Sr-90 Deposition Observed in Central and Northeast Honshu Island , Japan , after the Fukushima Dai-ichi Nuclear Power Plant Accident

Monthly depositions of anthropogenic radionuclides, which directly reflect atmospheric processes including emission from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), are an important probe to re-construct FDNPP accident sequence. Japanese radioactivity survey network monitored monthly 90Sr deposition after the FDNPP accident. The highest monthly 90Sr deposition (470±17 Bq m-2) was observed in March 2011 at Futaba near the FDNPP site, which is the same order of magnitude as that due to global fallout observed in 1963. The higher 90Sr deposition in March 2011 occurred in the Kanto and inland of the Tohoku areas. The monthly 90Sr deposition decreased with time, as did 137Cs. Atmospheric effect of the FDNPP-derived 90Sr continued more than 1 year. 90Sr/137Cs activity ratios in March 2011, which were in the range from 0.99 × 10-4 to 3 × 10-4, showed no large spatial variation. This result allowed us to estimate total atmospheric emission of 90Sr from the FDNPP accident from the total atmospheric release of 137Cs. The total release amount of 90Sr at an early stage of accident was calculated to be 2.7 – 3.6TBq. Sr-90 Deposition Observed in Central and Northeast Honshu Island, Japan, after the Fukushima Dai-ichi Nuclear Power Plant Accident Publication History: Received: March 20, 2017 Accepted: July 20, 2017 Published: July 22, 2017


Introduction
On 11 March 2011, the Richter scale 9.0 magnitude earthquake, so called "the 2011 Great East Japan Earthquake", and Tsunami struck the northeast coast of Japan, resulting in widespread injury and loss of life.These natural disasters caused severe accident in the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) [1].As a result, large amounts of radionuclides have been released in the environment [1,2].Japanese government, research institute and university have continuously monitored the FDNPP-derived radionuclides in the atmospheric samples [3][4][5][6][7][8].Just after the FDNPP accident, 131 I, 134 Cs and 137 Cs were detected as dominant FDNPP-derived radionuclides in atmospheric samples such as atmospheric dust and deposition, whereas refractory radionuclides such as 103 Ru and 95 Zr could not be detected [3][4][5].In contrast to volatile radionuclides such as radiocesium, there is only limited information on atmospheric behaviors of FDNPP-derived radiostrontium ( 89 Sr (half life: 50.53 d) and 90 Sr (half life: 28.79 y)) [8] because radiostrontium was a minor release component due to un-volatile properties of oxide [9].Total atmospheric emissions of 89 Sr and 90 Sr from the FDNPP accident were estimated to be about one and two orders of magnitude lower than 137 Cs, respectively [1].Although radiostrontium is a minor component within the FDNPP-derived radionuclides, knowledge on radiostrontium derived from the FDNPP is important to have better understanding of characteristics of the FDNPP accident.
The monthly 90 Sr deposition have been monitored by Japanese Radioactivity Survey project since the late 1950s, in which major part has been performed by Meteorological Research Institute (MRI) [10][11][12][13].After the FDNPP accident, the monthly 90 Sr deposition observed at Tsukuba was reported [8], in which the monthly 90 Sr deposition in March 2011 increased to 5.2 ±0.1 Bq m -2 , being more two orders magnitude greater than that in the late 2000s.However, there is no report about atmospheric dispersion of the FDNPP-derived 90 Sr in Japan.
Activity ratios of 89 Sr and 90 Sr are an important indicator to identify freshly released 90 Sr using 89 Sr with a short half-life.Although after the FDNPP accident 89 Sr was detected in airborne dust ( 89 Sr/ 90 Sr: ~15) [8] and seawater ( 89 Sr/ 90 Sr: 11.8±0.8)[14], there is very limited information on 89 Sr in environmental samples because of complicated and time-consuming radiochemical procedure.Activity ratios of 90 Sr to 137 Cs are a useful tool to identify sources of radioactive releases and to have better understanding of environmental behaviors of these radionuclides [15,16].The annual mean 90 Sr/ 137 Cs activity ratios of deposition samples in the period of 1980 to 1985 were a range from 0.43 to 0.77, which is due to radioactive fallout from the 26 th Chinese atmospheric nuclear test conducted in October 1980 [17].In May 1986 just after the Chernobyl accident, the 90 Sr/ 137 Cs ratio in a Tsukuba deposition sample decreased to 0.0094.The 90 Sr/ 137 Cs ratios in deposition samples varied in the 1990s and 2000s due to effects of resuspension and terrestrial processes [13,18,19].In March 2011 just after the FDNPP accident, the marked low 90 Sr/ 137 Cs ratio (about 2x10 - 5 ) was observed in the deposition sample collected at Tsukuba [8].To have better understanding of environmental behaviors of the FDNPPderived radionuclides and the FDNPP accident sequence, therefore, it is important to examine the spatial and temporal variations of the 90 Sr/ 137 Cs ratios in deposition samples.
In this paper, we describe spatial distributions and temporal variationsof the monthly 90 Sr deposition observed in the central and eastern Honshu Island, Japan, after the FDNPP accident, and discuss total atmospheric emission of 90 Sr from the FDNPP.

Sampling and Method
Monthly 90 Sr deposition data recorded in Environmental Radioactivity database [20] were used for analysis of its temporal variation.Monthly radioactivity deposition samples have been collected at radioactivity monitoring stations by Meteorological Research Institute (MRI, Tsukuba) and local government laboratories.The monitoring stations in central and eastern regions in Japan (latitudinal range: 35°N -40°N, longitudinal range: 138°E -141°E, Figure 1 ).Monthly deposition samples (rainwater and falling dust) were collected by rainwater samplers with surface areas of 0.5 m 2 (4 m 2 for MRI), which are usually installed on the roof of main monitoring building in each monitoring station.Monthly rainwater and falling dust samples was collected in appropriate bottles on the first day morning of every month.
We briefly describe the method of sample preparation and 90 Sr measurements in detail described elsewhere [21]. 90Sr analysis has been performed by MRI and Japan Chemical Analysis Center (JCAC).Water sample was dried on a hot plate.The resultant residues were weighted after drying in an oven at 110°C and then transferred to a plastic container.Dried residue sample in a plastic container was subjected by gamma spectrometry.After the gamma survey, the deposition samples collected by local government laboratories were transported to JCAC.A part of the deposition sample was subjected to 90 Sr measurements.The deposition samples were decomposed with conc.HNO 3 and H 2 O 2 , and dissolved in HNO 3 solution.To separate Sr fraction from other natural and anthropogenic beta-emitters, carbonate, oxalate, chromate and iron hydroxide precipitation techniques were performed successively, and the Sr fraction was finally purified by fumic HNO 3 technique or ion exchange technique basically according to MEXT manual [21,22].After that, Sr was precipitated as carbonate and fixed on a 5C filter paper, which was subjected to beta counting.After the activity measurement sources were left for several weeks to achieve 90 Sr-90 Y radioequilibrium, their beta-activities were measured using a low-background 2π gas-flow counter (Tennelec LB5100) and determined with a calibration curve, which was made from a relationship between known amounts of 90 Sr activity and SrCO 3 .Counting efficiency ( 90 Sr-90 Y) was in a range of 50 to 70%.Chemical yield was determined from recovery of added stable Sr.In this case, treatment to remove contribution of 89 Sr has been carried out [8].The 90 Sr activity was decay-corrected at mid-sampling.The detection limits of 90 Sr were approximately 3.5 mBq m -2 (MRI) and approximately 50 mBq m -2 (JCAC), respectively.

Results and Discussion
The spatial distributions of monthly 90 Sr deposition in March 2011 are depicted in Figure 1A.The monthly 90 Sr depositions in the sites within 300 km apart from the FDNPP, which were in the range from 0.89 ±0.023 Bq m -2 to 470 ±17 Bq m -2 , are more than one order of magnitude greater than pre-FDNPP accident level [8,13,23].On the other hand, in contrast of 137 Cs [5], 90 Sr activities in some locations of Kofu (35.65°N, 138.57°E; inland site) and the Japan Sea side sites (Niigata: 37.91°E, 139.04°N and Akita: 39.72°E, 140.10°N) were less than the detection limit.The highest monthly 90 Sr deposition was observed at Futaba about 5 km from the FDNPP, which is higher than the maximum monthly 90 Sr deposition (170 Bq m -2 ) originating from the 1961-1962 large-scale atmospheric nuclear testing observed at Koenji (Tokyo) in 1963 [10][11][12].The relatively high 90 Sr depositions occurred at Hitachinaka and Tsukuba, corresponding to one of major   pathway of the radioactive plume derived from the FDNPP accident [24].The spatial distribution of the 90 Sr deposition in March 2011 roughly corresponded to high 137 Cs deposition area depicted by aerial monitoring [1].These findings suggest that 90 Sr detected in the deposition samples collected in March 2011 is derived from the atmospheric emission due to the FDNPP accident, which is supported detection of 89 Sr (half life: 50.53 d) in aerosol samples [8].In April 2011 as shown in Figure 1B, a region detected 90 Sr in deposition samples spread to coastal zone of the central Honshu-Island and coastal sites in the Japan Sea, although the monthly 90 Sr depositions in the sites detected in March 2011 decreased.The temporal change of the affected area, where the FDNPP-derived 90 Sr was observed, is similar to that of 137 Cs [5].In May 2011 (Figure 1C), the monthly 90 Sr deposition in all sites decreased.The spatial distribution of the 90 Sr deposition suggests that the atmospheric deposition of the FDNPP-derived 90 Sr cannot result in marked enhance of 90 Sr deposition density in land surface except area adjacent to the FDNPP.Measurements of 90 Sr in soil revealed that a small increase of 90 Sr was observed in the exclusion zone near the FDNPP, in which an average 90 Sr concentration in surface soil was 17.5 Bq kg -1 with a range from 1.4 to 80.8 Bq kg -1 whereas out of the exclusion zone there was no significant increase of soil 90 Sr having an average 90 Sr of 4.6 Bq kg -1 with a range from 1.6 to 20.6 Bq kg -1 , which is the same level as that in the pre-FDNPP era [25,26].
In order to elucidate depositional behaviors of the Fukushimaderived radionuclides, it is important to examine the temporal change of the monthly 90 Sr deposition.Temporal variations of the monthly 90 Sr depositions during the period from March 2011 to March 2012 are shown in Figure 2. The monthly 137 Cs deposition except Morioka exhibited decrease tendency during the period of March to May.We calculated apparent atmospheric half-lives (AAHL) of the Fukushimaderived 90 Sr (Table 1).The AAHLs at Futaba and Yamagata are similar to that of the FDNPP-derived 137 Cs during the corresponding period [5], whereas the AAHLs in the Kanto plain (19 -44 days) are longer than that of the FDNPP-derived 137 Cs.This finding suggests that the atmospheric behavior and/or emission processes of the FDNPPderived 90 Sr were different from 137 Cs.After June 2011, the level of the monthly 90 Sr deposition except several sites decreased to less than detection limit.However, the monthly 90 Sr deposition at Futaba near the FDNPP with a range of 0.27 to 2.2 Bq m -2 showed no decrease until March 2012.The monthly 90 Sr deposition at Hitachinaka about 100 km south the FDNPP were measureable until November 2011, although 90 Sr was detected in several samples collected in 2012.For Tsukuba, the monthly 90 Sr deposition decreased until July 2011 and was kept at a constant level (0.01 to 0.077Bq m -2 ), which is about one order of magnitude larger than the pre-FDNPP level (2009-2010: ND to 0.0077Bq m -2 ).These findings suggest that the FDNPP-derived 90 Sr at least affected in atmosphere of the Kanto plain until early 2012, although its radiological contribution was very small.90 Sr/ 137 Cs activity ratios in deposition samples are an effective tool to have better understanding of emission and resuspension processes of the FDNPP-derived radionuclides [13,18,19].We examine the spatial distribution of the 90 Sr/ 137 Cs ratios in the deposition samples collected in Mach 2011.The result is shown in Figure 3.The 90 Sr/ 137 Cs ratios, which ranged from 0.88 × 10 -4 to 3.5 × 10 -4 with a median of 1.8 × 10 -4 , are approximately constant spatially.These ratios are similar with the 90 Sr/ 137 Cs ratios in soil of the exclusion zone with a range from 0.57 × 10 -4 to 3.0 × 10 -4 , although 90 Sr in soil contained contribution of 90 Sr derived from global fallout [25,27].The median of Page 3 of 5 the 90 Sr/ 137 Cs ratio in the deposition samples reflects that in the major radioactive emission into atmosphere just after the FDNPP accident.This finding allows us to estimate the atmospheric total 90 Sr emission from total 137 Cs release.Many researchers have estimated the total atmospheric release of 137 Cs derived from the FDNPP accident because of one of major emission radionuclides and its long physical and environmental half-lives, which has been summarized in several reviews [1,2,28].Aoyama et al. [29] concluded that the most reliable value of the total atmospheric 137 Cs release is 15 -20 PBq.The total atmospheric 90 Sr emission is calculated to be 2.7 -3.6 TBq from the median of the 90 Sr/ 137 Cs activity ratios and the total atmospheric 137 Cs release, which corresponds to 0.00052-0.00069% of the total core inventory (5.22 × 10 2 PBq).This estimated total atmospheric emission of 90 Sr is more than one order of magnitude lower than the previous estimate (139TBq) [1].The temporal variations of the 90 Sr/ 137 Cs activity ratios in deposition samples collected at Futaba, Hitachinaka, Tsukuba, Utsunomiya and Yamagata are shown in Figure 4.The 90 Sr/ 137 Cs ratios at Futaba near the FDNPP showed around 2 × 10 -4 , although relatively high and low ratios occurred on August -October 2011 and on January -March 2012, respectively.The 90 Sr/ 137 Cs ratios at Hitachinaka, Tsukuba and Utsunomiya, corresponding to the Kanto area, increased from April to June 2011 and have exhibited high values of 0.00055 -0.013 after April 2011, although its large temporal and spatial variations were observed.The 90 Sr/ 137 Cs ratios at Yamagata northwest of the FDNPP increased after May 2011.These findings suggest that atmospheric emission processes and dispersion behaviors of 90 Sr and 137 Cs after April 2011differed from the initial release in March 2011.To explain long-term atmospheric effects of the FDNPP-derived radionuclides, Page 4 of 5 resuspension hypothesis that 90 Sr and 137 Cs in deposition were supported by local suspension of surface soil particles has been proposed [6,30,31].However, it is difficult to explain long-term variations of the 90 Sr/ 137 Cs ratios in the deposition by this hypothesis because 90 Sr and 137 Cs in local soil are too low to support their deposition levels [6].One of other possible processes is atmospheric emission from highly contaminated area including the FDNPP [7].The relatively high 90 Sr concentrations (up to 1 kBq kg -1 ) were observed in surface soil near the FDNPP collected in the period of December 2011 to July 2012, which correspond to higher 90 Sr/ 137 Cs ratios in the major emission [32].The post accident sporadic release of radionuclides may be contained more enriched 90 Sr than in the initial release because TEPCO implemented selective removal of radiocesium in contaminated water at the first stage.In fact, one major sporadic release of radionuclides as a late emission was observed on August 19, 2013, in which higher 90 Sr contamination and higher 90 Sr/ 137 Cs ratio (0.04) in a soil sample corresponding to the simulated plume line [33].However, factors controlling the complicated temporal variations of the 90 Sr/ 137 Cs activity ratios are still unknown as mentioned by Igarashi et al. [8].

Conclusion
We summarized monitoring results of radiostrontium after the Fukushima Daiichi Nuclear Power Plant accident.Radiostrontium ( 89 Sr and 90 Sr) was released in the atmosphere due to the FDNPP accident.Since the maximum deposition on March 2011 just after the FDNPP accident, the monthly 90 Sr deposition in the sites decreased with time.The late effects of 90 Sr in deposition continued more than one year, although the area affected by 90 Sr is limited comparing with 137 Cs.For the initial emission in March 2011, the 90 Sr/ 137 Cs activity ratios in deposition samples with a median of 1.8 × 10 -4 were approximately constant.The total atmospheric emission of 90 Sr is estimated to be 2.7 -3.6 TBq from the 90 Sr/ 137 Cs ratio and the total release amount of 137 Cs.The 90 Sr/ 137 Cs activity ratios in deposition samples showed large spatial and temporal variability, which reflects complicated atmospheric behaviors of 90 Sr and 137 Cs including postaccident sporadic releases from the FDNPP site.

Figure 1 :
Figure 1: Spatial distributions of monthly 90 Sr deposition observed in Japan.(Unit: Bq m -2 ) A: March, B: April, C: May.Abbreviations of station names are shown in Table1.
Figure 1: Spatial distributions of monthly 90 Sr deposition observed in Japan.(Unit: Bq m -2 ) A: March, B: April, C: May.Abbreviations of station names are shown in Table1.

Figure 2 :
Figure 2: Temporal variations of monthly 90 Sr deposition during the period from March 2011 to March 2012.Closed red circle: Futaba, closed black square: Hitachinaka, open black circle: Tsukuba, open green square: Utsunomiya, closed blue circle: Yamagata