I. INTRODUCTION
On March 11, 2011, a serious 7-level nuclear accident happened at Japan Fukushima Daiichi nuclear power plant caused by tsunami and earthquake. A huge amount of radioactive materials were released causing a serious off-site consequence. China National Nuclear Accident Emergency Coordination Committee issued on March 31 announced that 25 provinces and districts in China had monitored trace amount of 131I or other artificial radioactive nucleus [1, 2]. The straight-line distance between Fukushima Daiichi nuclear power plant and Qinshan nuclear power base is about 2300 km. Due to the diffusion by wind and ocean currents, part of radioactive materials is very like to have spread to Qinshan area.
"Qinshan area" means the scope of a radius of 10 km and the center of the circle is the 300 MWe units around the QinShan nuclear power base. 3H, 14C, 90Sr, 137Cs, 131I and γ nuclides are analyzed [3-5]. The sampled mediums are as follow:
• Environmental γ radiation;
• Air and fallout (aerosol, fallout and rain);
• Soil (land surface soil, mud of seabed and beach soil);
• Agriculture products (tea, green vegetable, milk, herbage, rapeseed, mulberry, radish, rice, goat, green soybean);
• Aquatic products (mullet, hair tail, jellyfish, shrimp, spiral shell);
• Water (seawater, drinking water, pond water, underground water);
• Biological indicators (pine needle, oyster, moss) [6].
In addition, it also provides Qiaosi (WSW, 60 km), Hangzhou (WSW, 100 km) and Zhoushan (ESE, 140 km) as our environmental references, where we can collect herbage, milk, fallout, tea and seafood. They are located in the small frequency distribution range of wind direction from wind field of nuclear power plants. The distance from nuclear power plant is far enough and not affected by other radioactive facilities or industrial pollution. So, environmental references are not affected by the operation of nuclear facilities. While analyzing of the influence on environmental radiation level of Qinshan area caused by Fukushima nuclear accident, they are strong references.
II. ANALYSIS OF THE INFLUENCE ON ENVIRONMENTAL RADIATION LEVEL OF QINSHAN AREA CAUSED BY FUKUSHIMA NUCLEAR ACCIDENT
There are 5 pressurized water reactors (PWR) and 2 heavy water reactors (HWR) operated in Qinshan area. In order to identify the effects caused by Fukushima nuclear accident, a parallel contrastive analysis on the data was made, focusing on specific activity changes of 90Sr, 134Cs, 137Cs and 131I in the medium samples.
A. Emergency and intermediate phase
In order to monitor the diffusion and deposition of radioactive plume in Qinshan area after Fukushima nuclear accident, a continuous air sampling at 7 monitoring positions (Fig. 1) was did from March 25 to May 6, 2011. So the early effect of the Fukushima nuclear accident could be detected in Qinshan area.
1. 90Sr, 134Cs and 137Cs in aerosol samples
Aerosol and gas iodine are joint sampling, continuous pumping 72 h and the sample volume of air is about 600 m3 every time. To analyze 90Sr and 137Cs in aerosol samples, all of the samples in one quarter should be gathered and analyzed by radiochemical method. Obviously in second quarter of 2011, just after the nuclear accident, the specific activity of 137Cs in aerosol samples is tenfold higher than usual. The γ spectrum analysis of aerosol samples showed that a trace of 134Cs existed in air which can not be detected in air sample before the accident.
2. 90Sr and 137Cs in fallout samples
Fallout samples are collected monthly. Before and after the nuclear accident, the specific activity of 90Sr, 137Cs in fallout samples remained the same order of magnitude.
In second quarter of 2011 after the Fukushima nuclear accident, the specific activity of 90Sr in fallout samples is higher than usual. For example, the values of Xiajiawan (WSW, 1.25 km) and Qinlian (WNW, 2.4 km) are tenfold higher than 1st quarter of 2011, and the values of last three quarters of 2011 after the accident are all higher than MDC (MDC of 90Sr in fallout samples equals 1.3 mBq/dm2). The specific activity of 90Sr and 137Cs in fallout samples from Qiaosi (WSW, 60 km) remains the usual level.
3. 131I in air samples
From March 25 to May 6, according to the method from GB/T14584-1993, we have detected a trace of 131I in air samples (Fig. 2). Usually the specific activity of 131I in air samples is less than MDC which equals to 0.10 mBq/m3. From April 3 to 6, the specific activity of 131I in air samples has reached the top which equals 2.73 mBq/m3.
4. Other samples
The γ spectrum analysis of iodine boxes showed that not only 131I but also 134Cs and 137Cs existed in the activated carbon boxes. From April 3 to 18, the specific activity of 134Cs and 137Cs absorbed by iodine boxes are higher than MDC which equal to 0.04 mBq/m3 and 0.07 mBq/m3 ( Figs. 3 and 4).
In April of 2011, drinking water samples were analyzed. The radioactivity measurement results have no abnormal change. Through analysis of rain, fallout and air samples, the Fukushima nuclear accident had no effect on 3H, 14C and total α/β results.
Above all, after the Fukushima nuclear accident, the specific activity of 137Cs of air and fallout samples is significantly higher than usual. And especially in the second quarter of 2011, a trace of 134Cs and 131I have been detected in Qinshan area. At the same time, Qinshan nuclear power base is operating normally. It proves that part of radioactive plume released from Fukushima nuclear accident had diffused to Qinshan area.
III. LONG-TERM PHASE
In the long-term phase of nuclear accident, radioactive plume have been deposited into soil, plant and groundwater circulation system [7]. Radioactive materials diffused by sea have been taken by plankton into the food chain cycle or deposited in seabed. Therefore, internal exposure caused by ingestion of food and water is the most important monitoring object.
A. Soil samples
The specific activity of 137Cs and 90Sr in land surface soil samples from Qinlian (WNW, 2.4 km) and Qiaosi (WSW, 60 km) is just a little higher than usual which also has the same order of magnitude as usual. Till now the Fukushima nuclear accident had no abnormal effect on mud of seabed and beach soil. The migration trend of 90Sr and 137Cs in soil still need continuous monitoring in future.
B. Agriculture product samples
Agricultural product is an important ingestion pathway of internal exposure. A trace of 134Cs was detected in rapeseed, tea, herbage and green vegetable samples in 2011. The specific activity of 134Cs in rapeseed sample from Qinlian (WNW, 2.4 km) equals to (26±5) mBq/kg, and in tea sample from Hangzhou(WSW, 100 km) it equals to (190±10) mBq/kg. Also in herbage sample from Qiaosi (WSW, 60 km) it equals to (36±2) mBq/kg.
A trace of 131I was also detected in green vegetable, herbage and milk samples. The specific activity of 131I in herbage sample from Qiaosi (WSW, 60 km) and Zhapu (NE, 22 km) equals to (220±20) mBq/kg and (350±30) mBq/kg.
In the meantime, the specific activity of 137Cs in most green vegetable samples and herbage samples increased compared with the same period in past years. So did the rapeseed samples from Qiaosi (WSW, 60 km). The maximum specific activity of 90Sr in green vegetable samples collected in first half year of 2011 equals (657±40) mBq/kg which is nearly 4 times higher than usual. Especially the specific activity of 90Sr in green vegetable samples from Qiaosi (WSW, 60 km) equals to (370±3) mBq/kg.
The specific activity of 134Cs and 137Cs increased after the accident in 2011 and then felt to usual level in 2012 in herbage samples from Qiaosi (WSW, 60 km). So was the specific activity of 90Sr in green vegetable samples from Qiaosi (WSW, 60 km). This fully explained that migration of 90Sr, 134Cs and 137Cs had occurred in the environment.
The specific activity of 137Cs in radish samples of 2011 was slightly higher than usual. The specific activity of 90Sr and 137Cs in rice samples from Qiaosi (WSW, 60 km) also increased slightly. For other agricultural products such as bones of goat and mutton, their radioactive content did not change obviously.
C. Biological indicator samples
Biological indicator means that such organisms with large concentration factor for a particular radionuclide, which can indicate the pollution trend [8]. According to the ecological survey [9], pine needles, moss and oyster are specified as biological indicator of Qinshan area and nuclide such as 3H, 14C and 90Sr were analyzed on them. After the Fukushima nuclear accident, we detected a trace of 134Cs in moss of Baita Hill (NE, 2 km). The specific activity was about 180 mBq/kg in 2011, and then decreased to about 37 mBq/kg in 2012.
D. Seawater and aquatic products samples
The ocean is a huge, complex system. In the ocean, dispersion, mixing and transferring of the radioactive materials do not only depend on the physicochemical properties of radioactive material, but also depend on the emission of pollutants, sea hydraulic characteristics and hydrologic characteristics. In the ocean, radioactive substances will be accompanied by various physical, chemical and biological processes. That’s a complex series of changes [10, 11].
The data of two years after the accident showed that radioactive measurement results of seawater are stable, which means that the Fukushima nuclear accident had no influence on seawater of Qinshan area till now.
Seafood is also an important ingestion way for internal exposure. Except for that the specific activity of 90Sr in mullet bone samples increased year by year slightly, the rest of the radioactive measurement results have no obvious change.
IV. CONCLUSION
After Fukushima nuclear accident, a trace of 134Cs and 131I was detected, and the specific activity of 90Sr and 137Cs was bigger than usual in air. Due to the radioactive plume spread to Qinshan area is small and the diffusion duration is not long, 131I wasn’t detected in air samples any more from May 6, 2011. And the specific activity of 90Sr, 134Cs and 137Cs felt back to normal level from the 3rd quarter of 2011.
Through the analysis on the monitoring of the environmental radiation level two years after the Fukushima nuclear accident, the specific activity of 90Sr, 134Cs and 137Cs is a little higher than 2010. And 131I had transferred from air to vegetables, herbage and milk. Although these nuclide mentioned above has small specific activity, and significant change of environmental radiation level is unlikely to present now. Migration changes are still worthy of sustained attention in future.
Because the territorial sea is adjacent, spiral shell as freshwater aquatic product and jellyfish as seafood were added to analyze the specific activity changes of radionuclides in seawater and seafood in Qinshan area after the Fukushima nuclear accident.
Meanwhile, Fukushima nuclear accident enlightens us:
1. Medium types of environmental radiation monitoring samples should be increased after the Fukushima nuclear accident And according to the migration of radionuclides in environment, monitoring items should be adjusted [12, 13].
2. The radiation monitoring of seawater and seafood is still needed to be strengthen in future.
3. To make precise consequence accumulation, environmental radiation monitoring program for long-term phase is needed.
Fukushima nuclear accident impact on China and related countermeasures
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