I. INTRODUCTION
On March 11, 2011, a powerful earthquake of 9 in the Richter scale hit the eastern part of mainland Japan. It triggered gigantic tsunami, with waves in heights of over 10 m, damaged the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). When the backup electricity supply was lost, cooling system of nuclear reactors in the FDNPP was broken down, causing hydrogen explosions in No.1 and No. 3 reactor buildings on March 12 and 14, respectively. The explosions released a large amount of radioactive materials into the environment. The accident was classified on the INES (International Nuclear and Radiological Event Scale) scale at the maximum level of 7, the same as the Chernobyl accident. Radioactive nuclides, mostly produced by the nuclear fission of 235U and released into the atmosphere and oceans, were carried by global atmospheric and oceanic circulations all over the world [1-3]. Radioactive particles with different composition, size, shape and structure were identified worldwide in different biotic and abiotic media. The radionuclides detected included 131I, 137Cs, 134Cs, 132Te, 132I, 136Cs, etc. and radioactive noble gases (133Xe, 135Xe) [4, 5].
Lanzhou (36 N, 103 E, 1520 m a.s.l.), the capital of Gansu Province, is located on the upper reach of the Yellow River in north-west China, approximately 3200 km west of Fukushima, Japan. An atmospheric radionuclide monitoring station has been operated by Gansu Center for Disease Prevention and Control (GSCDC) in Lanzhou. It belongs to Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) International Monitoring System (IMS) and provides atmospheric radioactivity detection data to CTBTO International Data Center [6, 7]. The Fukushima radionuclides were first detected in the aerosol sample collected on March 26, 2011 by GSCDC.
II. MATERIALS AND METHODS
A. Materials
In order to quantitatively determine the radionuclide fallouts in Lanzhou due to the Fukushima accident, systematic analysis of aerosol samples was undertaken by GSCDC right after the accident. The samples were measured by low-background HPGe detector operated in low level background shield made of lead, iron and copper. The detector was an ORTEC liquid nitrogen cooled P type HPGe coaxial detector, with 70% relatively efficiency, energy resolution (FWHM) of 1.92 keV at 1332 keV, and the integral background counting rate of 1.97 cps in energy range of 5–2734 keV.
The aerosol samples were collected with high volume air sampler (Snow White, Senya Oy, Finland). The sampling air flow was controlled by the inverter which controls the speed of pump motor through changing the AC frequency. The initial sampling flow rate was about 700 m3/h, and the flow rate was > 500 m3/h during a sampling period. The typical air volume pass through the filter in daily sampling was about 16000 m3. The 3M filler product has a collection efficiency of > 80% for a particle size of 0.2 microns at operation conditions. The effective sampling area was about 420 mm×550 mm.
B. Methods
The aerosol sampling was carried out at sampling spot in rural in northeast of Lanzhou. The air sampler was installed on the top of a concrete platform which is 6 m height above the ground. The exposed filter was compressed into a Φ50 mm×6 mm cylinder for gamma spectrum measurement. The sampling frequency was daily.
As the operation procedure, the aerosol monitoring was initiated with 24 h filter exposure, followed by a cooling time of 24 h to let the short lived radon progeny decay. The gamma spectrum measurement lasted for 24 h.
The gamma spectrometer was calibrated with reference source, which is a cylinder of Φ50 mm×5 mm. The calibration source was provided by National Physical Laboratory, UK. The radionuclides in the source include 241Am, 109Cd, 57Co, 139Ce, 51Cr, 113Sn, 85Sr, 137Cs, 54Mn, 88Y, 65Zn and 60Co. The source substrate is a blank filter for collecting aerosol samples. No correction for self-absorption effects was performed, as the sample density is close to the calibration source. The spectra were analyzed using Gamma-Vision32 provided by ORTEC with the gamma spectrometer. The peaks at 364.5 keV (131I), 477.6 keV (7Be), 661.6 keV (137Cs), 604.7 keV (134Cs) and 818.5 keV (136Cs) were used for activity determinations. The activity of 131I was decay corrected to the start time of sampling.
III. RESULTS AND DISCUSSION
A. Radionuclide concentration in aerosol
The aerosol in Lanzhou was daily monitored before the Fukushima accident, background levels for fission isotopes such as 131I, 134Cs and 136Cs were invariably below the minimum detectable concentration (MDC). They represent anthropogenic radionuclides without natural contribution, which were released in large quantities after atmospheric nuclear weapons tests, or from the Chernobyl nuclear reactor accident, and after authorized releases from nuclear reprocessing facilities. Under normal conditions, only 137Cs, due to its long half-life, is still present in the atmosphere [8].
Starting from March 26, radioisotopes release from nuclear reactors was detected. In this way, we could establish the Fukushima’s cloud arrival time in Lanzhou, about 14 days after the accident. The radionuclide detected was 131I, with the activity concentration of 0.064 mBq/m3. On April 6, 2011, 131I, 137Cs, 134Cs and 136Cs in air were recorded, in activity concentrations of 1.194, 0.231, 0.173 and 0.008 mBq/m3, respectively. Considering about the meteorological condition, the maximum values are correlated with rainfall which accelerated the radionuclides falling from upper to lower atmospheric probably. Two days after, the activity concentrations fell to about 50%, while they followed a general decreasing trend day by day. After May 2, 2011 all the observed values were below the MDC. The average concentrations of Fukushima radionuclides measured by GSCDC were [separate-uncertainty]0.23 0.30 (131I), [separate-uncertainty]0.03 0.05 (137Cs) and [separate-uncertainty]0.03 0.04 (134Cs) mBq/m3.
As shown in Fig. 1, the activity concentration of 131I is much higher than 137Cs and 134Cs in daily monitoring results. This means the radioactive cloud was richer in 131I, as iodine is a more volatile element than cesium. Results of radioactivity measurements in aerosols following the Fukushima accident showed that at least three radioactive plumes arrived at Lanzhou as indicated by 131I activity concentration, on March 28, April 1 and April 6.
The atmospheric transport model of the radioactive materials released from FDNPP was simulated by BGR (Bundesanstalt für Geowissenschaften und Rohstoffe) according to the data from CTBTO. As shown in Fig. 2, movements of the released radionuclides were initially controlled by the westerly winds toward the Pacific and North America. Under influence of the cyclonic system prevailing over the Bearing Sea, an arm of the air masses moved toward the eastern part of Russia. It then entered north-east China following the northerly flow along the western flank of the cyclonic circulation. Another arm of the air masses was rapidly transported across the Pacific Ocean, North America, Europe and Asia, and entered north-west China. After March 26, the fission products could be detected over the northern hemisphere.
According to the report of MEP (Ministry of Environmental Protection, the People’s Republic of China), the average concentrations (in mBq/m3) of Fukushima radionuclides measured in China mainland were [separate-uncertainty]0.60 0.89 (131I), [separate-uncertainty]0.16 0.15 (137Cs) and [separate-uncertainty]0.15 0.14 (134Cs). The average concentrations (in mBq/m3) measured in China northern provinces were [separate-uncertainty]0.88 1.11 (131I), [separate-uncertainty]0.20 0.17 (137Cs) and [separate-uncertainty]0.19 0.16 (134Cs). The average concentrations (in mBq/m3) measured in China southern provinces were [separate-uncertainty]0.30 0.31 (131I), [separate-uncertainty]0.11 0.07 (137Cs) and [separate-uncertainty]0.10 0.06 (134Cs). The average concentrations measured by GSCDC were lower than the MEP data, because lower activity concentrations (of the order of 10-6 Bq/m3) were not reported or detected by the monitoring station of MEP.
From Fig. 3, average concentrations of the Fukushima radionuclides measured in different provinces in China show significantly decrease from north to south and from north-west to south-east. It complied with the simulation of BGR that two main radionuclide transport pathways of the Fukushima accident entered mainland China.
Before the Fukushima fallout, however, 137Cs in activity concentration closed to MDC (of the order of 10-6 Bq/m3) was occasionally detected in aerosol samples due to the residual radioactivity of the atmospheric atomic bomb tests and of the Chernobyl accident. In contrast to normal background conditions when the main source of 137Cs in ground level aerosols is its resuspension from soil. This contamination had been monitored for a long time and no evidence of large fluctuations in air was found. In March 2013 (Fig. 4), 137Cs was detected with significantly increase in activity concentration in aerosol samples during a sand storm period. The 137Cs activity concentration was of the order of 10-5 Bq/m3, one order of magnitude higher than pre-Fukushima accident level. This indicates that the Fukushima fallout increased the activity concentration of 137Cs in soil in Lanzhou.
B. Fukushima radionuclides vs. cosmogenic 7Be
The cosmogenic 7Be, produced in the lower stratosphere and upper troposphere, has been widely used as a tracer of atmospheric processes [9]. Although the production of 7Be in the atmosphere is very different from 131I, 137Cs and 134Cs, its presence in the troposphere, and specifically its transport from the stratosphere/troposphere to the ground-level air may give information on vertical transport of air masses, and could help better understand the behavior of Fukushima radionuclides in the troposphere.
The 131I, 137Cs and 134Cs vs. 7Be aerosol activity record (Fig. 5) shows that appearance of the 131I, 137Cs and 134Cs activity concentration maxima was accompanied by 7Be increases usually. But the correlation coefficients of 131I, 137Cs and 134Cs vs. 7Be, which are r=0.167 (p=0.324), -0.064 (p=0.732) and -0.073 (p=0.736), respectively, differ from the results studied by other authors. As p > 0.05 indicates no correlation between 7Be and 131I, 137Cs and 134Cs. Povinec [8] found the 131I vs. 7Be and 137Cs vs. 7Be correlation coefficients of r=0.55 (p=0.061) and 0.59(p=0.043), respectively; and Lujaniené [10] found a stronger correlation between 131I, 137Cs and 7Be (r=0.69 and 0.75, respectively). The different correlation coefficients were probably because of the small data set.
C. Activity ratios
The 131I/137Cs activity ratio in aerosol samples varied from 0.3 to 133.5 with the average of 13.5 from March 27 to May 2 (Fig. 6(a)). The average value is closed to the 131I/137Cs activity ratio in total atmospheric release which was estimated to be 11 [11]. The 131I/137Cs ratios in mainland China ranged 0.8 - 26.4 with an average of 6.3 according to the MEP data.
To compare the 131I/137Cs activity ratio at the initial release of radionuclides from the FDNPP, the 131I activity was decay-corrected to March 12, 2011 when the first emission of radionuclides occurred. As shown in Fig. 6(b), the decay-corrected 131I/137Cs activity ratios ranged from 18.9 to 486.3, with an average value of 122. From March 27 to April 20, the decay corrected 131I/137Cs activity ratio showed a minimum value of 36, on April 5. However, the 131I/137Cs activity ratio seemed to return gradually to the values observed before April 5 after the minimum was reached. The similar variation in concentration and temporal decreases in the 131I/137Cs activity ratio were monitored in the western regions of Japan such as Kanazawa, Nagoya, Osaka, and Tokushima [12].
The main long-lived constituents in the Fukushima fallout in Lanzhou are 137Cs and 134Cs, which have half-lives of 30.17 years and 2.06 years, respectively. The nuclides are utilized as time markers. The 134Cs/137Cs activity ratio can reveal information on radioisotope origin, such as deciding the fallout of a bomb or the fission products of a power reactor [13]. The relatively high activity concentrations of 137Cs and the very low values of activity ratio of 134Cs/137Cs, in combination with the absence of 134Cs in most of the cases indicate a strong contribution from "older" 137Cs, probably from the Chernobyl accident and past global fallout [14]. The contribution of 134Cs was similar to 137Cs, as the 134Cs/137Cs activity ratio was close to 1 in the air from the FDNPP accident [15]. Figure 7 shows the 134Cs/137Cs activity ratios in aerosols taken in Lanzhou. The ratios of 134Cs/137Cs observed from March 27 to April 19 ranged 0.3–1.4 with an average of 0.8; while the 134Cs /137Cs activity ratio based on the MEP data varied from 0.3 to 1.8 with the average of 0.9 in mainland China.
D. Back trajectories analysis
In order to simulate the pathway of air masses from Fukushima arriving at Lanzhou, China, the NOAA-ARL (National Oceanic and Atmospheric Administration-Air Resources Laboratory) HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory) model was used. The Fukushima radionuclides were detected in Lanzhou on March 26, 14 days after the Fukushima nuclear accident. The maximum hours that can be simulated in the HYSPLIT application are 312 h (13 days). Thus, 13 days back-trajectories were used to study the transport of air masses from Japan to Lanzhou.
The back-trajectory analysis (Fig. 8) shows a direct transfer from Fukushima started on March 13 across the Pacific Ocean, North America and Europe arriving in Lanzhou at the height of 9000 m AGL (above model ground level). Therefore, we can know that the air masses released from Fukushima accident on March 12 was transported to Lanzhou at the height close to 9000 m AGL.
E. Effective dose for inhalation
Particulate 131I collected on filter was only part of the total 131I present in the air. In the radioactivity from Fukushima, the ratio of gaseous to total over Europe averaged [separate-uncertainty]0.772 0.136 based on pooled measurements of both 131I forms, with a noticeable constant value, geographically and altimetrically [16]. Therefore, the total 131I concentration in the air mass over Lanzhou would be nearly four times the particulate 131I activity concentration measured on filters, i.e. at the peak it would be about 4.76 mBq/m3.
The effective dose for inhalation to the general population produced by the arrival of the radioactive plume to Lanzhou was calculated using the conversion factors set by the IAEA [17], an average inhalation flux of air of 22.2 m3/d per person and the results of the activity concentrations measured by GSCDC. The obtained values were 2.45 nSv and 8.86 nSv with the activity concentrations of 131I were corrected by gaseous/total concentration ratio respectively. The very low effective dose calculated from the data collected in the aerosol samples demonstrates that the FDNPP accident in Japan had no impact over the health of the population of Lanzhou.
IV. CONCLUSION
The radioactivity from the FDNPP accident, triggered by the earthquake and tsunami of March 11, 2011, was detected in the ground surface air of Lanzhou 14 days after the steam venting and explosions. From March 26 to May 2, 2011, at least one kind of the Fukushima radionuclide within 131I, 137Cs, 134Cs and 136Cs was detected above background level. On the basis of radionuclide observations carried out in the air of Lanzhou following the accident at the FDNPP we may summarize the obtained results as follows:
Results of radioactivity measurements in aerosols following the FDNPP accident shows that at least three radioactive plumes arrived at Lanzhou as indicated by 131I activity concentration.
The back-trajectory analysis simulated by NOAA-ARL HYSPLIT shows a direct transfer of the air masses released from Fukushima to Lanzhou across the Pacific Ocean, North America and Europe at the height close to 9000 m AGL.
The aerosol monitoring data of 2013 shows the higher activity concentration of 137Cs was of the order of 10-5 Bq/m3, one order of magnitude higher than pre-Fukushima accident levels. It demonstrates that long-lived 137Cs from FDNPP accident could be detected in ground level aerosols as its resuspension from soil and it will exist in the environment for a long time.
The risk of effective dose for inhalation to human health produced by the arrival of the radioactive plume to Lanzhou can be compared to the dose limit to the general public of 1 mSv/year. The value of effective dose is close to one millionth of the annual limit. It can be concluded that at Lanzhou the dose increase due to the Fukushima accident is entirely negligible.
Preface: Migration of radionuclides from the Fukushima Daiichi Nuclear Power Plant accident
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