Radioactivity of seawater nearby Daya Bay Nuclear Power Station

The radioactivity of seawater nearby Daya Bay NPPs was introduced in this paper. The average annual concentrations of 90Sr and 137Cs ranged from 0.55∼2.38 and 0.81∼4.68 Bq·m-3 respectively during 1993∼2015. The average annual concentration range of 3H is 0.8∼7 Bq·L-1. The concentration range of 14C is 7.6—8.6 Bq·m-3, and the concentration of 3H and 14C is higher than that of the reference location or the background, but with no cumulative effect. The concentrations of 90Sr and 137Cs in seawater have no obvious relationship with the nuclear liquid effluent discharging. The average concentrations of 238U and 232Th were 24.6±10.6 and 18.1±7.5 Bq·m-3 respectively during 2000∼2008.


Introduction
Natural nuclides 40 K, U, Th and their decay products, cosmogenic nuclides 14 C and 3 H, released nuclides 90 Sr and 137 Cs due to nuclear test or accident in last century; moreover, these radionuclides appear in the seawater if without the influence of artificial radionuclides sources.
Dayawan Nuclear Power Station (GNPS), Lingao Nuclear Power Station I (LNPS I) and Lingao Nuclear Power Station II (LNPS II) co-locate in the Daya Bay, 6 REP NPPs started to generate electricity in 1994, 2002 and 2010 respectively. In normal operating state, liquid effluent rarely contains 90 Sr, 137 Cs, 124 Sb, 58 Co and 54 Mn, and the activity concentrations of 124 Sb, 58 Co and 54 Mn are usually below the detection limit in West Daya Bay. 110m Ag is rarely detected since 2009 when silver flange was replaced by other materials, while because of the influence of nuclear explosion test and accident such as chernobyl, 90 Sr, 137 Cs could still be detected. 3 H and 14 C are accounted for a great proportion in liquid effluent, in the same time they naturally present in the environment.
In order to evaluate the effect of liquid effluent from nuclear power plants, 90 Sr, 3 H, 14 C, γ nuclides (such as 137 Cs, 124 Sb, 60 Co, 58 Co, 54 Mn, 110m Ag) and 238 U, 232 Th were monitored in seawater in Daya Bay.
In this paper, the activity concentrations of 90 Sr and 137 Cs are accounted and analyzed during different stages of 6 REP reactors in Daya Bay from 1993 to 2015. Also, the activity concentration of 3 H in 1993-2014, the concentration of U and Th, the activity of 14 C in 2015 were presented.  3 H was analyzed in all samples, while 90 Sr and γ nuclides were only analyzed in the locations of W2, W4, W9, W11, W12, W13 and W14. The distribution of the monitoring point diagram is shown in the left panel of figure 1.

Experimental
We optimized the sampling point location amount to 10 in 2002, which were marked as L1~L10. Every year we took samples 2 or 4 times. 3 H was analyzed in all samples. But 90 Sr and γ nuclides were only analyzed in L1, L4, L9 and L10. The right panel of figure 1 gives the distribution of the monitoring point location diagram.
The monitoring of 14 C began at 2015, for which the monitoring points locate in L1, L4, L10 and the liquid effluent discharge channel.

Analysis method
In this paper, 3 H, 14 C, 90 Sr, 137 Cs, U and Th have been monitored using different methods follow state standard or method published in paper. Table 1 showed the methods used in this paper. GB12375-90 analytical method of tritium in water was used to detect 3 H. The samples used for 3 H measurement are not concentrated by electrolysis. For monitoring 3 H, the low background liquid scintillation equipment Quantulus 1220 was used. For this equipment, the background turns to be lower than 0.7 cpm and the detection efficiency is greater than 60%. The detection limit was about 1.2 Bq·L -1 if the measurement time was set 24 h.
The monitoring method for 14 C followed Guo Guiyin's approach [1]: take 20 L pellucid samples to the four neck flask, stirring with magnetic stirrer when adding sulfuric acid. CO2 origins from inorganic carbon were carried into the collecting bottle by N2; K2S2O8, FeSO4 and H2O2 were added respectively and then collect CO2 from organic carbon. Finally, the sample was measured by method of liquid scintillation with Carb-Sorb E absorption CO2.
Radiochemical analysis of strontium-90 in water precipitation by fuming nitric acid GB6764-86 and Radiochemical analysis of strontium-90 in water extraction chromatography by di-(2-ethylhexyl) phosphoric acid GB6766-86 were used for the detect of 90 Sr, "GB6764-86"and "GB6766-86" were MnO2, produced by chemical reaction between H2O2 and KMnO4, along with K2CoFe(CN)6, were used for the precessing of γ radionuclides like 137 Cs, U and Th in seawater samples by adsorptioncoprecipitation method [2].
When processing statistical analysis, sample activity is taken into consideration by 1/2 of the detection limit in case where activity is smaller than detection limit. The number of calculating samples is m/n, while n represents total number of samples and m refers to the amount of the samples which possess an activity higher than detection limit. The activity would be provided in the form of A±X, where A is means activity and X represents to the uncertainty caused by the counting error of the instrument during the measuring process.

Artificial radionuclides 90 Sr, 137 Cs, et al
The activity concentration of 90 Sr and 137 Cs in the seawater of West Daya Bay among 1993~2016 is illustrated in table 2, which gives a value varies between 0.55~2.38, 0.81~4.68 Bq•m -3 respectively, while the maximum value in one sample turns out to be 4.70 and 7.05 Bq•m -3 respectively. These maximums appeared between 1993~1996 when NPPs was in pilot run. According to the NPP operation record, abnormal emissions took place during 1995~1997 which were mainly caused by maintenance and other activities. During this period, in some patches of seawater a trace of 124 Sb, Ag and 58 Co was detected. As for example, the activity of 124 Sb and 58 Co in four samples in July 1994 ranged 0.55~2.38 and 0.81~4.68 respectively, the 110m Ag activity of four water samples in April 1995 and three samples in 1997 varied between 1.1~23.3 and 1.52~1.67 respectively [3], while for the rest samples, all the activities were below detection limit. In conclusion, the phenomenon that 137 Cs activity turned out to be much higher during this period may relate to the abnormal emissions of NPPs. Also, the liquid effluent emissions of NPPs show no significant influence in 90 Sr and 137 Cs concentrations through figure 2, which well satisfied the natural decay law, in West Daya Bay.  Table 3 and figure 3 give the 90 Sr and 137 Cs activities for the effluent that has not been well diluted. The results in tables 3 and 2 do not tell much difference, which indicates that 90 Sr and 137 Cs give a trace amount in the NPPs liquid effluent. Figure 4 showed the relationship between annual emission and specific activity of 137 Cs in West Daya Bay seawater samples. When comparing data between tables 2 and 3 a conclusion could be made that the operation of LNPS I and LNPS II does not emit a detectable amount of 90 Sr and 137 Cs.  Cs-137 specific activity (Bq/kg)

3 H and 14 C in West Daya Bay seawater
Due to the lack of relevant method and standard, the 14  . The 14 C activity is higher in no well diluted discharge channel and in L4/L10 which have a weak dilution capacity, than in L1 which possesses a strong dilution capacity. Activity concentration of 14 C in West Daya Bay seawater is more than 50% higher than that in the reference location Maoming. The activity of 3 H in West Daya Bay seawater ranges from a value below 0.6 to 70.7 Bq·L -1 , while the annual average value ranges 0.8~7 Bq·L -1 . The annual average activities of 3 H are three to ten times higher than the background activities, which might be caused by the emission of liquid effluent from NPPs, the difference between sampling time and discharge time. No concentration effect of 3 H appears facing the operations of the 6 NPPs [4], which indicates that the dilution-exchange capacity of West Daya Bay seawater satisfies the requirements on the discharge of liquid effluent.

Concentrations of 238 U and 232 Th
The method used to monitor 238 U and 232 Th is introduced in the literature [2]. The average value of 238 U and 232 Th were 24.6±10.6, 18.1±7.5 Bq·m -3 in 177 samples sampled in West Daya Bay during 2000-2008 respectively. Convert to mass concentration of U and Th were 2.0±0.86, 4.47±1.85μg·L -1 respectively (The activity of 238 U in natural uranium is 12.35 kBq·g -1 , the activity of 232 Th in natural thorium is 4.046 kBq·g -1 ).The concentration of U is 3.1-3.9 g·L -1 reported in the literature [5,6], which is higher than the previous values. The main reason is that the samples were monitored before the decay balance between 238 U and its progeny 234 Th, but take the activities of 238 U deduced from 234 Th. The concentration of U was 2.82 μg·L -1 in seawater from West Daya Bay detected by using sodium polyphosphate as fluorescence enhancement agent while the verified value was 2.83 μg·L -1 by ICP-MS [7]. This value was a little lower than that in the literature [6].

Conclusion
The Average annual concentration of 90 Sr, 137 Cs and 3 H ranged 0.55~2.38, 0.81~4.68 and 0.8-7 Bq·L -1 respectively during 1993~2015, the concentration ranges of 14 C was 7.6-8.6 Bq·m -3 in 2015 in seawater nearby Daya Bay Nuclear Power Station. The concentrations of 3 H and 14 C were higher than that in the background or in the reference location, but with no cumulative effect. The trend of 90 Sr and 137 Cs concentrations was consistent with the natural decay law, with no relationship when comparing the discharging of liquid effluent from the NPPs. The Average concentration of 238 U and 232 Th were 24.6±10.6 and 18.1±7.5 Bq·m -3 respectively during 2000 -2008.
Artificial radionuclides could not be detected except for part of the samples sampled during 1995-1997 during which there was abnormal emission or nuclear power accident. Trace amounts of 124 Sb, 60 Co and 110m Ag were detected in seawater from West Daya Bay.