Levels, Variations and Sources of 90sr and 137cs in Environmental and Food Samples Around Qinshan Nuclear Power Plant, China in 2012-2019

Yiyao Cao Zhejiang Provincial Center for Disease Control and Prevention Zhixin Zhao Hangzhou Hospital for the Prevention and Treatment of Occupational Disease,China Xiaoming Lou Zhejiang Provincial Center for Disease Control and Prevention Shunfei Yu Zhejiang Provincial Center for Disease Control and Prevention Meibian Zhang National Institute for Occupational Health and Poison Control, Chinese Center for Disease Control and and Prevention Zhiqiang Xuan Zhejiang Provincial Center for Disease Control and Prevention Zhongjun Lai Zhejiang Provincial Center for Disease Control and Prevention Xiangjing Gao Zhejiang Provincial Center for Disease Control and Prevention Yaoxian Zhao Zhejiang Provincial Center for Disease Control and Prevention Hong Ren Zhejiang Provincial Center for Disease Control and Prevention Dongxia Zhang Zhejiang Provincial Center for Disease Control and Prevention Peng Wang (  pwang@cdc.zj.cn ) Zhejiang Provincial Center for Disease Control and Prevention


Introduction
Over the past decades, the proportion of energy sourced from nuclear power has increased rapidly worldwide. China has vigorously developed nuclear power in recent years [1] . Systematic monitoring of important anthropogenic radionuclides is crucial for revealing sources and transport pathways in case of sever accidental or deliberate releases of radioactive pollutants. Among the radionuclides released from the operation of nuclear facilities and nuclear accidents, the two high-yield ssion products 90 Sr (t ½ = 28.79 y) and 137 Cs (t ½ = 30.17 y) are recognized as the most important from the radiological perspective [2] . Due to their relatively long half-lives, 90 Sr and 137 Cs can be preserved in terrestrial and marine systems for a long time once entering the environment. Under natural environmental conditions, 137 Cs and 90 Sr mainly enter the human body through the food chain and respiration. Both 90 Sr and 137 Cs have long biological half-lives in the human body [3][4][5][6][7][8] , therefore it is important to continuously monitor 90 Sr and 137 Cs in environmental and food samples, especially those from the surroundings of nuclear facilities, to ensure the radiological safety of individuals and the environment. In contrast to 137 Cs, longterm monitoring data for 90 Sr activity in environmental and food samples worldwide are sparse. This fact is mainly attributed to the long and tedious sample preparation and measurement procedures for 90 Sr.
The Qinshan nuclear power plant (QNPP) is the rst nuclear power plant in China and o cially commenced commercial operation in December 1991. The QNPP is a multi-unit nuclear power plant, and consists of three phases NPPs operating two heavy water reactors (HWRs) and seven pressurized water reactors (PWRs) with a total installed capacity of ca. 6.5 GW. Arti cial radionuclides such as 3 H and 14 C in the vicinity of QNPP have been investigated to a very limited extend during the past decades [9,10] . Studies on primary ssion products in environmental and foods samples, especially long-term systematic studies have not been reported so far. In this work, we report for the rst time a long-term observation of 137 Cs and 90 Sr in environmental and food samples collected around QNPP during 2012-2019. The distribution levels, temporal variations and source terms of 137 Cs and 90 Sr in the study region were investigated. The annual effective dose was estimated for the local public based on our measurement data.

The study region and sample collection
The QNPP is located in Haiyan County, Jiaxing City, Zhejiang Province in China, close to Hangzhou Bay in the East China Sea, 130 km away from Shanghai, and 93 km away from Hangzhou (the capital of Zhejiang Province). All samples were collected within 30 km of QNPP during the period of 2012-2019 ( Fig. 1), and the detailed sampling information is listed in Table 1.
Total (dry and wet) atmospheric deposition samples were monitored quarterly at a location 9.8 km away from QNPP. A total deposition collector (ZJC-deposition automatic collector, Zhejiang Hengda technology co.) with a surface area of 0.25 m 2 was placed on the open space of a building roof at Center for Disease Control and Prevention of Haiyan for sample collection. The sample was collected at the end of each month and bulked quarterly for radioactivity measurement.
Surface waters were collected in Qianmudang reservoir, and tap waters were collected at the same place as for the total atmospheric deposition samples. The water samples were collected twice per year from each location in May (wet season) and October (dry season), respectively.
As the most popular food for Haiyan residents, mullet, salsola, rice and crucian carp were collected annually in this work for radioactivity monitoring. Mullet and salsola were locally produced in Haiyan, whereas rice and crucian carp were collected in the Haining City (the primary supplier of rice and crucian carp to local Haiyan population).

Sample preparation and measurement
All dried food samples were placed in a 10 L quartz crucible and ashed with a microwave-ashing furnace (MKX-R4HB, Qingdao Maikewei Microwave Technology Co. Ltd) according to the rapid pretreatment method [11] with gradual temperature increase from 100-150°C to 500°C (Table S1 in supporting information). All total atmospheric deposition samples were dried on a graphite heating plate (YKM-400C; Changsha Yonglekang Instrument Equipment Co., Ltd.) at 100℃ and then ashed in a mu e furnace (Thermo Fisher Scienti c Co., Ltd.) at 450℃ for 8 h.
The radioachemical analysis of 90 Sr in water and ash samples was according to the Chinese national standard procedures [12][13][14][15] . In general, Sr (100 mg) and Y (20 mg) carriers were added to water (adjusted to pH = 1.0 with concentrated HNO 3 ) and ash samples. For water samples (50 L of each), SrCO 3 /CaCO 3 precipitation was used to pre-concentrate Sr. The SrCO 3 /CaCO 3 precipitate was dissolved with 6 mol/L HNO 3 and the sample was adjusted to pH = 1 for further chromatographic puri cation.
The processed sample solution for water or ash was loaded onto a chromatographic column (1 cm diameter × 15 cm length) containing di-(2-ethylhexyl) phosphoric acid (HDEHP), at a ow rate of 2 ml/min to separate 90 Y. The column was washed with 40 mL of 1.5 mol/L HNO 3 at 2 ml/min, and Y was eluted with 30 mL of 6 mol/L HNO 3 at 1 ml/min. The separation time was recorded to calibrate the decay of 90 Y. Yttrium was nally prepared as oxalate precipitation for gravimetric measurement of Y chemical yields prior to detection with a low background α/β counter (LB790; German Berthold Technology Company). Each sample was counted for 10 cycles, with each cycle for 100 minutes [12][13][14][15] .
The determination of 137 Cs in water and ash samples were according to Chinese national standards [16][17][18][19] . Prior to 137 Cs determination, the samples were kept for one month to allow for a complete decay of 131 I. Each dried food sample was screened with gamma spectrometry to eliminate the interferences of other short-lived radiocesium ( 134 Cs, 136 Cs, and 138 Cs) on 137 Cs measurement by beta counting. Cesium carrier (20 mg) was added to the water (50 L, adjusted with nitric acid to pH = 2.0) and ash samples (5-20 g). Cesium contained in each ash sample was leached with 1.5 mol/L HNO 3 for several times.
Ammonium phosphomolybdate (AMP) was added to water or leachate sample to adsorb cesium. The precipitate was ltered and dissolved in NaOH solution. Cesium was nally separated as Cs 3 Bi 2 I 9 precipitate in citric acid and acetic acid medium for gravimetric determination of chemical yield and measurement of 137 Cs using the low background α/β counter. The measurement time was kept 100 min per cycle, in total of 10 cycles for each sample [16][17][18][19] . All the analytical results obtained in this work were summarized in Table S2 the different source input functions and transport processes between 137 Cs and 90 Sr. 137 Cs is particle reactive and readily attached to aerosols and dust, whereas 90 Sr is more water soluble and easier to be dissolved in rainwater [20] . The late occurrence of 137 Cs peak compared to 90  May and 90 Sr in October, respectively. Whereas the peak values for both radionuclides in tap water, as observed in May 2017, seem to appear one year later compared to the source water. This again re ects the human intervention in the water treatment and delivery process. It is also noted that the annual average of 137 Cs activity concentration in source water decreased rapidly after 2016 to reach similar level as in 2012, while 90 Sr activity concentration was decreasing slowly and still nearly two times higher than the level in 2012. All the measured values for 90 Sr and 137 Cs activity concentrations in water samples around QNPP during 2012-2019 were below the concentration limits recommended by WHO and Chinese national standards [21,22] and also comparable to the reported values for waters collected around other NPPs in China (Table 2). Table 2 Activity concentrations of 90 Sr and 137 Cs in water and food samples collected around NPPs in various regions of China
In the freshwater systems, the high particle/colloid loads may effectively scavenge 137 Cs, whereas 90 Sr is kept stable due its highly conservative behavior. Freshwater in some rivers has been found to include high concentrations of 90 Sr [29,30] . Besides, higher salinity in brackish water may facilitate the uptake of 137 Cs (similar to potassium) into the sh. The values of concentration factors for Cs and Sr in marine shes are 100 and 3, respectively [31] . Therefore, it is reasonable to expect a larger difference in 137 Cs activity concentration between mullet and crucian carp compared to that in 90 Sr activity concentration.
The annual effective dose (AED) due to the ingestion of 137 Cs and 90 Sr in foods (i.e., mullet, salsola, rice and crucian carp) were estimated based on the Chinese national standard [32]  of Sr than Cs [35] . For example, very high 137 Cs/ 90 Sr ratios have been reported in the atmospheric fallout from Fukushima accident (∼1000) [36] and Chernobyl accident (∼1000) [37] . In seawater, 137 Cs/ 90 Sr ratios reached 39 ± 1 beyond the coast of Japan due to massive liquid releases of cooling water in spring 2011 [36] . The 137 Cs/ 90 Sr activity ratios before the Fukushima accident were reported in the range of 2.6-18 in Japanese sh whereas the 137 Cs/ 90 Sr activity ratios ranged from 98 to 480 after the accident [38] .
As 137 Cs is more readily adsorbed and immobilized on clay minerals while 90 Sr exhibits a higher mobility, it is reasonable to foresee a decrease in global fallout derived 137 Cs/ 90 Sr activity ratios in some freshwater and food samples. This has been approved in earlier studies where activity ratios of 137 Cs/ 90 Sr in wheat and polished rice from Japan increased by nearly 20 and 3 times, respectively, from 1959 to 1995 [39] . 137 Cs and 90 Sr in the atmospheric deposition should re ect their signature in the ground-level air, which is mostly from resuspension of the deposited 137 Cs and 90 Sr in soil [40] . Sr is chemically easier to elute than Cs in the soil column by rainwater [41] , therefore 137 Cs/ 90 Sr activity ratio in the typical surface soil usually becomes higher than the typical global fallout value [42,43] . However, lower 137 Cs/ 90 Sr activity ratios compared to the global fallout level were observed in the total atmospheric deposition in this work. This might be related to the relative high annual precipitation rate in the study region, therefore the total atmospheric deposition signal are predominated by wet precipitation associated to 137 Cs/ 90 Sr activity ratios comparable to fresh waters.
The peak 137 Cs/ 90 Sr activity ratio (1.14 ± 0.18) in the total atmospheric deposition in the second quarter of 2016 was ca. 2.5 times higher than the average values (0.46 ± 0.15) in 2015. This might suggest an additional radioactive input with higher 137 Cs/ 90 Sr activity ratio in the study region in 2016. As a potential consequence of this additional radioactive source, maximum 137 Cs/ 90 Sr activity ratios were observed in the source water (1.18 ± 0.05) in 2016, rice (1.14 ± 0.26) in 2017 and salsola (1.08 ± 0.09) in 2018, respectively. The occurrence of peak 137 Cs/ 90 Sr activity ratio following the sequence of atmospherewater-biota pinpoints the additional radioactive source might be a direct atmospheric fallout either from local nuclear facilities (e.g., QNPP and other local NPPs) or global sources (e.g., Fukushima accident and others).
It is virtually impossible that the Fukushima fallout arrived to the study region in 2016, ve years after the accident. At the rst several days of the accidents, air transport in the mid-latitudes was dominated by prevailing westerly winds, which could circle around the globe in 2 weeks [44] . For example, several pulses of radioactive emission from Fukushima were observed in Northern Taiwan 14 days after the accident [45] . We suspect the potential additional radioactive input in 2016 is from a local source. It was reported that the two units in Fangjiashan NPP (FJSNPP) as an expansion of Phase I in QNPP, were put into operation in December 2014 and February 2015, respectively [10] . The commencement of the two units could potentially introduce to some extent increased release of 137 Cs and 90 Sr. However, this does not support the decrease of 137 Cs and 90 Sr concentrations and 137 Cs/ 90 Sr activity ratios in the total deposition for the following years. Therefore, further con rmation is needed due lack of operational and discharges data from QNPP in this work.

Conclusions
This study presents the rst long-term systematic study of levels, variations and sources of 90 Sr and 137 Cs in environmental and food samples around QNPP in 2012-2019. The concentrations of 90 Sr and 137 Cs obtained in this work represent the background level, with all the values below the values recommended by WHO and Chinese national standard. Moreover, the peak concentrations of 90 Sr and 137 Cs appeared in 2016 were suspected to be related with an additional input from the local facility, but it requires further con rmation. This study indicate the high sensitivity of 90 Sr and 137 Cs, especially the 137 Cs/ 90 Sr activity ratio for detecting any radioactive release in the region. In the future, 90 Sr and 137 Cs monitoring is recommended as regional safeguard measure against accidental release from the local nuclear power plant.

Supporting information
The supporting information includes four tables summarizing the programmed heating conditions for microwave-assisted ashing of food samples in this work (Table S1), 137 Cs and 90 Sr activity concentrations and 137 Cs/ 90 Sr activity ratios in total atmospheric deposition (Table S2), source and tap waters (Table S3) and food samples (Table S4)