In this work, fish, shrimp, shellfish, crab and algae were selected around nuclear power plants in Zhejiang Province in 2021–2023. Moreover, to facilitate a more accurate comparison of ⁹⁰Sr activity levels, the city of Zhoushan City was selected as the control point for seafood monitoring. This city does not possess a nuclear power plant and has a high consumption of seafood. Each of the five types of seafood around the NPP and in Zhoushan was collected once a year and used for ⁹⁰Sr detection. The relevant regions and detailed sampling information of seafood are shown in Figure 1 and Table 1.

The following chemicals were used in the experiment: SrCl₂·6H₂O and Y(NO₃)₃·6H₂O (Shanghai Macklin Biochemical Technology Co., Ltd.); Bi(NO₃)₃·5H₂O (Sinopharm Chemical Reagent Co., Ltd.); sodium sulfide, hydrochloric acid, hydrogen peroxide, and nitric acid (Shanghai Lingfeng Chemical Reagent Co., Ltd.), absolute ethanol (Anhui Ante Food Co., Ltd.), oxalic acid (Changshu Yonghua Chemical Technology (Jiangsu) Co., Ltd.), ammonia solution (Hangzhou Longshan Fine Chemicals Co., Ltd.), and di-(2-ethylhexyl)phosphoric acid (60–80 mesh, Beijing Research Institute of Chemical Engineering and Metallurgy, CNNC). The water used in this experiment complies with the Grade 2 water standards specified in GB/T 6682-2008, and all reagents used were of analytical grade.

Low background total α and total β measuring instrument (LB790 ten-way low background αβ measuring instrument, Germany Berthold Technologies), graphite hot plate (HTL-400EX, NanoHeat), muffle furnace (DC-B125/13, Beijing Original Technology Co., Ltd.), electronic balance (AE 200, Mettler Toledo Instruments (Shanghai) CO. Ltd.), drying oven (Binder, Binder Environmental Test Equipment (Shanghai) Co., Ltd.), centrifuge (5810, Eppendorf, Germany).

All seafood was processed without delay following its arrival at the laboratory, having been transported in a frozen state. The procedure involved washing, selecting edible portions, draining and weighing the samples, which were then placed in an oven at 105°C for drying. After drying, they were carbonized and ashed in a muffle furnace to decompose the organics, a process which requires step-segmental warming. The maximum temperature was maintained at 500°C during the ashing of the samples. Once the ashing process was complete, the sample ash was allowed to cool to room temperature and weighed again in order to obtain the weight of the ash sample and the ash-fresh ratio. In this work, none of the ⁹⁰Sr analytical procedures involved composite samples, and all of our experimental subjects were individual specimens.

For sample preparation, 5–10 g of sample ash was placed in an evaporation dish, followed by 0.5 mL of strontium carrier (50 mg/mL) and 1 mL of yttrium carrier (20 mg/mL). Then 10 mL of HNO₃ and 6 mL of H₂O₂ were added to remove organic matter and impurities, and after evaporation on a graphite hot plate, the samples were transferred to a muffle furnace at 600°C for calcination for 2–3 h. Once the sample had cooled to room temperature, it was leached with 2 mol/L hydrochloric acid solution (40 mL × 2) and centrifuged at 4,000 rpm for 5 min to retain the supernatant. Subsequently, the residue was washed two times with deionized water and the supernatant was mixed with the previous supernatant, which was retained in a beaker. 10 g of oxalic acid was added to the mixture, and the pH of the solution was adjusted to 2.5 with ammonia. The precipitate was collected by filtration, and then transferred to a muffle furnace for calcination at 800°C for 1 h. The product was completely dissolved with 6 mol/L nitric acid, followed by the addition of 1.0 mol/L HNO₃, bismuth carrier (20 mg/mL) and 0.3 mol/L sodium sulfide solution, the insoluble matter was filtered out, the filtrate was collected, and its pH was adjusted to 1.

The pretreated filtrate was passed through the Di-(2-ethylhexyl) phosphoric acid (HDEHP) column, and washed with 1.0 mol/L hydrochloric acid (30 mL) at a controlled flow rate of 1.5 s/drop, then rinsed with 1.5 mol/L nitric acid (40 mL). Finally, the filtrate was collected into a Teflon beaker with 6 mol/L nitric acid (30 mL) at a flow rate of 3 s/drop.

The saturated oxalic acid solution (5 mL) was added to the eluent, and its pH was adjusted to 2.5 with ammonia. After aging, the eluate was filtered to produce the precipitate. The dried precipitate was taped to the sample tray and determined with the low background α-β counter, counting 10 cycles of 100 min each.

The specific activity of ⁹⁰Sr in seafood was calculated using the following formula:

where C is the specific activity of ⁹⁰Sr, Bq/kg; N_s is the net count rate of the sample, min⁻¹; M is the ash freshness ratio, g/kg; ŋ is the instrumental efficiency, %; W is the sample ash, g; Y_Y is the yttrium chemical recovery, %; λ is the decay constant of ⁹⁰Y; and t is the intermediate moment from the separation of ⁹⁰Sr, ⁹⁰Y to the measurement, h.

The radioactivity produced by ingestion accumulates in the body and affects the body’s tissues and organs over a long period of time, causing varying degrees of internal radiation damage (21–23). The annual effective dose (AED, mSv/y), as a radiation protection quantity, is considered an effective tool for radiation exposure risk assessment (15). The resident intake of seafood can be calculated on the basis of the frequency and quantity of seafood consumed by the population, allowing for the estimation of AED.

The AED associated with internal exposure through seafood consumption was calculated by the following equation:

where I_Sr is the respective annual intake of ⁹⁰Sr in seafood (Bq/y), e(g)_Sr is the age-dependent dose conversion coefficient for ingestion of ⁹⁰Sr (2.8 × 10⁻⁸ Sv/Bq).

Referring to the relevant literature on the status of annual consumption of seafood by residents of coastal and island areas in Zhejiang Province (24), the annual per capita consumption of fish, shrimp, shells, crabs and seaweeds for the whole population was shown in Table 2.

Prior to utilizing the LB790 ten-channel low background αβ counter to detect the sample, the electroplating source of ⁹⁰Sr–⁹⁰Y with radioactivity intensity of 1.20 × 10³ (Number of particles on the surface/2π·min) inspection and standard source scale were employed, and the background measurement cycle and measurement time were consistent with the actual detection of the sample. All instruments utilized were qualified within the verification cycle. The ⁹⁰Sr–⁹⁰Y standard solution (9.78 Bq/g of ⁹⁰Sr in 0.1 mol/L nitric acid) was added to the blank sample for pretreatment and detection, and the discrepancy between the detection results and the reference value of the reference material was within 10%. Ten percent of the samples were taken for parallel sample analysis, and the relative standard deviation of the measured values of the parallel samples was within 10%. The analysts are certified to work, regularly participate in on-site quality control, and the experimental data are accurate and reliable.

The specific activities of ⁹⁰Sr in five types of seafood consumed by residents of Zhejiang are presented in Table 3. The results of the three-year monitoring demonstrate that the specific activities of ⁹⁰Sr in the five types of seafood sampled around the NPP have remained stable and have not undergone significant change. The mean specific activities of ⁹⁰Sr in fish, shrimp, shellfish, crabs, and algae in the vicinity of the nuclear power plant were found to range from 0.09 to 0.76 Bq/kg, 0.03–0.30 Bq/kg, 0.01–0.22 Bq/kg, 0.11–1.04 Bq/kg, and 0.02–0.95 Bq/kg, respectively, which were much lower than the national standards for specific activity of ⁹⁰Sr in seafood and close to the results reported by other literature (25–28). It is noteworthy that ⁹⁰Sr specific activities were significantly higher in fish and crab compared to the other three types of seafood, whereas ⁹⁰Sr specific activities in shellfish were at a lower level, due to the removal of the shells of the shellfish samples used in this study, taking into account the osteophilic nature of ⁹⁰Sr and the fact that it accumulates in the shells of bivalves, together with calcium (29). In comparison to other seafood products, crabs possess a relatively low muscle content. Consequently, during the processing of crabs, the shells were retained, while the inedible parts were removed. However, the main component of crab shells is chitin which bonds calcium strongly (20). Due to the similar chemical properties of strontium and calcium, strontium may be bound to crab shells, leading to the higher specific activity of ⁹⁰Sr in crabs. Moreover, ⁹⁰Sr is metabolized slowly in fish tissues; the effective half-life of ⁹⁰Sr in fish is about 500 days and up to 97% of ⁹⁰Sr is present in fish tissues such as bones, scales, and gills (30). On the other hand, fish and crabs are at a higher trophic level in the food chain compared to the other three types of seafood. As a result, fish and crab exhibit higher ⁹⁰Sr specific activity.

In this study, the specific activities of ⁹⁰Sr in different types of seafood (fish, shrimp, shellfish, crab, and algae) monitored from 2021 to 2023 were analyzed by the Kruskal-Wallis test and the results are presented in Table 4 and Figure 2. The results demonstrated statistically significant differences (p < 0.001) in the ⁹⁰Sr radioactivity levels of the five seafood species. A comparison of any two groups of the five seafood products revealed that the p-values between fish and shellfish, crab and shellfish, shrimp and crab were all less than 0.05. In particular, the p-values between fish and shellfish, crab and shellfish were all less than 0.001, indicating that there were significant differences in the specific activities of ⁹⁰Sr radioactivity in these groups of seafood. The source of these differences may be due to the different environmental conditions under which the biota grows, as well as to differences in the selectivity and accumulation of ⁹⁰Sr for different types of foodstuffs.

Seafood is considered to be an important source of protein for people living near nuclear power plants. From the monitoring of seafood from Qinshan NPP, Sanmen NPP, San’ao NPP and the control site Zhoushan, it can be found that the ⁹⁰Sr specific activities of seafood around each nuclear power plant fluctuate normally within the background level and are close to those of the control site Zhoushan from 2021 to 2023, as shown in Figure 3A. A comprehensive investigation was conducted to assess the levels of ⁹⁰Sr in various seafood types at multiple monitoring points (Figure 3B). The findings revealed that the specific activities of ⁹⁰Sr in all seafood categories were markedly below the established limit value, as defined by the national standard. The ⁹⁰Sr specific activity in shrimp and shellfish around Zhoushan is higher than in the other three NPPs. This can be attributed to the unique geographical location of the Zhoushan fisheries at the confluence of the Yangtze River, Yong River and Qiantang River and the estuary of the three rivers (31), has turned the waters of Zhoushan into an important outlet for radionuclides from the Yangtze River, resulting in the accumulation of certain radionuclides in seafood. On the other hand, Zhoushan Fishery is known for its rich biodiversity. The area is shaped by complex interactions of ocean currents and topography, creating a habitat that is favorable for a wide range of marine organisms. It serves as a vital spawning and feeding ground for a multitude of shrimp species (32). It was also observed that there were minor fluctuations in the specific activities, which were attributed to a number of objective factors, including fluctuations in the detection instruments and changes in background levels. However, it was noted that the specific activities remained at the background level and did not exhibit any significant increase.

The statistical analyses (Kruskal-Wallis test) of the three NPPs and the control site demonstrated that the specific activities of ⁹⁰Sr in seafood from disparate locations were analogous (Table 5). Furthermore, no significant difference was observed (p > 0.05), which provides additional evidence that the three nuclear power plants were operating stably.

The specific activities of ⁹⁰Sr in the five types of seafood monitored in different years are presented in Figure 4. The results demonstrate that there was no significant inter-annual variation in the levels of ⁹⁰Sr radioactivity in the five types of seafood during the three-year period, and that the results exhibited a tendency toward stability. The Kruskal-Wallis test was carried out to analyze the ⁹⁰Sr specific activities of fish, shrimp, shellfish, crab and algae over the period from 2021 to 2023. The results demonstrated no statistically significant difference (p > 0.05), as illustrated in Table 6. This finding indicates that there was no notable alteration in specific activities of ⁹⁰Sr among the five seafood types between 2021 and 2023. Furthermore, the specific activities of ⁹⁰Sr remained stable and remained below the national standard limits. This indicates that the nuclear power plant operated in an effective and environmentally responsible manner, with no discernible adverse effects on marine life.

Our team has been engaged in a prolonged period of research, with the objective of monitoring the ⁹⁰Sr specific activity in foodstuffs in the vicinity of three nuclear power plants in Zhejiang Province (33–35), as shown in Table 7. Furthermore, the specific activities of ⁹⁰Sr in seafood detected in 2020 at the Ningde and Fuqing nuclear power plants, which are located in close proximity to the latitude and longitude of the three nuclear power plants in Zhejiang Province, ranged from 0.03 to 0.75 Bq/kg and 0.03–0.39 Bq/kg, respectively (22). The specific activities of ⁹⁰Sr in fish around Rooppur Nuclear Power Plant in the People’s Republic of Bangladesh, which is also on the Asian plate, ranged from 0.02 to 1.55 Bq/kg (36). The results of this study for ⁹⁰Sr specific activity in seafood did not exhibit a significant change when compared to the results of the team’s previous monitoring and monitoring results around other nuclear power plants. Furthermore, the ⁹⁰Sr specific activity we observed in seafood were similar to those reported in the global monitoring studies (20, 26, 37, 38), indicating stable operation of nuclear power plants in Zhejiang Province.

Based on the survey of relevant literature on the annual consumption status of seafood by residents of coastal cities in Zhejiang Province (24), the AED due to the intake of ⁹⁰Sr from a single type of seafood by the residents around the nuclear power plant and Zhoushan City in 2021–2023 was estimated, as shown in Figure 5. It is noteworthy that among the five categories of seafood, marine fish contributed the largest AED, which may be associated with the dietary habits of residents of Zhejiang Province, where fish has the highest annual per capita consumption. Additionally, this can be attributed to the practical considerations for residents, for whom fish is more readily accessible. From 2021 to 2023, the AED due to individual types of seafood in the vicinity of the three nuclear power stations and Zhoushan remained at a stable level, with no significant fluctuations observed. Furthermore, the AED accumulated from total seafood consumption over the period from 2021 to 2023 was estimated (Table 8). The results demonstrated that the AED due to total seafood at Qinshan Nuclear Power Station, Sanmen Nuclear Power Station, San’ao Nuclear Power Station and Zhoushan were 0.55 × 10⁻⁴ to 4.36 × 10⁻⁴ mSv/y, 0.76 × 10⁻⁴ to 1.20 × 10⁻⁴ mSv/y, 1.01 × 10⁻⁴ to 2.81 × 10⁻⁴ mSv/y, 5.72 × 10⁻⁴ to 5.85 × 10⁻⁴ mSv/y, respectively. The AEDs from seafood in Zhoushan City were higher than those in the three nuclear power plant regions of Zhejiang Province, primarily because Zhoushan is an island area, while the Qinshan NPP, Sanmen NPP and San’ao NPP are located in coastal areas—leading to distinct differences in seafood consumption habits. Furthermore, the Zhoushan city encompasses China’s largest fishing ground, Zhoushan Fishery, renowned for its high marine biodiversity and abundant fishery resources. This ecological advantage ensures greater accessibility of diverse seafood for local residents, leading to significantly higher seafood consumption compared to populations near nuclear power plants. Consequently, the AED attributed to seafood in Zhoushan exceeds that in the nuclear power plant vicinity. However, these values are considerably below the acceptable level of 1.0 mSv/y set by the International Commission on Radiological Protection (ICRP) for the general public, indicating that the dose burden on the population is slight and that the operation of the nuclear power stations has no discernible impact on public health. Furthermore, the anthropogenic radionuclide ¹³⁷Cs appears in about 40% of seafood dose assessments due to its radiological significance in routine and accidental releases from nuclear facilities, its background presence due to global fallout and its relative ease of measurement (39). By comparing the AED due to ¹³⁷Cs in seafood reported in the relevant literature (40), we found that the AED due to ⁹⁰Sr in seafood monitored in this work is close to the reported value, which is consistent with the results reported by IEAE on the contribution to annual ingested dose from radionuclides in diet (41).