospheric plutonium had been removed from the atmosphere before injection of Mt. Pinatubo debris. This interpretation is further supported by 137Cs and 7Be surface air data, which show no increase during 1991 (Pham et al., 2013). Additionally, the 239þ240Pu surface air concentration data for the 1990s showed no noticeable increase (Fig. 2). Further studies are needed to obtain a better understanding regarding the role of volcano debris in redistributing plutonium in the atmosphere. 3.5.5. Sea-spray effects Atmospheric transport of plutonium via sea-salt aerosols is another potential source of plutonium in the atmosphere. Studies conducted in the vicinity of the British reprocessing facility at Sellafield and French nuclear reprocessing facility near La Hague showed that the sea spray transport of marine discharged radioactivity transfers significant levels of long-lived radionuclides to the land, much of it in the respirable size range, and that this material can be carried to large distances from the sea. The sea salt aerosol consists primarily of chlorides (~88.7%) and sulfates (~10.8%), while other chemical substances constitute about 0.5% of the total mass. The 239þ240Pu concentration in sea water is significantly lower than that in soil and therefore the contribution of plutonium from sea salt is much smaller than that from the soil. Hirose et al., 1983, estimated that the concentration of plutonium from sea salt aerosol in surface air of Tsukuba was less than 1mBq/ m3. Plutonium-bearing sea-spray aerosols have been observed 9 km inland from the Irish Sea. The activity levels of plutonium in sea-spray diminished at least by a factor of three with distance from shore (Eakins and Lally, 1984). The highest concentration of 239þ240 Pu reported in the surface air was 814 mBq/m3. From an estimation of the deposition rate of sea salt at the coast and the radionuclide levels present at sea, these authors estimated enrichment factors of 10e20 for Pu and 241 Am and 1e3 for 137 Cs. Other studies indicate that the sea spray borne radioactivity can be detected at large distances from the sea. For example, Cambray and Eakins (1982) found that levels of 239þ240Pu and 241Am in the soil cores taken 20 km inland were still in excess of those expected from nuclear fallout. It has also been reported that the sea spray transport of marine discharged radioactivity transfers significant levels of long-lived radionuclides to the land, much of it in the respirable size range, and that this material can be carried to large distances from the sea. An alternative pathway for plutonium transport from sea to atmosphere is flooding, followed by deposition on soil matrix and resuspension, as observed in the shorelines of Irish Sea (McKay and Pattenden, 1993). Flooding, deposition and re-dissolution cycle of soil is regular at tidal zones. A sheep liver sample from a flooding area was reported to contain about 9.3 Bq/g of 239þ240Pu, whereas

49

the corresponding liver sample from a non-flooding area contained only about 0.012 Bq/kg of 239þ240Pu (McKay and Pattenden, 1993). In the long run, the marginal effect of sea spray could become significant due to the enrichment of sea-spray with sediments in the surf zones. 4. Conclusions In this study, historical data collected by various national and international monitoring networks were used to describe the source terms, levels and long-term behavior of plutonium in the atmosphere. The atmospheric nuclear weapons testing of 1950s1960s and the burn-up of the SNAP-9A satellite have been the major sources of plutonium in the atmosphere. However, the amount of plutonium in the atmosphere today is small and comparable to the levels generally found at surface-level air. Long-term variations (1963e2015) of plutonium isotopes in the upper atmosphere show that the plutonium in the atmosphere decreased with an apparent residence time of about 1.5 ± 0.5 years, and that the temporal variations of plutonium in surface air followed the stratospheric trends until the early 1980s. The 239þ240Pu air concentration post-1984 is still frequently detectable, whereas the detection of 238Pu is less frequent. The major source of plutonium in the surface air today is not the stratospheric plutonium, which was injected into the atmosphere in 1950e1960s, instead most plutonium in the air today is associated with resuspended soil, which is contaminated from nuclear weapons fallout. Additionally, other environmental processes such as global dust storms and biomass burning/wildfire also play an important role in redistributing plutonium in the post fallout air. References Alvarado, J.A.C., Steinmann, P., Estier, S., Bochud, F., Haldimann, M., Froidevaux, P., 2014. Anthropogenic radionuclides in atmospheric air over Switzerland during the last few decades. Nat. Com. 5, 3030. http://dx.doi.org/10.1038/ncomms4030. Arimoto, R., Kirchner, T., Webb, J.L., Conley, M., Stewart, B., Schoep, D., Walthall, M., 2002. 239,240Pu and inorganic substances in aerosols from the vicinity of the Waste Isolation Pilot Plant: the importance of re-suspension. Health Phys. 83, 456e470. Arnold, D., Wershofen, H., 2000. Plutonium isotopes in ground-level air in Northern Germany since 1990. J. Radioanal. Nucl. Chem. 243, 409e413. Beck, H.L., Bennett, B.G., 2002. Historical overview of atmospheric nuclear weapons testing and estimates of fallout in the continental United States. Health Phys. 82, 591e608. Bennett, B.G., 1978. Environmental Aspects of Americium. Rep. EML-348. Environmental Measurements Laboratory, U.S. Department of Energy, New York, New York. Bortoli, de M.C., Gaglione, P., 1969. SNAP plutonium -238 fallout at ISPRA Italy. Health Phys. 16, 197e204. Bourlat, Y., Millies-Lacroix, J.C., Dunoyer, B., 1994. Plutonium-239,240 atmospheric radioactivity measurements at Mururoa from 1986 to 1991. J. Environ. Radioact. 177, 107e120. Bu, W., Ni, Y., Guo, Q., Zheng, J., Uchida, S., 2015. Pu isotopes in soils collected downwind from Lop Nor: regional fallout vs. global fallout. Sci. Rep. 5, 12262. http://dx.doi.org/10.1038/srep12262. Cambray, R.S., Playford, K., Lewis, G.N.J., 1985. Radioactive Fallout in Air and Rain: Results to the End of 1984. AERE R 11915. Environmental and Medical Sciences, Division Harwell Laboratory, Oxfordshire, England. Cambray, R.S., Eakins, J.D., 1982. Pu, 241Am and 137Cs in West Cumbria and a maritime effect. Nature 300, 46e48. Carlsbad Environmental Monitoring and Research Center.www.cemrc.org/Annual Report.New Mexico State University. Carlsbad, NM. n, M., Enamorado, S.M., Jime nez-Ramos, M.C., Wacker, L., Chamizo, E., García-Leo 2010. Measurement of plutonium isotopes, 239Pu and 240Pu, in air-filter samples from Seville (2001-2002). At. Environ. 44, 1851e1858. Choi, M., Lee, D., Choi, J., Cha, H., Yi, H., 2006. 239þ240Pu concentration and isotope ratio(240Pu/239Pu) in aerosols during high dust (Yellow Sand) period, Korea. Sci. Total Environ. 370, 262e270. Douglas, R.L., 1978. United States Environmental Protection Agency Office of Radiation Programs-Las Vegas Facility Technical Note ORP/LV-78-3. Eakins, J.D., Lally, A.E., 1984. The transfer to land of actinide-bearing sediments from the Irish Sea by spray. Sci. Total Environ. 35, 23e32. EML (Department of Energy, Environmental Measurements Laboratory), 2000. Surface Air Sampling Program. Available from: http://www.eml.doe.gov/

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