The safety of cooling water intake in nuclear power plants (NPPs) has gradually become an important factor affecting the safety of NPPs. Marine flora and fauna outbreaks are one of the main types affecting the safety of cooling water intake in NPPs, and the driving mechanism is more complicated and difficult to predict. This paper mainly analyzes the main types and typical cases of marine organisms that cause cooling water intake blockage in NPPs, and analyzes the key factors such as hydrodynamic conditions, timing of organism's outbreak, characteristics of species, and design of the screen systems. The results show that strong hydrodynamic conditions or strong tides caused by special meteorological conditions are important factors. Considering the time of organism's outbreak, spring and summer (May–August, may change slightly according to different latitude) is key period because of the reproduction and growth of most marine organisms, according to the growth law of marine organisms. In terms of biological characteristics, it is sensible to focus on those species with fast growth, strong reproductive capacity, short life cycle, weak swimming ability, and cluster distribution. As to the design of screen systems, the blockage mainly occurs in revolving filter screens. The grid spacing of normal mechanical grille is too large to block the small marine organisms. It is necessary to add trash interception nets according to the type and size of the marine species. Finally, a case is used to confirm the factors needed to pay more attention.
With the development of the economy and the progress of society, the demand for energy by human beings continues to increase, and energy shortage has become a bottleneck restricting the development of human society. As a new type of energy, nuclear power has obvious advantages in solving resource shortages and improving environmental quality because of its characteristics of economy, high efficiency, and environmental protection. It has become an important energy source for ensuring China's sustainable development and has made considerable progress.
On the website of National Nuclear Safety Administration of China, until March 2018, more than a dozen nuclear power plants (NPPs) are operating along the coast of China, and about 18 units are under construction. According to the “13th Five-Year Plan” for energy development of China, in 2020, nuclear power installations will strive to reach 58 × 106 kW, and nuclear power installations under construction will reach 30 × 106 kW.
However, in recent years, with the changes of marine ecological environment, coastal nuclear power plants often meet incidents in which sea creatures and floating objects block the water intake. Most coastal nuclear power plant sites choose once-through cooling water system. During normal operation, a large amount of cooling water needs to be taken from the sea. This can easily cause blockage of the water intake. A 2006 study by the World Association of Nuclear Operators (WANO) found that there were 44 such events at nuclear plants since 2004 . The most common material causing blockages was aquatic life, including algae and aquatic grasses, mussels, jellyfish, crustaceans (shrimp and crabs), and fish. These materials contributed to 37 of the 44 events. The remaining events were created by accumulations of frazil ice (ice crystals forming in a body of water), depositions of sand and silt and ingress of crude oil. The survey of Electric Power Research Institute (EPRI) found that debris issues are common . Nearly, 50% of facilities have de-rated due to debris issues and slightly more than 10% have shut down due to debris. In China, according to incomplete statistics, since 2014, there have been many incidents of sea creatures gathering and affecting the operation of the unit . For example, a nuclear power plant in the north of China, in July 2014, a large number of jellyfish caused a rotating filter blockage. Units 1 and 2 of the circulating pumps jumped and power plants shutdown finally. In addition, in August 2015, a large number of Acaudina (100–200 mm) invaded a nuclear power in the southeast of China, which caused the circulating pumps out of work and unit 3 of the nuclear power plants shutdown emergently. These incidents not only led to huge economic losses of nuclear power plants, but also affected the safe and stable operation of nuclear power plants, which caused extensive attention of the regulator departments, nuclear power plant owners, and related designing institutes.
Ruan  analyzed the cause of the cooling water intake blockage, and put forward the corresponding experience feedback and improvements, based on the investigation and analysis of intake blockage events on the global nuclear power plants in service. Li et al.  analyzed effective marine life detection methods for different marine life invasions. Meng et al.  proposed an early warning and decision-support system for marine organisms in water cooling system in a nuclear power plant in order to control invasive marine organisms and to ensure safety of cooling water in a nuclear power plant. Kang et al.  analyzed the measured data of the ocean currents in the small area around the water intake and found that the sea area within 300 m from the water intake has a net flow of residual current pointing to the water intake, which is the key area for preventing the blockage. These studies mainly focused on the introduction of intake water blockage events in domestic and foreign nuclear power plant, the experience feedback, and suggestions on early warning and prevention of intake water blockage events. There was not much research on the mechanism of the blockage events of nuclear power plant caused by the outbreak of marine organisms.
This paper aims to the issue of the cooling water intake safety of nuclear power plants caused by marine organisms, and analyzes four key factors about hydrodynamic conditions, the outbreak timing of organisms, design of the filter systems and characteristics of species which could impact the safety of cooling water intakes of nuclear power plants. It also provides technical support for rationally solving the water intake safety of nuclear power plants and ensuring the safe operation of nuclear facilities.
2 Classification of Cooling Water Intake Blockage
The blockage of the water intake of nuclear power plants can be divided into two categories, one is the debris blockage, and the other is the blockage of marine organisms. For debris blockage, it mainly includes the blockage caused by foreign garbage (such as wheat straw), sea ice, sediment or silt, leakage of foreign crude oil, and cultures (such as kelp). The driving factor mainly is disaster weather, which is related to flow conditions such as currents and wind waves. Therefore, the prevention of such incidents can be concerned with the forecast of disaster weather. In addition, the plants need to pay attention to the possible sources of foreign objects. If the sites near the river mouth, it is more possibly blocked by garbage.
For marine organism's blockage, the organisms mainly include fish, shrimp, jellyfish, green tide, and red tide. The mechanism of the outbreak of such organisms is complicated, and it is quite difficult to make early warning and prevention. According to the current research results, the measures used in prevention work can be carried from the aspects of hydrodynamic conditions, the outbreak timing of organisms, design of the filter systems, and characteristics of species, which are describe as follow.
3 Key Factors Analysis
3.1 Hydrodynamic Conditions.
The aquatic organisms in the ocean caused cooling water intake blockage almost have weak ability of movement and basically drift with the water. When they are carried by the currents around the water intake, they could enter the water intake structure under the huge pumping water flow, if there were not enough debris prevention measures. Therefore, considering the safety of water intake of nuclear power plants, it is necessary to analyze the hydrodynamic conditions of the water intake area to understand whether it is easy to gather debris near the water intake or whether it is on the debris transportation route.
3.1.1 Tide Impact.
Marine organisms that cause the water intake blockage of the nuclear power plant are usually some creatures with weak moving ability. Therefore, the strength of the water flow will obviously affect the possibility of water intake blockage. The coastal tides in China vary widely, and the tidal dynamic conditions are different in different regions.
It can be seen from the figures that the tidal power in the coastal areas in Liaoning and Shandong provinces in China where the nuclear power plants are located is relatively weak. While the coastal tide power in the southeastern provinces of China such as Zhejiang and Fujian is much stronger. Therefore, from the perspective of tidal dynamic conditions, for the same intensity of marine life outbreaks, the water intakes of nuclear power plants in Liaoning, and Shandong provinces are more likely to be blocked. In addition, it also can be seen that the coastal tidal power of Guangdong, Guangxi, and Hainan provinces in southern China, is much weaker, and the water intake of nuclear power plants is easily blocked. Meanwhile, the strong tidal power can also inhibit the organism's outbreak.
3.1.2 Wave and Residual Flow Impact.
Except of the tide power, the wave and residual flow could also affect the blockage of the water intake by marine organisms. Some nuclear power plants will setup dikes on the outside of the water intake to reduce the influence of waves and reduce the blockage of the water intake (see Fig. 3). Many nuclear power plants have a herringbone intercepting net at the water intake, which is to use the power of the tidal current to clean the floating objects attached to the net.
3.2 Outbreak Time of Organisms.
The outbreak time of marine organisms is mainly related to the growth law of organisms. The growth and reproduction of organisms are closely related to temperature. Together with the setting of the national ban on fishing, it provides the basis for marine life outbreaks.
According to the statistics of incidents with sea creatures gathering and affecting the operation of the unit (see Table 1), most of the marine life outbreaks occur in May to August, which would cause water intake blockage.
In addition, according to the Bulletin of China Marine Ecological Environment Status 2017 , from May to July 2017, the green tide disaster affected the most coastal waters of the Yellow Sea in China (see Table 2), and there are two nuclear power plant sites in this area, where their latitude positions are 36°42′ and 36°58′, respectively.
From the above analysis, it can be seen that the time of marine life outbreaks will vary with latitude, but it is highly related to temperature. From the results of biological outbreaks and temperature analysis in various sea areas, we assess the temperature range of marine life outbreaks is approximately between 15 °C and 25 °C.
Other times when marine life is prone to outbreaks are the time of the disaster weather, the time of the national ban on fishing, and the peak of the aquatic culture, which was almost in May to August in China.
3.3 Design of the Filter Systems.
The layout of the water intake system of a typical nuclear power plant is shown in Fig. 4. The filtration system for the water intake is mainly composed of three parts. The first part is coarse grid with filter size 200 mm, the second is fine grille with filter size 50 mm, and rotating filter is the last with filter size 4 mm.
From the current feedback, most of the blocking events occur at the rotating filter. It indicates that the filtration load at the rotating screen of these sites is the largest, while the front coarse grid and fine grille do not play a good role in sharing the dumping load. Therefore, in the design of the size of the interception system, the size distribution of marine organisms in the waters should be fully considered.
Due to the overfishing of offshore fisheries, the big individual fish populations are extremely rare. The offshore fish is mainly composed of some small individual fishes. Therefore, the design of the front coarse grid and fine grille size needs to be adjusted with the size distribution of marine organisms appropriately.
According to the results of the offshore fishery resources survey at a nuclear power plant site in 2017–2018 (see Table 4), the average body length of the catch is less than 100 mm. The average body length of the fish is among 70–83 mm, the average body length of the crustacean is among 41–53 mm, and the average body length of the Cephalopod is among 12–38 mm. It indicated that the grid size should also be among 10–100 mm, and mostly, the size should be smaller than 50 mm, because the body length is normally greater than the body width.
3.4 Characteristics of Species.
According to the current research, most marine organisms in the coastal waters are r-strategy organisms, which are characterized by strong adaptability and vitality, rapid reproduction and growth, short life cycle, easy recovery of resources, and prone to burst growth.
Studies have shown that when the seabed water temperature is 10–15 °C, it is especially suitable for the hatching, growth, and reproduction of otter . In this condition, the otter can create a new jellyfish through a large number of transverse cracks. The jellyfish grows very fast, and many new jellyfish are produced in just a few days. In addition, studies also have shown that the water temperature is too high in summer or autumn, and the temperature difference between the sea surface and the sea floor is extremely large, which will greatly accelerate the growth rate of the otter.
The other case is Chinese prawn which has the characteristics of fast growth, short growth cycle, strong reproductive ability, weak swimming ability and cluster distribution, and has the basic elements of population outbreak. External hydrodynamic conditions, such as ocean currents and storm surges, tend to exacerbate their intrusion to the cooling water intake system.
4 Case Study
4.1 Haiyang Nuclear Power Plant.
The Haiyang Nuclear Power Plant is located on the Lengjiazhuang and Dongjiazhuang villages, Liugezhuang Township, Haiyang city, east of the headland surrounded by sea on three sides (see Fig. 5). The plant site is 10 km from Haiyang's Liugezhuang Township, 22 km from Haiyang city, 93 km from Yantai city, and 107 km from Qingdao city. The site is planned to have six 1000 MW pressurized water reactor units, with a proposed project for expansion of two 1250 MW units.
4.2 Tide Force Analysis
The tidal vibration in the engineering sea area is controlled by half-day rotating tide wave system in the South Yellow Sea. The tidal type is a regular half-day tide, which rises and falls twice within one day. Table 5 shows the multiyear average tidal level data of the site based on 1-year observation data from the site's station and 13 years' tidal data from Rushankou Station.
Based on the tidal range, the tide can be divided into weak tide (tidal range less than 2 m), middle tide (tidal range 2–4 m), and strong tide (tidal range greater than 4 m). Considering the tidal range, the project sea area belongs to the middle tide area. The tidal force is relatively normal, which could not take the drift objects out of the water intake channel.
According to the analysis of multisections' full-tidal hydrological survey data, the tidal currents in the project sea area are mainly composed by regular half-day tidal currents. The movement of the tidal current is mainly reciprocating. The direction of the tidal current changes with the seasons. During the high tide, the direction is WSW-WNW, which was parallel with the direction of the cooling water intake channel.
4.3 Species Characteristics
4.3.1 Main Catchment Composition
184.108.40.206 Fishery resources.
According to fishery resources survey results in 2016 (see Fig. 6), the season with the largest density of fish eggs and larvae is spring (May). Although fish eggs and larvae do not block the water intake, the amount of fish eggs and larvae is a prerequisite for causing biological outbreaks, it is necessary to pay attention to prevent the blockage.
Meanwhile, according the results, the average fish density and biomass was highest in summer (August). Although fishes are not the main species on blocking the water intake, but they will make the blockage worse. In some extreme condition, they also can cause problems on cooling water intake of nuclear power plants.
220.127.116.11 Jellyfish and Entermorpha.
Jellyfish and Entermorpha are the main species which could cause the cooling water intake blockage in Haiyang nuclear power plant.
According to the fishery resources survey results, jellyfish were only found in spring (May) and summer (August). There were only small jellyfish in spring, which were only 50–140 g each. However, they grow up quickly. In summer, there were lots of large jellyfish discovered, and the weight was about 5–10 kg each. It became a big threat for safety of cooling water intake.
Entermorpha was not sampled in the survey, because it was drifted from the south of Yellow sea, and it just appeared in stable time in project sea area, most in July. Sometimes it would last to August.
In July 2013, a large area of Entermorpha appeared in the water intake of the open channel of the Haiyang Nuclear Power Plant, covering an area about 50,000 m2. The thickness of the Entermorpha floating at the water inlet is about 5–7 cm .
4.4 Outbreak Time.
According to field investigations in July 2019, a large area of Entermorpha appeared in the water intake of the open channel of the Haiyang Nuclear Power Plant. The plant immediately activated the emergency plan and launched salvage work. The invasion of Entermorpha in the channel last about a month and then faded gradually until it disappeared.
At the end of August 2019 and early September, Entermorpha faded gradually but the jellyfish invasion began. Especially, at the end of August, the daily salvage weight was about 4000–4500 kg, and there were about 700 jellyfishes on the nets. However, at the end of September, the daily salvage weight was dropped to 500 kg. Jellyfish varied in size, with the largest about more than 50 kg, and the smallest about 5 kg. The average was about 7.5–10 kg.
Therefore, as to Haiyang Nuclear Power Plant, June to August is the outbreak time of Entermorpha, and the peak period is around the end of July. As Jellyfish, the outbreak time is from August to September, and the peak period is at the end of August.
4.5 Filter Systems.
Haiyang nuclear power plant has designed the normal filter systems, such as coarse grid with filter size 200 mm, fine grille with filter size 50 mm, and rotating filter with filter size 4 mm. Meanwhile, according to the feedback based on experience, Haiyang nuclear power plant enhanced the filter systems with interception nets. There were one permanent net and some temporary interception nets added in the water intake channel (see Fig. 7). In the emergency condition, there would be seven interception nets in the water intake channel. The permanent net was designed to intercept Entermorpha and small lost power boats, which was 3 m high with 2 m under the water. The mesh size was 120 mm × 120 mm, which was suitable for intercepting Entermorpha. Most of the temporary interception nets were with mesh size 50 mm × 50 mm, and some of them were sink bottom nets. They could be effective supplement to the permanent net.
In this study, we mainly analyze the key factors which may influence the possibility of the cooling water intake blockage. They are hydrodynamic conditions, the outbreak timing of organisms, design of the filter systems, and characteristics of species.
The results show that in the north and south of China sea, the tide is weaker and the cooling water intake may be blocked easier by the bloom organisms than the east of China sea.
Considering the time of organism's outbreak, spring and summer (May–August, may change slightly according to different latitude) is key period because of the reproduction and growth of most marine organisms, according to the growth law of marine organisms. The time of national ban on fishing is almost the same period which all could increase the possibility of the blockage of water intake.
As to the design of screen systems, the blockage mainly occurs in revolving filter screens. The grid spacing of normal mechanical grille is too large to block the small marine organisms. The suitable size for the grid spacing is 10–50 mm. We hope new studies could be carried out to confirm the effectiveness of finer spacing grilles. It is also necessary to add trash interception nets according to the type and size of the marine species.
In terms of biological characteristics, it is sensible to focus on those species with fast growth, strong reproductive capacity, short life cycle, weak swimming ability, and cluster distribution.
The case of Haiyang nuclear power plant also confirmed the four factors above which could explain the happening of water intake blockage, and it also introduced some measures to forecast and prevent the blockage.