Water Chemistry in Nuclear Power Plant

Abstract

Water quality has long been an important part of the operation of nuclear power plants. Water is used as a working and cooling fluid in power plants. The quality of source waters to be used in the power plants after treatment should conform to the prescribed values of Physicochemical properties like pH, EC, TDS, alkalinity, hardness, presence of chloride content, silica, and heavy metals as recommended by technical guidelines. The Physicochemical properties of water must be recovered the recommended values of the World Health Organization (WHO), United States of Public Health (USPH), and power plant water chemistry guidelines. But the values of raw water are very far from the recommended values of Nuclear Power Plants operation. So, it needs to treat to use in the boilers. Gravitation, Carbon filtration, Ion exchange method, and Reverse Osmosis (RO) are good ways to treat the water before use in power plants. The aim of this chapter is to explore the water chemistry of the source water quality parameters values and those of the recommended values of technical guidelines.

1. Introduction

Vapor and water are used as heat transmission fluids in a number of heat transportation systems, both as a coolant and for heating, because of their accessibility and high heat capacity. A lake or the sea are two potential natural sources of chilly water [1, 2, 3, 4, 5]. Because of the high heat of vaporization, condensing steam is a highly effective heating fluid. Water and steam are corrosive, which is a drawback. Water is the coolant in practically all electric power plants, where it makes steam, and the steam spin the turbines that turn generators. Water is used extensively in the US to cool power plants. Water may also be utilized as a neutron moderator in nuclear power plants. Water serves as a condenser and a controller in most nuclear reactors. Removing water from the reactor inhibits the nuclear process.; nonetheless, alternative means for preventing a fission reaction are preferred, and it is preferable to have the nuclear reactor core roofed with water to provide sufficient cooling.

1.1 Water utilization in electric power generation

Thermoelectric or “thermal” power plants boil water to generate steam, which is then used to generate electricity. Hydropower plants, which employ dams and other methods to create energy in moving water, rely heavily on water. The flowing water drives the rotating blades, which spin a generator, and mechanical energy is converted into electrical energy by the spinning of turbines [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]. Hydroelectric power generates a significant amount of the world’s electricity.

1.2 Water chemistry in nuclear power plant

Cooling water: Most of the water is utilized to keep things cold. Vapor is accustomed to spinning the turbines that create electric power in power plants, which boil water to make steam. The steam is then cooled by drawing massive amounts of water from surrounding rivers, lakes, and seas, which is then utilized to generate additional power.

Boiler water: Thermal power plants use boilers to produce pressurized steam, which is used to spin turbines to generate electricity. The theory of the Rankin Cycle does this. To produce a team, every boiler needs a huge amount of fuel. Coal, gas, oil, and nuclear fuel are used to heat boiler water.

Process water: A thermal power production plant that converts heat energy into electricity. A steam-driven turbine transforms heat into mechanical power as an intermediary to electrical power. Water is heated, converted to steam, and then used to drive a steam turbine, which moves a power generator to produce electricity.

Consumptive water: A condenser removes heat from the water cycle in a conventional thermoelectric power plant. Cooling water is utilized to get rid of the heat. The USGS (United States Geological Survey) also measured the quantity of water consumed by thermoelectric power plants (amount of water evaporated, transpired, or absorbed into products).

1.3 The layout of nuclear power plant

There is various equipment used in a nuclear power plant. The most important parts are:

1. Fuel rod (Uranium is the basic fuel).

2. Blades/ Control rods.

3. Moderator /coolant.

4. Reactor pressure vessel.

5. Steam/vapor Generator

6. Turbine/generator

7. Containment

In steam turbines and steam turbine generators, superheaters are commonly utilized (HRSGs). Their goal is to elevate the temperature from saturation to the appropriate ultimate temperature, which in certain situations can be as high as 1000°F. When utilized in steam turbines, superheated steam lowers the turbine’s heat rate and, as a result, lowers the turbine’s steam heat rate, improving the turbine and related to plant power and output efficiency. Also, based on the pressure ratio, steam situations at the steam turbine exhaust will have no moisture; nonetheless, moisture in the last few phases of a steam turbine might harm the turbine blades. Some design approaches and performance elements of superheaters that might be of relevance to power station engineers may be found in the study that are listed/mentioned below. The most important components of a nuclear power plant are:

1. Water storage tank

2. Water circulating pump

3. Boiler/steam generator

4. Turbine

5. Generator

In a pressurized water reactor (PWR), a nuclear power plant contains three main circuits:

• The primary circuit absorbs heat from the reactor core and transfers it to the secondary to produce steam.

• In this circuit, the water turns into steam.

• The tertiary circuit is called the cooling circuit. Here the steam condenses and turns into water (Figure 1).

Figure 1.

Layout of nuclear power plant [14].

1.3.1 Primary circuit

PWRs are a type of pressurized water reactor that uses ordinary water as a controller and coolant. A main cooling circuit that runs under tremendous pressure through the reactor core and a secondary circuit that creates steam to power the turbine define the architecture [15, 16, 17]. The heat from the reactor’s fuel rods was transported to a primary circuit that held water. This water gets incredibly hot (about 300°C), yet it does not boil since it’s kept under pressure (around 155 bar). As a result, the moniker “pressurized water reactor” was coined [14].

1.3.2 Secondary circuit

The water in the secondary circuit is at reduced pressure, so it blooms in the heat transfer, which acts as a steam/vapor generator. The vapor is condensed and transferred to the heat transfer in connection with the previous (primary) circuit then it drives the turbine and produces energy. The steam generator receives the heated water via a heat exchanger. Heat is transmitted from the reactor cooling water to a different secondary circuit (the steam-water circuit). Water is transformed to steam in this circuit due to the reduced pressure; the vapor is then utilized to turn the turbine linked to a generator [14].

1.3.3 Tertiary circuit

At last, the steam that exits the turbine is cooled and turned back into the water. The condenser is cooled using a different tertiary cooling circuit that uses water from an external device [14].

2. Methodology

Water quality from diverse sources was studied and compared to the standard value of boiler water from a typical thermal power plant and the secondary circuit of the VVER-1200 PWR Reactor. To determine pH a digital pH meter (Model HT 2002-0, S/N CO316002, Hanna instrument) was utilized. The chemical properties of the water samples were determined using various analytical tests and a digital EC and TDS meter (Model S/N: Co127A, Hanna-2003-02). Heavy metals were also investigated using the Atomic Absorption Spectrophotometric (AAS) process, and the chemical properties (Hardness, Alkalinity, Chloride, and Silica) of the water samples were determined using various analytical tests.

2.1 Water purification method in power plant

This chapter briefly describes all experimental observations that are presented in the form of numerical data relevant to the individual samples used in the current experiments. It is representing the study on the analysis of water quality of the source waters and the recommended value of boiler water of Nuclear Power Plant and secondary circuit of VVER-1200 PWR reactor. The deviations between the obtained values and the recommended values are briefly discussed in the following section. Before and after treatment, several water quality parameters such as pH, Electrical Conductivity (EC), Total Dissolved Solids (TDS), Chloride, Total Hardness (TA), Total Alkalinity (TA), Silica (SiO₂), and Heavy Metals were meticulously monitored. Then, to utilize for power generation, we picked the optimal condition of the best quality of source waters (Table 1).

No	Parameter	Unit	Recommended values of PWR secondary circuit
1	pH		4.5–10
2	Electrical conductivity (EC) at 25°C	μS/cm	00
3	TDS	ppm	00
4	Chloride	ppm	<0.1
5	Total hardness	ppm	00
6	Total alkalinity	ppm	00
7	Silica (SiO2)	ppm	—

Table 1.

Recommended values of water quality for PWR secondary circuit.

2.1.1 Distillation

Distillation has several advantages. The technology is relatively affordable, except for glassware and warming components, there are no extra costs, and it generates water of generally acceptable quality. The water of Type II or III quality is commonly produced by distillation, with a resistivity of around 1.0 mega-ohm/cm. However, distillation has several disadvantages, and as a result, it is not as commonly employed as it once was. Distillation is not just a process that can be done whenever you want it. As a result of this, a certain amount of water must be purified and saved for future use. Charge particles or plasticizers will filter outside of the water pot if the storage vessel is not composed of an inert substance, decontaminating the water. Bacteria have been shown to thrive in stagnant water. The water sample was autoclaved, and the bottles could be sterilized. The bottle, however, is exposed to microorganisms once it is opened, and contamination occurs. Distillation has other disadvantages, such as wasting a lot of water and energy. It must be cleaned regularly since mineral deposits from the feedwater have built up.

2.1.2 Deionization

In laboratories, deionization is a common procedure for producing distilled water on demand. Most deionization systems consist of one-to-four-cylinder cartridges linked to plumbing lines and hanging on a wall opposite a sink. Deionization exchanges hydrogen ions for cationic pollutants and hydroxyl ions for anion exchange impurities in the input water. The desalination resins, which are little spherical plastic beads, filter the feed water. Over time, water cations and anions replace all of the fertile hydrogen and hydroxyl groups in the seeds, necessitating the replacement or regeneration of the resin. Deionization has several advantages when it comes to producing clean water (over distillation). Because it is an on-demand technique, purified water may be delivered on demand. Nuclear standard desalination resin or polishing mixed bed resin eliminates virtually all ionic particles in water to a maximum resistance of 18.2 mega ohm/cm (at 25°C). Deionization, on the other hand, does not ensure that the water is completely free of contaminants. Small particles of ion exchange resin are pushed out of the system during operation, and stagnant water in the cartridges may encourage bacterial development. Deionization does not remove all dissolved organic molecules from the feedwater, and these chemicals might foul the ionic liquid.

Finally, for laboratories that want to replace rather than regenerate their deionization cartridges, deionization cartridges might be a costly choice. Many attempts have been made to address the drawbacks of deionization and distilled. The cartridges survive significantly longer in some configurations where distillation precedes deionization, but the concerns of bacterial contamination persist [15].

2.1.3 Reverse osmosis

Reverse osmosis is a water purification technique that avoids many of the disadvantages of distillation and deionization. The natural mechanism of osmosis may be used to explain reverse osmosis. Osmosis is the process of water moving from the less concentrated (purer) side of a semipermeable membrane to the more saturated (saltier) side [18, 19, 20]. This movement continues until the concentrations reach equilibrium, or until the pressure on the more concentrated side rises to the point where the flow is halted. Osmosis, which is also the natural technique by which water travels from one cell to another in human bodies, is used to get water into a plant’s root. When a high-pressure pump is used to apply pressure to the more concentrated solution that is greater than the osmotic pressure, water molecules are driven back over the membrane to the less concentrated side, resulting in purified water. This is an example of reverse osmosis in operation. Most pollutants are normally removed by reverse osmosis, which eliminates 90–99% of them [2].

Table 2 shows the technical specifications of reverse osmosis.

Impurities	Extraction efficiency (%)
Dissolved solids	100
Bacteria	99.5
Pyrogens	99.5
Viruses	99.5
Monovalent inorganics	94–96
Divalent inorganics	96–98
Trivalent inorganics	98–99
Organic substances	97–99.5

Table 2.

Performance of reverse osmosis [2].

Reverse osmosis is a cost-effective technology that is widely used in fresh tap water before it is cleansed further using other technologies due to its high purification efficacy. Reverse osmosis is widely used in combination with the ion exchange process to increase the life of deionization “polishing” cartridges since it removes a substantial percentage of bacteria and pyrogens. Furthermore, a system that allows for the dispensing of reverse osmosis water provides a source of high-quality pre-purified water that is suitable for several laboratory applications.

2.1.4 Activated carbon filtration

Activated carbon filtration, which uses strong interaction and desorption to remove chlorine and soluble organic substances from water, is frequently found in two regions of a purifier. Even though chlorine and, to a smaller extent, dissolved organics contaminate thin-film composite reverse osmosis membranes, activated carbon is commonly utilized before the Ro system to remove these contaminants. In the polishing loop of a water purifier, a solid activated carbon filter is typically utilized to extract trace quantities of dissolved organics, resulting in water suitable for HPLC tests.

2.1.5 Ultrafiltration

Ultrafiltration employs a membrane that is essentially represented by RO systems, with the exception that the holes in the ultrafilter are somewhat bigger. Pyrogens and other big-chain biological substances or organic compounds such as RNase are removed from cleaned water using an ultrafilter. Because a large portion of the water delivered to the ultrafilter travels through it, if it is not maintained, it will ultimately block. The ultrafilter is routinely and tangentially cleansed free of impurities in a suitably constructed system. Ultrafiltration is an excellent technique for assuring highly consistent, very clean water quality with this sort of construction.

2.1.6 Ultraviolet oxidation

Ultraviolet oxidation kills bacteria by emitting ultraviolet light with a biocidal wavelength of 254 nm. It also split and ionizes some organics at 185 (nm), which are then removed by the polishing loop’s deionization and organic adsorption cartridges. In modern water purification techniques, ultrafiltration is widely used for drinking water.

2.1.7 Electrodialysis

Electrodialysis (ED) eliminates pollutants from water by drawing charging contaminates via charge-selective membranes and out of the cleaned water using an electrical current. ED is cost comparable with reverse osmosis for producing potable water from fresh brackish source water. However, ED has several disadvantages when it comes to producing laboratory-grade water, and as a result, it is rarely employed in labs. To begin with, ED’s ability to remove pollutants is restricted. Because they are not driven to the membranes, pollutants with weak or nonexistent charge density, such as some organics, pyrogens, and elemental metals, cannot be removed by ED [21, 22, 23, 24, 25]. Second, ED necessitates the use of a professional operator as well as frequent protection. Greater molecules with a substantial charge, such as colloids and detergents, can clog membrane pores, limiting their capacity to transport ions and necessitating regular cleaning. ED releases caustic soda, which can cause scaling, as well as potentially hazardous hydrogen gas. Finally, ED is a somewhat costly procedure. The electrical resistance of water rises as ionic pollutants are eliminated, requiring a greater electrical current to complete the purification process. Because of the higher power usage, purification above the potable level is considered uneconomical. Platinum and stainless steel, for example, are both costly component materials [2].

3. Conclusion

Water quality has long been an important factor in nuclear power plant (NPP) operation A proper water chemistry program is important for the safe operation of power plants. It ensures the integrity, reliability, and availability of the main plant structures, systems, and equipment that are essential for safety, by the assumptions and intentions of the design. Water is used as a working and cooling fluid in power plants. The chemistry of water coolants and corrosion concerns is particularly important in nuclear power plants. As a result, a water regime for commercially water-cooled equipment must be created to prescribe the values of water quality parameters such as Turbidity, pH, Electrical Conductivity (EC), Total Dissolved Solids (TDS), Total Hardness, Alkalinity, Chloride Content, Silica, and Heavy Metals. Hence, this chapter has been undertaken to study the water characteristics of nuclear power plants.