Introduction

Amid the rapid growth of coastal industries and the dwindling of fresh water supplies, coastal nuclear power plants increasingly rely on large seawater cooling towers with secondary cooling cycles. During operation, these towers release small, salt-containing water droplets. Due to their small size and aerodynamic properties, these droplets evade capture by water collectors and are carried outside the tower by the exhaust air, forming floating droplets. These salt-laden droplets can corrode equipment surfaces and pipelines1,2,3, increasing anti-corrosion expenses. Additionally, when they settle on the ground, they contribute to the salinization of the soil and the withering of nearby vegetation, thus affecting the local ecology4,5,6.

The study of the environmental impacts of floating droplets and salt deposition is crucial for the adoption of seawater cooling towers. The prediction of salt deposition in new power plants currently relies on numerical simulations. In 1972, the United States began a series of studies on floating droplets in cooling towers, leading to the development of the SACTI calculation program in 1990 by the U.S. National Laboratory Environmental Impact Research Center, using earlier models and field data7,8. However, SACTI has limitations in simulating complex buildings and terrain. Recently, researchers have employed computational fluid dynamics (CFD) to analyze the flow near cooling towers, studying fog plumes and floating droplets9,10. Comparing CFD and SACTI with field observations shows that CFD more accurately predicts flow fields by considering factors like fog evaporation, local circulation, and building structures, thus providing more reliable results for droplet and salt deposition11.

The assessment of salt deposition’s environmental impact is typically guided by the "Impact of Cooling Towers of Nuclear Power Plants" from the US Nuclear Regulatory Commission NUREG-1555, along with the “Technical Specification for Environmental Impact Assessment of Cooling Towers of Nuclear Power Plants (NB/T20307-2014).” These guidelines indicate that salt deposition levels between 100 and 200 kg/km2/month are generally harmless to plants. However, deposition exceeding 1000 kg/km2/month can damage plant leaves. The effects of deposition levels ranging from 200 to 1000 kg/km2/month on plant health remain unclear. Moreover, the severity of salt stress varies considerably among different plant varieties and developmental stages12,13,14. For instance, under a low salt concentration (0.15%), the yield of the salt-tolerant rice varieties Yanxian 156 and Yanjing 456 increased by approximately 10%, while the yield of Yanfeng 47 decreased by approximately 16%15. It is essential, therefore, to assess the impact of salt deposition based on specific plant and crop varieties. Historically, studies on the environmental impact of cooling towers have focused predominantly on vegetation, with less attention given to the effects on surrounding soil and water.

To address this, a CFD numerical simulation model of six seawater cooling towers was developed to analyze the distribution of salt deposition and its environmental impacts, including the effects on building dynamics and crosswinds. The simulation explored how salt deposition influences the salinity of surface water, water quality in paddy fields, and soil salt content. Experimental studies were also conducted to determine the effects of soil salinity on major crop growth. These findings will aid in evaluating the environmental impact of salt deposition from naturally ventilated seawater cooling towers and offer a technical reference for power plant site selection, enhancing the understanding of cooling towers’ environmental impacts.

Research object

The project site is located in Jiangsu Province, China, near the Shanhou River, which has a salinity range of 981.5–1023.2 mg/L, averaging 998.8 mg/L. According to the water salinity classification standard (Table 1), the Shanhou River’s surface water is classified from fresh to brackish. The area is primarily agricultural and residential, with minor industrial presence. The primary crops grown are rice in the summer and wheat in the winter.

Table 1 Classification standard of water salinity16.
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Local climate data of 2022 from Xizou Mountain Weather Station includes an average dry bulb temperature of 16.0 °C, a wet bulb temperature of 13.0 °C, and an atmospheric pressure of 1016.6 hPa. Windroses are detailed in Fig. 1, showing hourly statistics for wind speed and direction.

Fig. 1
Fig. 1
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Frequency distribution of wind speed and direction (Windrose).

The power plant houses six cooling towers, detailed in Table 2. On the west side, four high-level water collection cooling towers (T1–T4) are aligned north to south in an "I" configuration. Additionally, two standard cooling towers (T5–T6) are positioned on the east side in a similar north–south arrangement, as shown in Fig. 2. A building is situated between the two sets of towers, as indicated in Table 3.

Table 2 Main dimensions of 6 cooling towers.
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Fig. 2
Fig. 2
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The surrounding environment of the project and the layout diagram of cooling towers and buildings in the factory.

Table 3 Main size of buildings.
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The cooling towers utilize S-wave plastic water fillers. The high-level water collection towers are designed to achieve an outlet temperature of 28.15 °C, with a wind speed of 2.01 m/s in the filler section and a circulating water volume of 46.1 m3/s per tower. The standard towers have a designed outlet temperature of 28.30 °C, a wind speed of 1.47 m/s, and a circulating water volume of 20.1 m3/s per tower. The circulating seawater in the towers has a salinity of 28‰, a concentration ratio of 1.7, and a drift rate from the water collector of 0.001%.

Methods

Control function

During stable operation of the cooling tower, the air flow inside and outside the tower is modeled under steady conditions. The flow dynamics are referenced in Zhao17. In the filler section, the fluent porous media model is used to determine resistance loss, as expressed in Eq. (1). Additionally, the discrete phase model (DPM) analyzes air resistance to raindrops in the rain zone, using Eqs. (2) to (4). The MERKEL model is applied to examine the heat exchange process between the filler and rain zone.

$$\xi_{f} = \frac{{2A_{p} V^{M} \gamma_{a} }}{{\rho V^{2} H_{f} }}$$
(1)
$$\frac{{d\vec{u}_{p} }}{dt} = F_{D} (\vec{V} - \vec{u}_{p} ) + \frac{{\vec{g}_{x} (\rho_{p} - \rho )}}{{\rho_{p} }}$$
(2)
$$F_{D} = \frac{18\mu }{{\rho_{p} d_{p}^{2} }}\frac{{C_{D} {\text{Re}} }}{24}$$
(3)
$$C_{D} = \frac{24}{{\text{Re}}}\left( {1 + b_{1} {\text{Re}}^{{b_{2} }} } \right) + \frac{{b_{3} {\text{Re}} }}{{b_{4} + {\text{Re}} }}$$
(4)

where ξf is the resistance coefficient of filler; Ap is the experimental constant; V is the air velocity vector, m/s; M is the experimental constant; γa is the specific weight of air, N/m3; ρ is the density of air, kg/m3; Hf is the height of filler, m; t is the time, s; up is the particle velocity vector, m/s; FD is the drag force, N; gx is the acceleration vector of the particle, m/s2; ρp is the density of raindrops, kg/m3; dp is the raindrop diameter, m; μ is the dynamic viscosity, Pa·s; CD is the raindrop resistance coefficient; and bi is the experience coefficient.

The commercial computational fluid software application FLUENT solves the three-dimensional numerical model of the cooling tower. In natural ventilation towers, buoyancy is essential for the momentum equation. The modeling integrates air flow, heat transfer, raindrop evaporation, and water vapor diffusion. Variable separation solves the discrete equations, while the SIMPLEC algorithm decouples velocity and pressure. The QUICK format is used for velocity and temperature fields. FLUENT’s secondary interface facilitates calculations for outer boundary conditions, filler resistance characteristics, and raindrop evaporation, utilizing user-defined functions (UDF).

In this study, we calculated operating conditions involving wind speeds from 0 to 14 m/s and 16 different wind directions (from 0° to 337.5° in 22.5° increments). We conducted a weighted analysis of floating droplet sizes combined with hourly wind data to determine the distribution of salt deposition on the ground.

Grid establishment

The GAMBIT grid division software application was used to create a three-dimensional model. The computational domain measures 12,500 m × 4500 m × 2500 m, depicted in Fig. 3. The tower is segmented into various areas: above the throat, below the throat, filler area, bottom of the tower, and air inlet area. The grid outside the tower is sparsely distributed, while the filler area has the densest grid with approximately 8 million grids.

Fig. 3
Fig. 3
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Grid of the computational domain.

To track particle trajectories across different wind directions, grid models were established at 22.5° intervals between the line of towers and the far-field direction. We then calculated the 360° salt deposition around the tower group using statistical methods.

Boundary condition

The computational domain is split into two main sections: inside and outside the cooling tower. The outer region of the tower has an adiabatic boundary at the bottom, and when there is no wind, it uses a pressure outlet boundary. When there is wind, the air inlet has a velocity inlet boundary while other surfaces maintain a pressure outlet. The cooling tower’s shell is adiabatically bounded, and both the inlet and outlet of the tower are treated as internal boundaries. Additionally, the filler area is considered a porous medium to establish a resistance coefficient.

Floating droplets are released from either the water collector or the throat section of the cooling tower. Their initial velocity, temperature, and quantity are defined using a discrete particle model. A droplet spectrum is outlined in Table 4 to examine motion characteristics under varying particle sizes. Equations (2) to (4) are used to calculate the trajectories of floating droplets. During the salt deposition analysis, evaporation from these droplets is ignored.

Table 4 Droplet spectrum generated by cooling tower.
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The ambient wind speed correlates with height, as defined in Eq. (5), with a wind shear index of 0.1918. A user-defined function (UDF) captures the variations in wind speed with height. The effects of ambient humidity, air moisture content, and temperature are considered at either the pressure outlet or the velocity inlet boundary.

$$v_{w,y} = v_{w} \left( {\frac{y}{{h_{ref}^{{}} }}} \right)^{0.19}$$
(5)

where vw,y is the wind speed of height y, m/s; vw is the wind speed of reference height, m/s; href is the reference height, usually considered as 10 m; and y is the height, m.

Droplet data statistical method

The distribution of droplet settlement is influenced by the characteristics of droplet movement, the frequency distribution of wind, and the mass distribution of droplets. Statistically, the area around cooling towers can be divided into several 50 × 50 m regions arranged on a grid, with droplet settlement assumed to be uniform in each region. The settlement in each region is calculated using Eq. (6).

$$q_{i} = \sum\limits_{j = 1}^{j = N} {\frac{{m_{{d_{j} }} }}{{50 \times 50 \times \rho_{{d_{j} }} }} \cdot 3600 \cdot 1000 \cdot f_{{d_{j} }} \cdot f_{wind} }$$
(6)

where qi is the amount of droplets in the i region, mm/h; mdj is the mass of droplets with particle diameter dj falling in this region, kg/s; ρdj is the density of droplets with particle diameter dj, kg/m3; fdj is the frequency of droplets with particle diameter dj, kg/kg; fwind is the wind frequency of the corresponding direction in the region.

Calculation method for salt effects on the environment

The impact of salt deposition from cooling towers on surface water, rice field water quality, and soil was analyzed using the following calculation method.

(1) Increase in surface water salinity

The increase in surface water salinity due to cooling tower salt deposition was calculated based on the Shanhou River basin area, design discharge, and salt deposition.

$$\Delta CS=\frac{{A \times Sd \times 1000}}{{v \times 30 \times 24 \times 3600}}$$
(7)

where A is the area of the Shanhou River, km2, equal to 0.6; v is the mass flow rate of the Shanhou River, m3/s, equal to 15; Sd is the maximum salt deposition, kg/m2/month; ΔCs is the increase in salinity, mg/L.

(2) Salt accumulation in paddy field water

Rice is cultivated once a half year in the study area, therefore the impact of salt deposition on the water quality of the paddy field is evaluated over a six-month period.

$$\Delta C_{SH} = \frac{Sd \times 6}{dw \times 1000}$$
(8)

where dw is the standing water depth of the paddy field, m, equal to 0.02–0.05, m; ΔCSH is the salinity increase in half year, mg/L.

(3) Soil salt content

The increase in soil salt content caused by cooling towers on per unit area basis over T years can be calculated based on salt deposition:

$$\Delta ms = \frac{Sd \times 12 \times T}{1000}$$
(9)

where Δms is the increase in salt content caused by the cooling tower, g/m2; and T is the accumulated time, year.

In many parts of our country, the depth of the soil layer is 30 cm. The mass of soil per unit area and the content of soluble salts can be determined using the soil’s wet density and the percentage of soluble salts.

$$ms = \rho \times d_{t} \times \alpha \times 1000$$
(10)

where ms is the background of salt content in soil, g/m2; ρ is the soil wet density, g/cm3, equal to 1.861; dt is the soil layer, m, equal to 0.3; and α is the proportion of soluble salts in the soil.

Considering the increase in soil salt content and the baseline value of salt content, the soil salinity (αT) can be computed assuming that the salt deposition from the cooling tower is considered, and the losses due to irrigation and rain erosion are excluded.

$$\alpha_{T} { = }\frac{\Delta m_{s}}{m_{s} + \Delta m_{s} } \times 100\%$$
(11)

The classification of salinization degrees, according to reference Bao19, is as follows:

  • Non-salinized soil: salinity of < 0.1%

  • Salinized soil: salinity of 0.1–0.3%

  • Medium saline soil: salinity of 0.3–0.5%

  • Heavily saline soil: salinity of 0.5–1.0%

  • Very heavily saline soil: salinity of > 1.0%.

Experimental method for wheat salt stress

Salt deposition primarily affects the farmland of Dongxin Farm and Tongxing Town, where wheat is a key crop. Hence, the wheat variety Cangmai 6002 was selected for salt stress breeding experiments. The study examined the impact of soil salinity on the growth of Cangmai 6002 by planting 30 wheat seeds in soils with equal mass but varying salt levels.

In order to reflect the inclusion of the experiment, the soil salt content of different salt deposits (250–3000 kg/km2/month) under different cumulative years was first calculated. The maximum salt content of soil was about 0.51%. Therefore, 11 soil samples with salt content were prepared. 0.1%, 0.15%, 0.2%, …, 0.55%, 0.6% were tested. After pulverizing the soil, a 100 g sample is sifted through a 20 mesh/inch screen and placed in a petri dish. Sea salt is added according to the soil’s salt content, and enough water is added to moisten the soil and even out the surface of the planting dish. Subsequently, 30 wheat seeds are placed in each dish and the surface is levelled again.

The moisture levels of each planting dish should be monitored daily, and appropriate watering should be conducted to maintain moisture. After 10 days, the germination rate, bud length, root length, number of roots, fresh weight of the buds, fresh weight of the roots, and the root-to-shoot ratio should be statistically analyzed using Excel and SPSS software. This experiment for each soil sample will be repeated twice to ensure accuracy.

Results

Flow field

Figure 4 illustrates the pathline of hot and humid air at the cooling tower outlet when the wind direction is 270° (west wind) and the wind speed increases from 1 to 12 m/s. At a wind speed of 1 m/s, the hot air on the windward side at the outlet of T1–T4 is deflected by the crosswind. Conversely, the hot air at the outlet of T5–T6 is drawn towards and deflected by the airflow from T1 to T4. At a wind speed of 3 m/s, the deflection of the hot air at the outlets of T1–T6 increases and is shielded by the airflow from T1 to T4, with the deflection angle at the outlets of T5–T6 being smaller than that at T1–T4. When the wind speed exceeds 5 m/s, the outlet air from the cooling tower shows significant deflection, and the deflection amplitude of the air at the outlets of T1–T6 is roughly the same.

Fig. 4
Fig. 4
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Pathline when the wind direction is vertical to tower row (m/s, wind direction 270°).

Figure 5 shows the path of hot and moist air at the cooling tower outlet under various wind directions with an ambient wind speed of 3 m/s. When the wind direction is parallel or nearly parallel (within ± 22.5°) to the line connecting the towers, the airflow from T1 to T4 overlaps; for other wind directions, there is no overlap in the airflow at the tower outlets.

Fig. 5
Fig. 5
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Pathline from the tower outlet when the wind speed is 3 m/s (K).

Droplets trajectory with different particle sizes

Figure 6 illustrates the trajectories of droplets with varying particle sizes exiting the cooling tower at a wind speed of 3 m/s and a direction of 270°. It is noted that the quantity of airborne droplets decreases as particle size increases. Particularly, at a particle size of 450 µm, droplets struggle to escape the standard tower, though they can still exit a high-level water collection tower. When particle size reaches 550 µm, escaping the high-level water collection tower becomes exceedingly difficult.

Fig. 6
Fig. 6
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Droplet trajectories of different particle diameters at cooling tower openings.

The interaction force between air and raindrops depends on their size and relative velocity of droplets. If the air velocity within the tower is too low or the droplet size too large, the droplets fail to exit the tower. Theoretical analysis indicates that droplets can only escape if Eq. (2) is greater than 0. Theoretically, at a wind speed of 2 m/s in the filler (T1–T4), the largest particle size that can exit is approximately 500 µm. Conversely, at a wind speed of 1.5 m/s in regular towers (T5–T6), the maximum particle size reduces to approximately 400 µm. These observations confirm the numerical model’s accuracy in predicting droplet trajectories.

Distribution of salt deposition

As outlined in Section "Control function", monthly, seasonal, and annual salt deposition statistics were compiled, with peak values shown in Table 5. The highest salt deposition occurred in spring, totaling about 700 kg/km2/month, and the lowest in winter, at roughly 550 kg/km2/month. The highest seasonal deposition consistently occurred approximately 15° south by west, within 2.5 km of the tower center (Fig. 7). Additionally, Fig. 8 displays the annual cumulative salt distribution, with a peak of 600 kg/km2/month around 15° south by west, about 2 km from the site.

Table 5 Maximum of monthly, seasonal, and annual salt deposition (kg/km2/month).
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Fig. 7
Fig. 7
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Seasonal cumulative distribution of salt deposits.

Fig. 8
Fig. 8
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Annual cumulative distribution of salt deposits.

In March and July, the maximum salt deposition was 900 kg/km2/month, within safe limits. However, it remains uncertain whether this level of deposition impacts plant health in the 200–1000 kg/km2/month range. Section "Breeding experiments on the impact of soil salinity on wheat growth" will detail breeding experiments to assess the effects of varying soil salinity on local plant growth.

Impact of salt deposition on the salinity of surface water

For conservative estimations, maximum values of monthly salt deposition (900 kg/km2/month), seasonal salt deposition (700 kg/km2/month), and average annual salt deposition (600 kg/km2/month) were chosen to assess the impact of cooling tower salt deposition on surface water salinity. Assuming that the dissolved salts in river water are NaCl, the increase in the salinity of river water caused by seawater cooling towers is presented in Table 6.

Table 6 Effects of salt deposition on river channels.
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The cooling towers’ maximum projected increase in the salinity of the Shanhou River is approximately 0.0139 mg/L, the effect on river salinity is less than 0.0014%, which is really small for the river. According to standard GB3838-2002, the limit for chloride (Cl-) in surface water for concentrated drinking purposes is 250 mg/L, and total salinity should be less than 1000 mg/L. The river’s average chloride concentration is about 128.6 mg/L, peaking at 205.3 mg/L, and the average total salinity is approximately 998.8 mg/L. Therefore, the cooling tower’s salt deposition is unlikely to exceed regulatory limits and will minimally impact the river’s water quality.

Impact of salt deposition on the salinity of the paddy field

Paddy fields irrigated by the Shanhou River assume that all dissolved salts are NaCl. The effects of maximum monthly (900 kg/km2), seasonal (700 kg/km2), and annual average (600 kg/km2) salt deposition on paddy field water salinity are shown in Table 7. Water depths of 0.02 m and 0.05 m were considered, within the typical range for paddy fields.

Table 7 Effects of salt deposition on the salinity of the paddy field.
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At water depths of 0.02 m and 0.05 m, the salinity of paddy field water can reach 1269 mg/L and 1107 mg/L respectively after 6 months. Despite the cooling tower’s influence, the water quality remains within the fresh to brackish range within this period, indicating a minimal impact on salinity classification and overall water quality.

Impact of salt deposition accumulation on the salinity of soil

Based on geotechnical investigations at the plant site, it was determined that the ground is composed of silty clay with an average salinity of soluble salts at 0.12%. The wet density of the soil was measured to be 1.861 g/cm3. Following guidelines typical in China, a cultivation depth of 30 cm is recommended. The calculated soil salinity is 558 kg/m2, with a soluble salt content of 670 g. It is assumed that all dissolved salts in the water are sodium chloride (NaCl). The impact of salt deposition on soil salinity was estimated using the highest average monthly salt deposition value of 900 kg/km2/month.

The local soil is categorized as salinized soil (salinity of 0.12% without the influence of cooling tower salt deposition). With salt deposition from the cooling tower, the salinity increases to 0.197% over 40 years, the salinity increases to 0.236% over 60 years, without a change in salinization degree. This estimation uses peak salt deposition values and does not account for the effects of precipitation and irrigation on reducing salinity over time, making the conclusion conservatively estimated.

Breeding experiments on the impact of soil salinity on wheat growth

Initially, the soil salt content was calculated for various deposition rates (250–3000 kg/km2/month) over different years, with a maximum recorded soil salinity of about 0.51%. Consequently, 11 soil samples with varying salinity levels (from 0.1 to 0.6%) were prepared for experimentation, as described in section "Experimental method for wheat salt stress". The growth outcomes after 10 days are depicted in Fig. 9.

Fig. 9
Fig. 9
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Record of experimental germination of wheat with salt stress.

From Fig. 9, it is evident that the wheat exhibits relatively high germination rates and shoot lengths at salinity levels from 0.1 to 0.2%. However, both metrics decline with increasing salinity from 0.25 to 0.35%. The trend continues, with significant reductions in germination rates and shoot lengths as salinity levels reach 0.4–0.55%. At a salinity of 0.6%, variety Cangmai 6002 fails to germinate altogether.

Table 8 illustrates the impact of varying soil salinity levels on the morphological indexes of Cangmai 6002 at the bud stage. The table uses asterisks to denote significance: * indicates a significant difference at the 0.05 level, and ** indicates an extremely significant difference at the 0.01 level. Among the 11 treatments, there were significant or extremely significant differences in germination rate, bud length, bud fresh weight, and root-to-shoot ratio, while the other three morphological indexes showed no significant differences.

Table 8 Effects of soil salinity on Cangmai 6002’s morphological traits.
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Table 9 presents the results of multiple comparisons where the same letter denotes no significant differences, and different letters indicate significant differences. The findings suggest that soil salinity levels between 0.10 and 0.30% do not significantly affect the germination rate, bud length, bud fresh weight, and root-shoot ratio.

Table 9 Multiple comparison results of key parameters of wheat growth.
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In conclusion, soil salinity ranging from 0.10 to 0.20% does not impact the germination rate and bud-related indexes of Cangmai 6002 at the bud stage20. A conservative analysis estimates that the maximum salt accumulation in the cooling tower deposited in the wheat field over 40 years would not significantly affect yield. According to Section "Impact of salt deposition accumulation on the salinity of soil", soil salinity is expected to reach 0.24% after 60 years of salt deposition from six cooling towers, a level at which wheat can still complete its entire growth cycle, though yield may decrease.

Given the demand in northern China for drought and alkali-resistant wheat due to their robust flavor, the higher prices of these wheat flour and related products can provide favorable economic and social benefits, potentially offsetting any production losses from drought and alkali conditions.

Conclusion

In this paper, we employ a three-dimensional numerical simulation method to examine the dynamics of floating droplets in seawater cooling towers and to quantify salt deposition. We assess the theoretical impact of salt deposition from cooling towers on the surrounding soil and water properties through numerical and mathematical analyses. Furthermore, breeding experiments were conducted to evaluate the effects of increased salinity, due to salt deposition, on wheat growth under salt stress. Our findings are summarized as follows:

  1. (1)

    Peak salt deposition occurs in March and September, reaching 900 kg/km2/month, which remains below the critical threshold for damaging plant foliage.

  2. (2)

    The maximum salt deposition from cooling towers, 900 kg/km2/month, raises the Shanhou River’s salinity by approximately 0.0139 mg/L, maintaining its classification as fresh to brackish water.

  3. (3)

    Salt deposition increases the total salinity of paddy field water to 1269 mg/L and 1107 mg/L at depths of 0.02 m and 0.05 m, respectively, without altering its classification over 6 months.

  4. (4)

    Conservative estimates indicate that after 60 years of maximum salt deposition from cooling towers, accumulated soil salinity will remain at 0.24%, without reclassification as salinized soil.

  5. (5)

    Soil salinity levels ranging from 0.10 to 0.20% do not impact the germination rate or bud-related indices of Cangmai 6002 during the bud stage, nor do they significantly affect wheat field yield over a 40-year period despite the maximal salt accumulation from six cooling towers.