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 years1. 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 fission products 90Sr (t½ = 28.79 y) and 137Cs (t½ = 30.17 y) are recognized as the most important from the radiological perspective2. Due to their relatively long half-lives, 90Sr and 137Cs can be preserved in terrestrial and marine systems for a long time once entering the environment. Under natural environmental conditions, 137Cs and 90Sr mainly enter the human body through the food chain and respiration. Both 90Sr and 137Cs have long biological half-lives in the human body3,4,5,6,7,8, therefore it is important to continuously monitor 90Sr and 137Cs 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 137Cs, long-term monitoring data for 90Sr 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 90Sr.

The Qinshan nuclear power plant (QNPP) is the first nuclear power plant in China and officially 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. Artificial radionuclides such as 3H and 14C in the vicinity of QNPP have been investigated to a very limited extend during the past decades9,10. Studies on primary fission products in environmental and foods samples, especially long-term systematic studies have not been reported so far. In this work, we report for the first time a long-term observation of 137Cs and 90Sr in environmental and food samples collected around QNPP during 2012–2019. The distribution levels, temporal variations and source terms of 137Cs and 90Sr in the study region were investigated. The annual effective dose was estimated for the local public based on our measurement data.

Materials and methods

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.

Figure 1
figure 1

Map showing the geographical location of QNPP (A, scale: 1:42,000,000) and (B, scale: 1:4,000,000), and sampling sites in the surroundings of QNPP marked with filled circles and numbers (C, scale: 1:500,000). The map was produced using software MapInfo Professional using data from https://bajiu.cn/ditu/.

Full size image
Table 1 Samples collected from QNPP in this work during 2012–2019.
Full size table

Total (dry and wet) atmospheric deposition samples were monitored quarterly at a location 9.8 km away from QNPP. A total deposition collector (ZJC-VI deposition automatic collector, Zhejiang Hengda technology co.) with a surface area of 0.25 m2 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).

Reagents and standards

Anhydrous alcohol, sulfuric acid, acetone, hydrochloric acid, hydrogen peroxide, nitric acid, oxalic acid, ammonia, and di (2-ethylhexyl) phosphoric acid used for this study were all analytically grade. A 90Sr-90Y standard solution (9.78 Bq/g of 90Sr in 0.1 mol/L nitric acid) and a 137Cs standard solution (1.47 Bq/g of 137Cs in 0.1 mol/L nitric acid) were purchased from National Institute of Metrology, China. Electroplating source of 90Sr-90Y with radioactivity intensity (Number of particles on the surface/2π·min) of 1.20 × 103 was used for calibrating the detection efficiency of the instrument.

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 method11 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 muffle furnace (Thermo Fisher Scientific Co., Ltd.) at 450℃ for 8 h.

The radioachemical analysis of 90Sr in water and ash samples was according to the Chinese national standard procedures12,13,14,15. In general, Sr (100 mg) and Y (20 mg) carriers were added to water (adjusted to pH = 1.0 with concentrated HNO3) and ash samples. For water samples (50 L of each), SrCO3/CaCO3 precipitation was used to pre-concentrate Sr. The SrCO3/CaCO3 precipitate was dissolved with 6 mol/L HNO3 and the sample was adjusted to pH = 1 for further chromatographic purification.

For ash samples (5–30 g), after treated with 3–10 mL of concentrated HNO3 and 3 mL of H2O2, and evaporated to dryness, a second ashing was performed at 600 ℃ to ensure the complete decomposition of organic substances. Each ash sample was digested with 30–83 mL of 6 mol/L HCl for two times, the combined leachate was proceeded with CaC2O4/SrC2O4 precipitation for Sr pre-concentration. The CaC2O4/SrC2O4 precipitate was ashed in a muffle furnace (600℃, 1 h), and dissolved in 1.5 mol/L HNO3 for further chromatographic purification.

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 flow rate of 2 ml/min to separate 90Y. The column was washed with 40 mL of 1.5 mol/L HNO3 at 2 ml/min, and Y was eluted with 30 mL of 6 mol/L HNO3 at 1 ml/min. The separation time was recorded to calibrate the decay of 90Y. Yttrium was finally 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 min12,13,14,15.

The determination of 137Cs in water and ash samples was according to Chinese national standards16,17,18,19. Prior to 137Cs determination, the samples were kept for one month to allow for a complete decay of 131I. Each dried food sample was screened with gamma spectrometry to eliminate the interferences of other short-lived radiocesium (134Cs, 136Cs, and 138Cs) on 137Cs 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 HNO3 for several times. Ammonium phosphomolybdate (AMP) was added to water or leachate sample (from the ash sample) to adsorb cesium. The precipitate was filtered and dissolved in NaOH solution. Cesium was finally separated as Cs3Bi2I9 precipitate in citric acid and acetic acid medium for gravimetric determination of chemical yield and measurement of 137Cs using the low background α/β counter. The measurement time was kept 100 min per cycle, in total of 10 cycles for each sample16,17,18,19. All the analytical results obtained in this work were summarized in Tables S2, S3, S4 in the supporting information.

Results and discussion

90Sr and 137Cs activity concentrations in total atmospheric deposition

The 90Sr and 137Cs activity concentrations in total atmospheric deposition are widely used indicators for revealing potential radioactive pollution in the atmosphere. The inter-annual and seasonal variations in activity concentrations of 90Sr and 137Cs in total atmospheric deposition collected around QNPP during the period of 2012–2019 are presented in Fig. 2. The observed annual average activity concentrations range within 0.44–1.68 Bq/m2 (mean ± sd = 0.97 ± 0.51 Bq/m2) for 90Sr and 0.10–1.17 Bq/m2 (mean ± sd = 0.40 ± 0.36 Bq/m2) for 137Cs, respectively. 90Sr and 137Cs activity concentrations demonstrate similar inter-annual variabilities across the entire study period, with the lowest annual average constantly obtained in 2012–2015 and the highest values in 2016 followed by a gradual decreasing trend until 2019. The appearance of peak 90Sr and 137Cs activity concentrations in 2016, which are up to ca. 4 and 11 times higher compared to the other years, respectively, potentially indicates additional radioactive plume in the study region in 2016 (see discussion later).

Figure 2
figure 2

Variation of 90Sr (A) and 137Cs (B) activity concentration in total atmospheric deposition collected around of QNPP during 2012–2019.

Full size image

No clear seasonal variations of 90Sr and 137Cs in total atmospheric deposition were observed in this study. Though the highest activity concentrations of 90Sr and 137Cs both occurred in 2016, 90Sr peaked in the first quarter of 2016 whereas 137Cs peaked in the second quarter of 2016. This might be associated to the different source input functions and transport processes between 137Cs and 90Sr. 137Cs is particle reactive and readily attached to aerosols and dust, whereas 90Sr is more water soluble and easier to be dissolved in rainwater20. The late occurrence of 137Cs peak compared to 90Sr peak in 2016 could be due to the association of 137Cs to fine particles suspended in the atmosphere which prolonged the residence time of 137Cs in the atmosphere; and the plum rain in the second quarter (mostly in May) of 2016 flushed the fine particles and thereby prompted the deposition of 137Cs onto the ground. Nevertheless, both 90Sr and 137Cs activity concentrations in the atmosphere around QNPP were generally low in the past decade, and can be considered as baseline concentrations. This low background presents an opportunity to observe any future changes in the 90Sr and 137Cs concentrations and therefore can serve as baseline data for any future inputs from QNPP and other potential nuclear activities.

90Sr and 137Cs activity concentrations in water samples

The time-series of activity concentrations of 90Sr and 137Cs in water samples collected around QNPP during the period of 2002–2019 are plotted in Fig. 3. Except for a few cases, no significant differences are observed for 90Sr and 137Cs activity concentrations in the two type waters between May and October in each year. 90Sr and 137Cs annual average activity concentrations in tap water (5.0–8.2 mBq/L for 90Sr and 1.4–4.2 mBq/L for 137Cs) are relatively stable compared to those in source water (4.3–11.1 mBq/L for 90Sr and 0.9–7.0 mBq/L for 137Cs) during 2012–2019. This should be a consequence of the additional water treatment process (e.g., filtration, disinfection) from raw water to tap water.

Figure 3
figure 3

Variation of 90Sr and 137Cs activity concentration in source and tap water collected around QNPP during wet (1st) and dry (2nd) seasons in 2012–2019.

Full size image

The highest 90Sr and 137Cs activity concentrations in source water are observed in 2016, with 137Cs in May and 90Sr 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 reflects the human intervention in the water treatment and delivery process. It is also noted that the annual average of 137Cs activity concentration in source water decreased rapidly after 2016 to reach similar level as in 2012, while 90Sr activity concentration was decreasing slowly and still nearly two times higher than the level in 2012. All the measured values for 90Sr and 137Cs activity concentrations in water samples around QNPP during 2012–2019 were below the concentration limits recommended by WHO and Chinese national standards21,22 and also comparable to the reported values for waters collected around other NPPs in China (Table 2).

Table 2 Activity concentrations of 90Sr and 137Cs in water and food samples collected around NPPs in various regions of China.
Full size table

90Sr and 137Cs activity concentrations in food samples

The activity concentrations of 90Sr and 137Cs in different types of food samples collected around QNPP during 2012–2019 are presented in Fig. 4. Activity concentrations in rice, salsola, mullet and crucian carp were in the ranges of 0.04–0.5 Bq/kg fresh weight (f. w.), 0.3–1.1 Bq/kg f. w., 0.4–1.1 Bq/kg f. w. and 0.6–1.3 Bq/kg f. w. for 90Sr, and 0.03–0.09 Bq/kg f. w., 0.02–0.07 Bq/kg f. w., 0.04–0.3 Bq/kg f. w, and 0.04–0.6 Bq/kg f. w. for 137Cs, respectively. Our results are comparable to the activity concentrations of 90Sr and 137Cs in similar samples collected from other different NPPs in China (Table 2), and far below the Chinese regulatory limit for general foodstuffs.

Figure 4
figure 4

Activity concentrations of 90Sr (A) and 137Cs (B) in different food samples (mullet, rice, salsola, and crucian carp) collected around QNPP during 2012–2019.

Full size image

Though no general trends in the temporal variations of 90Sr and 137Cs activity concentrations in the four food species can be seen during 2012–2019, the lowest activity concentrations for both radionuclides are mostly observed in rice and the highest values in crucian carp. Interestingly, 90Sr activity concentration is often higher in mullet compared to salsola, whereas the opposite is obtained for 137Cs. Even between the two fish species, the difference in 137Cs activity concentration (up to 10 times) is much higher than that in 90Sr activity concentration (max. 1.5 times). These features are linked to 90Sr and 137Cs levels in the environment where different biota are growing and their different physiological mechanisms for the incorporation and excretion processes of Sr and Cs28,29. For example, mullet inhabits salt water and brackish waters whereas crucian carp is a freshwater fish that inhabits lakes, ponds, and slow-moving rivers.

In the freshwater systems, the high particle/colloid loads may effectively scavenge 137Cs, whereas 90Sr is kept stable in the water due its highly conservative behavior. Freshwater in some rivers has been found to include high concentrations of 90Sr30,31. Besides, higher salinity in brackish water may facilitate the uptake of 137Cs (similar to potassium) into the fish. The values of concentration factors for Cs and Sr in marine fishes are 100 and 3, respectively32. Therefore, it is reasonable to expect a larger difference in 137Cs activity concentration between mullet and crucian carp compared to that in 90Sr activity concentration.

The annual effective dose (AED) due to the ingestion of 137Cs and 90Sr in foods (i.e., mullet, salsola, rice and crucian carp) were estimated based on the Chinese national standard33 using the following equation:

$$AED = \, (I_{Cs} + I_{Sr} ) \cdot e(g)$$
(1)

where ICs and ISr is the annual intake of 137Cs and 90Sr in foods (Bq/y), respectively. e(g) was the dose conversion coefficient of ingested radionuclide 137Cs and 90Sr in foods. Dose conversion coefficients of 1.3 × 10–8 Sv/Bq for 137Cs and 2.8 × 10–8 Sv/Bq for 90Sr were used in this study according to the Chinese national standards .

The obtained AED through ingestion of 137Cs and 90Sr in foods for the local population around QNPP was calculated to be in the range of 1.3 × 10–4–1.1 × 10–3 Sv/y during 2012–2019, with a mean value of (3.9 ± 4.0) × 10–4 Sv/y. In this calculation, the annual per capita human food consumption (0.19, 0.085, 67.84 and 0.40 kg/year/person for mullet, salsola, rice and crucian carp, respectively) by the local public were according to the survey in 2019 and assumed to be constant during 2012–2019. The estimated mean AED for these eight years is considered negligible in respect to the recommended limit (1 mSv/y) established by International Commission on Radiological Protection (ICRP). This reveals that the foods around QNPP were at a safe level during the period 2012–2019.

Source term identification via 137Cs/90Sr activity ratio

The temporal variations of 137Cs/90Sr activity ratios in the investigated samples during 2012–2019 are presented in Fig. 5. For total atmospheric deposition, the annual average 137Cs/90Sr activity ratios ranged from 0.24 to 0.81, with notable peaks observed in 2016. Narrower distribution ranges of 137Cs/90Sr activity ratios were observed in the water and food samples compared to the total atmospheric deposition, varying within 0.085–1.18 for source water, 0.11–0.80 for tape water, 0.055–0.13 for mullet, 0.069–1.14 for rice, 0.078–1.08 for salsola, 0.052–0.72 for crucian carp, respectively.

Figure 5
figure 5

137Cs/90Sr activity ratio in total atmospheric deposition (A), source water (B), tap water (C) and food samples (D) during 2012–2019.

Full size image

The background 137Cs and 90Sr levels on the world surface are mostly due to global fallout of atmospheric nuclear weapons testing during 1945–1980. The activity ratio of 137Cs/90Sr from the global fallout deposition was estimated to be about 1.634,35. Our results indicate that all the 137Cs/90Sr activity ratios are lower than the global fallout signal. This might be associated to faster removal of 137Cs compared to 90Sr originated from global fallout and/or additional input of 90Sr in the surrounding environment of the study region. The latter is very unlikely, because 90Sr concentrations measured in this work are in the background level and there is no record of significant radioactive releases in the study region. Besides, as any potential release of 90Sr (such as civil nuclear industry) would normally be accompanied by 137Cs release and often with much higher 137Cs/90Sr activity ratios compared to global fallout, owing to the lower volatility of Sr than Cs36. For example, very high 137Cs/90Sr ratios have been reported in the atmospheric fallout from Fukushima accident (∼1000)37 and Chernobyl accident (∼250)38. In seawater, 137Cs/90Sr ratios reached 39 ± 1 beyond the coast of Japan due to massive liquid releases of cooling water in spring 201137. The 137Cs/90Sr activity ratios before the Fukushima accident were reported in the range of 2.6–18 in Japanese fish whereas the 137Cs/90Sr activity ratios ranged from 98 to 480 after the accident39.

As 137Cs is more readily adsorbed and immobilized on clay minerals while 90Sr exhibits a higher mobility, it is reasonable to foresee a decrease in global fallout derived 137Cs/90Sr activity ratios in some freshwater and food samples. This has been approved in earlier studies where activity ratios of 137Cs/90Sr in wheat and polished rice from Japan increased by nearly 20 and 3 times, respectively, from 1959 to 199540.

137Cs and 90Sr in the dry atmospheric deposition was reported to reflect their signature in the ground-level air, which is mostly from resuspension of the deposited 137Cs and 90Sr in soil41. Sr is chemically easier to elute than Cs in the soil column by rainwater20, therefore 137Cs/90Sr activity ratio in the typical surface soil usually becomes higher than the typical global fallout value30,31. However, lower 137Cs/90Sr 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 lower 137Cs/90Sr activity ratios than global fallout, but are comparable to those measured in fresh waters in this work.

The peak 137Cs/90Sr 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 137Cs/90Sr activity ratio in the study region in 2016. As a potential consequence of this additional radioactive source, maximum 137Cs/90Sr 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 137Cs/90Sr activity ratio following the sequence of atmosphere-water-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, five years after the accident. At the first 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 weeks42. For example, several pulses of radioactive emission from Fukushima were observed in Northern Taiwan 14 days after the accident43. 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, respectively10. The commencement of the two units could potentially introduce increased release of 137Cs and 90Sr to some extent. However, this does not support the decrease of 137Cs and 90Sr concentrations and 137Cs/90Sr activity ratios in the total deposition for the following years. Therefore, further confirmation is needed due lack of operational and discharges data from QNPP in this work.

Conclusions

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