The formation of the carbonate system of circulating cooling water of the Rivne NPP and its influence on changes in the surface waters pH levels of the Styr river

The data on pH changes and corresponding equilibrium shifts of the carbonate system of process and return waters of the circulating cooling system (RCS) of the Rivne NPP during water treatment were analysed. In the additional cooling water previously clarified by liming, there is no dissolved carbon dioxide and no residual free alkalinity. The effects that occur during heating, cooling, and aeration in the RCS cause a shift in the equilibrium of the carbonate cooling water system with a decrease or increase in the content of carbon dioxide, bicarbonate, and carbonate ions, and pH level. The influence of the shift of the carbonate system and the changes in pH level during the discharge of return water from the Rivne NPP into a natural water body (Styr River) was analysed.


Introduction
The water is an essential component for the operation of nuclear power plants (NPPs), as it is needed for cooling in the processes of the steam-water cycle [1].However, wastewater discharges into natural reservoirs can lead to changes in the chemical equilibrium of their components, which is a potential technogenic hazard and requires constant monitoring in the operation of nuclear power plants [2,3].Carbon dioxide, bicarbonate and carbonate ions are important parameters for monitoring the water-chemical regime of the NPP cooling system.
These are the main components of the carbonate buffer system of natural waters providing the ability to neutralize acids and determine the alkalinity of water.That is why, in water treatment technologies of the circulating cooling system (RCS) of nuclear power plants, the components of the carbonate system shall meet a number of environmental standards [2], which is also essential in a view of the sustainable development of the entire energy sector [4].

Literature review
We know that in natural waters, the ratio of carbon dioxide, bicarbonate, and carbonate ions is a forming factor in the pH level of the aquatic environment.Each of the components of the carbonate system of water exists in a certain pH range.If any of these forms is in a given amount, this determines a certain pH value of water, which in turn determines the presence of co-existing 1254 (2023) 012102 IOP Publishing doi:10.1088/1755-1315/1254/1/012102 2 forms of carbonates.Dissolved carbon dioxide interacts with water and forms bicarbonate and carbonate ions, and an increase in the water pH level, as an important component of the carbonate equilibrium, leads to a shift in the carbonate equilibrium towards the formation of calcium carbonate [5][6][7].When studying river carbonate systems, it was found that in waters with higher alkalinity, atmospheric equilibrium occurs more slowly, since part of the carbon dioxide entering the river stream passes into bicarbonate ions [8].The result of this direct buffering is a constant preservation of pH, alkalinity index, and an increase in dissolved inorganic carbon index.In addition, the components of the carbonate water system are influenced by the hydrobiological processes.In particular, during the active vegetation season of the planktonic organisms, due to the consumption of carbon dioxide during the aquatic plants' photosynthesis, the carbon dioxide is assimilated with the formation of bicarbonate ions, which is manifested in the increase in water pH levels [9].
In the technological cycles of water cooling systems, the pH value of the water is also influenced by such factors as water temperature, water movement speed, time spent in the system, the intensity of aeration and evaporation of water, as well as the addition of chemical reagents and metallurgy of the system components [10,11].
For example, in a model study of carbon dioxide release and changes in the carbonate system when heated without aeration, with the formation of calcium carbonate scale, the conditions of multi-stage RCS evaporation were reproduced.It was found that when evaporating without aeration, the pH shifts to higher values and significantly affects the concentration of bicarbonate and carbonate ions in cooling water [12].The effect of concentration of soluble salts and precipitation of calcium carbonate in return waters was also observed when the cooling water was repeatedly heated in the turbine condenser, which indicated a shift in the equilibrium of the RCS carbonate system [13].It is also proved that the other chemical components present in industrial cooling systems, after discharges into natural water bodies, make the chemical composition of surface waters more complex and can change their pH value [14].An important element of safety is scientific studies and engineering achievements aimed at improving design solutions and increasing the reliability of NPP systems and components.The essential service RCS NPP whose reliability, safety, performance of functions and efficiency are influenced by the established water chemistry [15].

Relevance and goals of research
It is obvious that when operating cooling systems of nuclear power plants, in each specific case it is important to monitor the water-chemical regime of both the RCS itself and the natural water bodies to which their discharge waters fall.This will improve the understanding of the conditions to protect the natural surface waters, and therefore contribute to the environmental safety of the operation of nuclear power plants.
That is why the goal of our research was to analyze the formation of the carbonate system of the Rivne NPP and its influence on changes in the pH levels of the surface waters of the Styr River in order to optimize the use of chemical reagents in water treatment technology minimizing the ecological impact of discharged water on surface waters and improving the effectiveness of the water and chemical regime RCS.

Object and methods of research
The carbonate system of process and return water of the Rivne NPP was the object of this study.Water treatment of RCS cooling water is carried out by liming with bicarbonate regime dosage, followed by stabilization treatment with sulphuric acid.The water balance of the RCS of the Rivne NPP is given in (figure 1).Note that in accordance with the national standards of Ukraine requirements regarding the composition and properties of drinking water objects usage, the water of recreation areas reservoirs, as well as the water of fisheries reservoirs should not exceed the values of 6.5-8.5 for pH levels.
The pH was measured with the "I-160" ionometer according to the standard method [16].The analysis of the chemical control data of RCS was carried out in accordance with the reports on the assessment of the impact of non-radiation factors of the Rivne NPP [17].

Results
The dynamics of changes in the pH levels of technological waters of the Rivne NPP and the water of the Styr River are shown in the table 1 and at figure 2, figure 3, respectively.The data shown in the table indicate that the pH level of Styr River meets the requirements of national standards of Ukraine and this is determined by the content of bicarbonate ions.
During water treatment by liming, the pH level increases to 9.4-9.8units, there is a shift in the carbonate equilibrium with the formation of carbonate ions and precipitation of calcium carbonate, the overall hardness is significantly reduced compared to incoming water, and the free alkalinity is formed.During stabilization treatment with sulphuric acid, the alkalinity is neutralized to form bicarbonates.The need for stabilization treatment with sulphuric acid is determined by the quality indicators of RCS cooling water.Within the work of Rivne NPP RCS, during the period of our research, there were periods of recharge with additional water, neutralized or not neutralized with the sulphuric acid.When feeding with water neutralized with sulphuric acid, an increase in pH level was observed in the RCS cooling water, and the non-neutralized one has a decrease in pH level, which in both cases is due to the accumulation of bicarbonate ions (figure 2, figure 3).Regardless of the method of stabilization treatment, the stabilization of the pH values of RCS cooling water is observed in the range of 8.3-8.7.In all cases, the variation coefficients of the pH change ranges were less than 10%, which indicates their low variability and, consequently, relative constancy of the values.The discharge of return water with a pH level of 8.3-8.7 of the Rivne NPP does not affect the pH values of water in Styr River and compliance with the requirements of national standards of Ukraine (figure 2, figure 3).The data of pH control of the Styr River given in the figure show that the indicator is subject to seasonal fluctuations throughout the year.Thus, in winter, the pH level decreases to 7.5-8.0units, and in summer it increases up to 7.8-8.5 units.

Discussion
When operating RCS, it is important to ensure the water-chemical regime (WCR), in order to prevent the formation of scale, corrosion, and biological interference [2].According to the results of our research, it was found out that in the technological cycle of the RCS of the Rivne NPP, additional cooling water is completely devoid of carbon dioxide during the pre-liming, and the formation of carbon dioxide occurs when neutralized with sulphuric acid and when heated in heat exchangers by consumers with the release of calcium carbonate sediment.In the cooling tower, when cooling and aeration of cooling water with pH level 9.6 ± 0.2, without pre-stabilization treatment with sulphuric acid, occurs carbon dioxide absorption from the air can be observed to establish the equilibrium of the carbonate system by neutralizing the residual free alkalinity of the pre-limed water, which confirms the control data of cooling and additional water figure 3.
When using untreated inlet water to feed RCS, the scale formation calcium carbonate cannot be prevented even at low levels degree of evaporation for cooling water [13].Therefore, pre-treatment methods are used, which may include physical, physico-chemical and chemical methods of water treatment.Liming is one of the most common technological solutions for water treatment [10].The dose of lime used for water treatment determines the value of the pH values of the additional cooling water.
According to the results of our research, during the stabilization treatment with constant sulphuric acid (table 1, figure 2), the free alkalinity of pre-limed cooling water with pH 7.5-7.8values is already neutralized, the absorption of carbon dioxide from the air with neutralization of the residual free alkalinity in the cooling tower does not occur, when cooling in the cooling tower, it is possible to release carbon dioxide formed during the deposition of calcium carbonate by reaction with an increase in the hydrogen pH index, which is due to the concentration of the sample during evaporation and accumulation of bicarbonate ions for cooling water that has been limed and treated with sulphuric acid and does not contain free alkalinity.
The properties of the carbonate system of water during the stabilization treatment with factorial dosage sulphuric acid (table 1, figure 3) depend on the course of two competing reactions that cause the dissolution of carbon dioxide during aeration with the formation of bicarbonate ions, with a decrease in pH level, and, when evaporation in RCS by the decomposition of bicarbonate ions and the formation of carbonate ions, which increases pH level [12].That is, the initial distribution state of the carbonate system components determined by the initial pH level is the determining factor for shifting the equilibrium of reactions.
We have noticed seasonal fluctuations in the pH value in Styr River.In particular, a decrease in its values in winter and an increase in summer can be explained by seasonal features of the phytoplankton groups development and an increased manifestation of their photosynthetic activity during the warm period [9].That is, the change in the pH balance of the surface waters of the Styr River during the year is primarily related to the natural factors.

Conclusions
In the study of the equilibrium of the carbonate system in the process waters of the RCS of the Rivne NPP and the natural waters of the Styr river, it was found out that the use of stabilization treatment with sulphuric acid in water treatment (solely to ensure environmental standards for the pH value) is impractical.
Results of factorial and constant sulphuric acid dosing have comparable pH values of the cooling water, therefore accumulation of bicarbonate ions and neutralization of carbonate ions from an ecological point of view does not compensate for the artificial introduction of sulphate ions into natural objects for the use of sulphuric acid only to reduce the pH level of water in order to ensure environmental standards.It is advisable to use sulphuric acid only to normalize the water-chemical regime.Taking this fact into account will optimize the RCS water treatment technology of the Rivne NPP with a guarantee of compliance with the environmentally safe water-chemical regime of the discharge water on the Styr river.RCS are widespread project decision for cooling components and systems of power plants, these results of the study can be also applied to any power plant with RCS, where the pre-treatment of cooling water by liming and stabilization treatment with mineral acid is implemented.

Figure 1 .
Figure 1.Cooling water balance of the Rivne NPP RCS.

Figure 2 .
Figure 2. Change of pH in the process water of the Rivne NPP and the water of the Styr River for 2012 -2016 (diagram with accumulation).

Figure 3 .
Figure 3. Change of pH in the process water of the Rivne NPP and the water of the Styr River for 2017 -2021 (diagram with accumulation).

Table 1 .
Descriptive statistics of the results distribution of average annual pH measurements in the process water of the Rivne NPP and the water of the Styr River during water treatment with constant and factorial dose H 2 SO 4 .
Note: M is the arithmetic mean pH value; SE is the standard error; Min-Max are minimum and maximum pH values; CV is the coefficient of variation for pH values.