A chemical cleaning agent and cleaning method for a nuclear power plant secondary circuit system

Abstract

The application discloses a kind of for nuclear power plant two loop systems chemical cleaning agent and cleaning method, belong to nuclear power plant equipment chemical cleaning technical field.The cleaning agent is by cleaning main agent ethylenediaminetetraacetic acid, cleaning auxiliary agent (citric acid and / or glycollic acid), corrosion inhibitor N-108, pH regulator ammonia water, reducing agent (hydrazine hydrate or D-sodium isoascorbate), organic silicon defoaming agent and solvent (deionized water or desalted water) compound is formed.The cleaning agent cleaning stage is carried out at pH 5.5~6.5, temperature 110~130 DEG C;Passivation stage is completed by supplementing ammonia water at pH 8.5~9.5, temperature 80~95 DEG C.The cleaning agent of the application has high dissolution rate (up to 96.5%) to common scale type such as Fe3O4, has very low corrosion rate (<0.005 g / m 2 ·h) to austenitic stainless steel and nickel-based alloy, and seamless conversion of cleaning and passivation can be realized by pH adjustment, the operation process is simple, the waste liquid amount is less, the environmental protection is good, and the safety, efficient operation of nuclear power unit two loop system can be effectively guaranteed.

Description

Technical Field

[0001] This invention belongs to the field of nuclear power plant equipment cleaning, specifically relating to a chemical cleaning agent and cleaning method for the secondary loop system of a nuclear power plant. Background Technology

[0002] During the operation of the secondary loop of a nuclear power plant, harmful impurities carried in the feedwater enter the evaporator. After concentration and precipitation, they deposit on the surfaces of the heat exchange tube bundles, support plates, and baffles inside the evaporator. Simultaneously, the evaporator, operating under high temperature and pressure, inevitably undergoes oxidation reactions with the metal materials, resulting in oxide formation on the surfaces of the heat exchange tubes, support plates, and baffles. When these impurities or oxides accumulate to a certain level, they significantly affect the water flow conditions and heat exchange efficiency inside the evaporator. Furthermore, harmful ions beneath the deposits become highly concentrated during operation, easily triggering intergranular corrosion or intergranular stress corrosion of the heat exchange tubes inside the evaporator. This can lead to corrosion, cracking, and leaks in the 600, 690, or 800 series stainless steel tube bundles, support plates, and baffles inside the evaporator, seriously affecting the safe operation of the nuclear power plant. According to the Electric Power Research Institute (EPRI) Steam Generator Degradation Database in 1999, the main cause of evaporator heat transfer tube repairs in nuclear power plants between 1995 and 1999 was intergranular corrosion or intergranular stress corrosion of the stainless steel heat exchange tubes on the secondary side, accounting for about 48% of all tube repairs. Therefore, scaling and corrosion of the secondary loop evaporator is one of the main causes affecting the safe operation of nuclear power units, and effective methods must be adopted to solve this problem.

[0003] Currently, my country has over 25 operating nuclear reactors, with new units under construction. To ensure the safe operation of these reactors, the secondary loop of nuclear power plants must undergo regular descaling and cleaning. Extensive research and experimentation have been conducted both domestically and internationally to effectively address the scaling problem in the secondary loop. Early methods primarily involved physical and mechanical cleaning or high-pressure nitrogen bubbling, but these proved incomplete and unsatisfactory. Subsequently, the United States, Germany, and France proposed the SGOG-EPRI chemical cleaning method, the KWU chemical cleaning method, and the EDF chemical cleaning method, respectively, using ethylenediaminetetraacetic acid (EDTA), nitroglycerin triacetic acid (NTA), and citric acid as primary cleaning media. This ushered in the era of chemical cleaning for the secondary loop of nuclear power plants. Each of these three methods has its advantages and disadvantages. The SGOG-EPRI chemical cleaning method uses a corrosion inhibitor containing sulfur, which can corrode Inconel tubes. It also involves many cleaning steps and generates a large amount of waste liquid. The KWU chemical cleaning method uses NTA as the main cleaning medium. The agent has poor environmental performance. Although the cleaning agent does not contain corrosion inhibitors, it is easy to corrode the heat exchange tube bundle. The EDF chemical cleaning method also contains sulfur in the cleaning agent, which can easily cause adverse corrosion to the heat exchange tubes.

[0004] CN120209942 A discloses a chemical cleaning agent and method for steam generators based on composite EDTA. This technology uses EDTA as the main cleaning agent, supplemented with corrosion inhibitors, reducing agents, and solvent promoters, and performs cyclic cleaning under specific temperature (120–140℃) and pH (8.5–9.5) conditions. While this method is effective in dissolving scale and inhibiting corrosion, its high cleaning temperature requires strict control of system thermal stress, and the cleaning process necessitates multiple batches of agent addition, resulting in a complex operation and a relatively long overall cleaning cycle, which is not conducive to rapid and efficient field application. CN118407053 A provides a novel environmentally friendly steel oxide scale cleaning agent, based on sulfuric acid and containing hydrogen peroxide, surfactants, corrosion inhibitors, etc., which can remove oxide scale from the surface of hot-rolled steel in a short time (20–30 minutes) at room temperature. However, this cleaning agent, primarily composed of strong inorganic acids, while effective at removing scale from steel, is highly acidic and corrosive. Furthermore, the waste liquid produced after cleaning contains a large number of inorganic anions (such as sulfate), making it difficult to rinse thoroughly. If such a strong acid system is applied to the secondary loop system of a nuclear power plant, especially inside the complex evaporator, the residual corrosive ions can easily cause localized corrosion or stress corrosion cracking in materials such as stainless steel and nickel-based alloys, seriously threatening the long-term operational safety of the equipment.

[0005] Therefore, there is an urgent need to develop a chemical cleaning agent that has both high scale dissolving ability and minimal corrosion to various metal materials in the secondary loop of nuclear power plants, is easy to operate, produces little waste liquid, and is easy to rinse thoroughly, so as to meet the needs of safe, environmentally friendly, and efficient cleaning of nuclear power units. Summary of the Invention

[0006] The purpose of this invention is to solve the problems of evaporator corrosion and leakage or poor heat exchange efficiency and reduced load operation caused by scaling in the secondary loop of nuclear power units. It provides a high-efficiency, low-corrosion, easy-to-operate, environmentally friendly and safe chemical cleaning agent for the secondary loop system of nuclear power plants. This cleaning agent successfully solves the bottleneck problem in existing nuclear power plant cleaning technologies, which make it difficult to balance efficiency, corrosion, environmental protection and operational complexity. It not only has a good cleaning and scale dissolution effect, but also causes little corrosion damage to metal materials and leaves little chemical residue. Moreover, it produces a small amount of waste liquid and has excellent environmental performance. It has important practical significance for ensuring the safe, stable and economical operation of nuclear power units.

[0007] The chemical cleaning agent for the secondary loop system of a nuclear power plant provided by this invention is a compound of a cleaning agent, a cleaning aid, a corrosion inhibitor, a pH adjuster, a reducing agent, a defoamer, and a solvent. The addition amounts of each component per liter of solvent are: 50-100g of the cleaning agent, 10-25g of the cleaning aid, 2-5g of the corrosion inhibitor, 1-2g of the reducing agent, and 0.1-0.2g of the defoamer. The pH adjuster controls the pH of the cleaning agent to 5.5-6.5. The cleaning agent is ethylenediaminetetraacetic acid (EDTA); the cleaning aid is one or a mixture of two of citric acid and glycolic acid; the pH adjuster is ammonia; and the reducing agent is hydrazine hydrate or sodium D-isoascorbate.

[0008] Furthermore, in the above-mentioned cleaning agent, the corrosion inhibitor is corrosion inhibitor N-108.

[0009] Furthermore, in the above-mentioned cleaning agent, the defoamer is an organosilicone defoamer.

[0010] The preparation method of the cleaning agent of the present invention is as follows: the solvent is heated to 60-70°C, the corrosion inhibitor is added first and stirred to dissolve; then the main cleaning agent and part of the pH adjuster are added and stirred until completely dissolved; then the cleaning aid, reducing agent and defoamer are added in sequence; finally, the pH adjuster is added to adjust the pH value of the solution to 5.5-6.5, and stirring is continued for 30-60 minutes to obtain a uniform and stable cleaning agent.

[0011] The present invention also provides a method for cleaning the secondary loop system of a nuclear power plant using the above-mentioned cleaning agent, comprising the following steps:

[0012] (1) Cleaning stage: Inject the cleaning agent into the system, control the temperature at 110-130℃, and perform cyclic cleaning.

[0013] (2) Passivation stage: After cleaning, add ammonia water as a pH adjuster to the system to adjust the pH value of the liquid in the system to 8.5-9.5 and the temperature to 80-95℃ for passivation treatment.

[0014] (3) Flushing stage: After passivation is completed, drain the liquid in the system and use demineralized water to thoroughly flush the system.

[0015] In the above cleaning stage (1), if the pH value of the cleaning solution is higher than 6.5, the main cleaning agent ethylenediaminetetraacetic acid is added to adjust the pH.

[0016] In the above passivation stage (2), if the pH value of the liquid in the system is lower than 8.5, ammonia water is added as a pH adjuster to adjust the pH.

[0017] The beneficial effects of this invention are as follows:

[0018] 1. This invention uses ethylenediaminetetraacetic acid (EDTA) as the main cleaning agent, combined with citric acid or glycolic acid as auxiliary agents, to form a highly efficient organic chelate cleaning system. This system has extremely strong complexing and dissolving capabilities for common iron oxide (such as Fe3O4) scale in the secondary loop system of nuclear power plants, with a scale dissolution rate of 86.7% to 96.5%. It can effectively remove deposits from critical parts such as evaporator heat transfer tubes, restore heat exchange efficiency, and has high cleaning efficiency and excellent scale dissolution performance.

[0019] 2. This invention uses corrosion inhibitor N-108, which provides excellent protection for austenitic stainless steels (such as 0Cr18Ni9Ti) and nickel-based alloys (such as Inconel 600) widely used in the secondary loop of nuclear power plants. It exhibits extremely low corrosion and high equipment safety. Tests show that the corrosion rate for these critical materials is less than 0.005 g / m³. 2 The corrosion rate was measured at a uniform level, with no signs of intergranular corrosion or stress corrosion, fundamentally avoiding the material degradation risks that might be induced by traditional sulfur-containing corrosion inhibitors or inorganic acid cleaning. For ordinary carbon steels or low-alloy steels such as 20G and WB36, the corrosion rate remained at a low and uniform level (approximately 0.25–0.38 g / m²). 2 (·h), which is far below the engineering permit standard, ensuring the overall integrity of the system.

[0020] 3. This invention achieves integrated cleaning and passivation through ingenious formulation design. By simply adjusting the pH value (5.5–6.5 for the cleaning section and 8.5–9.5 for the passivation section) and temperature, cleaning and passivation functions can be seamlessly switched within the same reagent system without the need to change the cleaning medium or perform complex neutralization and rinsing steps. This greatly simplifies on-site operation procedures, significantly reduces downtime for cleaning, and improves the availability of nuclear power units.

[0021] 4. The cleaning agent of this invention uses only biodegradable or high-temperature decomposable organic components, making it environmentally friendly and avoiding the highly corrosive anions (such as Cl-) from strong inorganic acids (such as sulfuric acid and hydrochloric acid). - SO4 2- This method avoids the problems of residues and the environmental risks associated with sulfur-containing components. Because it eliminates the need for multi-step replacement and rinsing and has high reagent utilization, the total amount of waste liquid generated is significantly less than that of traditional stepwise cleaning processes (such as the SGOG-EPRI method), reducing the cost and burden of subsequent waste liquid treatment and storage.

[0022] 5. The cleaning agents of this invention are all water-soluble organic components. After passivation is completed and the solution is discharged, they can be quickly and thoroughly removed from complex system structures (such as dense heat transfer tube bundles) by conventional demineralized water rinsing, leaving virtually no harmful residues. This eliminates the risk of localized concentration and subsequent corrosion caused by cleaning agent residues during operation, ensuring the long-term safe operation of the unit after cleaning.

[0023] 6. The cleaning agent formulation and method of this invention are specifically designed for various materials (carbon steel, alloy steel, stainless steel, nickel-based alloy) and typical scale types in the secondary loop of nuclear power plants. It has universality and can be applied to online or offline chemical cleaning of the entire secondary loop system, and has high promotional value. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited to these embodiments.

[0025] The corrosion inhibitor N-108 used in the following examples was provided by Xi'an Xieli Power Technology Co., Ltd., and the defoamer AFZ-10A was provided by Shanghai Salaf Chemical Co., Ltd.

[0026] Example 1

[0027] Heat 500 mL of deionized water in a beaker to 60–70 °C. First, add 1.5 g of corrosion inhibitor N-108, then add 25 g of EDTA and 15 g of ammonia. Stir until the EDTA is completely dissolved. Next, add 5 g of citric acid, 5 g of glycolic acid, 0.5 g of hydrazine hydrate, and 0.05 g of defoamer AFZ-10A. Finally, adjust the pH of the solution to 5.6 with ammonia and continue stirring for 30 min to prepare a homogeneous and stable cleaning agent.

[0028] Add 6g of Fe3O4 scale to the above beaker, and simultaneously attach three test pieces each of the following materials to the solution: 20G (common auxiliary system material), 20KCr (common low-pressure water supply system material), WB36 (common high-pressure water supply pipe material), T91 (common main steam pipe material), 0Cr18Ni9Ti (evaporator auxiliary system material), and Inconel 600 (common evaporator heat exchanger tube material). Then, place the beaker in a sealed autoclave and put it in an oven at 120℃ for constant temperature cleaning for 24 hours. After cleaning, cool down the solution to 90℃, add ammonia to adjust the pH to 8.9, and passivate for 6 hours. The experiment is then completed, and the total iron content in the solution and the corrosion rate of the test pieces are measured. The results are shown in Table 1.

[0029] Example 2

[0030] Heat 500 mL of deionized water in a beaker to 60–70 °C. First, add 2 g of corrosion inhibitor N-108, then add 40 g of EDTA and 24 g of ammonia. Stir until the EDTA is completely dissolved. Next, add 8 g of citric acid, 1 g of hydrazine hydrate, and 0.1 g of defoamer AFZ-10A. Finally, adjust the pH of the solution to 5.9 with ammonia and continue stirring for 30 min to prepare a homogeneous and stable cleaning agent.

[0031] Add 8g of Fe3O4 scale to the above beaker, and simultaneously attach three test pieces each of the following materials to the solution: 20G (common auxiliary system material), 20KCr (common low-pressure water supply system material), WB36 (common high-pressure water supply pipe material), T91 (common main steam pipe material), 0Cr18Ni9Ti (evaporator auxiliary system material), and Inconel 600 (common evaporator heat exchanger tube material). Then, place the beaker in a sealed autoclave and put it in an oven at 110℃ for constant temperature cleaning for 24 hours. After cleaning, cool down the solution to 90℃, add ammonia to adjust the pH to 9.2, and passivate for 6 hours. The experiment is then completed, and the total iron content in the solution and the corrosion rate of the test pieces are measured. The results are shown in Table 1.

[0032] Example 3

[0033] Heat 500 mL of deionized water in a beaker to 60–70 °C. First, add 2.5 g of corrosion inhibitor N-108, then add 50 g of EDTA and 30 g of ammonia. Stir until the EDTA is completely dissolved. Next, add 5 g of citric acid, 1 g of hydrazine hydrate, and 0.1 g of defoamer AFZ-10A. Finally, adjust the pH of the solution to 6.5 with ammonia and continue stirring for 30 min to prepare a homogeneous and stable cleaning agent.

[0034] Add 10g of Fe3O4 scale to the above beaker, and simultaneously attach three test pieces each of the following materials to the solution: 20G (common auxiliary system material), 20KCr (common low-pressure water supply system material), WB36 (common high-pressure water supply pipe material), T91 (common main steam pipe material), 0Cr18Ni9Ti (evaporator auxiliary system material), and Inconel 600 (common evaporator heat exchanger tube material). Then place the beaker in a sealed autoclave and put it in an oven at a constant temperature of 130℃ for 24 hours for cleaning. After cleaning, cool down the solution to 90℃, add ammonia to adjust the pH to 9.5, and passivate for 6 hours. The experiment is then completed, and the total iron content in the solution and the corrosion rate of the test pieces are measured. The results are shown in Table 1.

[0035] Example 4

[0036] Heat 500 mL of deionized water in a beaker to 60–70 °C. First, add 2.5 g of corrosion inhibitor N-108, then add 50 g of EDTA and 30 g of ammonia. Stir until the EDTA is completely dissolved. Next, add 5 g of glycolic acid, 1 g of sodium D-isoascorbate, and 0.1 g of defoamer AFZ-10A. Finally, adjust the pH of the solution to 5.5 with ammonia and continue stirring for 30 min to prepare a homogeneous and stable cleaning agent.

[0037] Add 10g of Fe3O4 scale to the above beaker, and simultaneously attach three test pieces each of the following materials to the solution: 20G (common auxiliary system material), 20KCr (common low-pressure water supply system material), WB36 (common high-pressure water supply pipe material), T91 (common main steam pipe material), 0Cr18Ni9Ti (evaporator auxiliary system material), and Inconel 600 (common evaporator heat exchanger tube material). Then place the beaker in a sealed autoclave and put it in an oven at 120℃ for constant temperature cleaning for 24 hours. After cleaning, cool down the solution to 90℃, add ammonia to adjust the pH to 9.5, and passivate for 6 hours. The experiment is then completed, and the total iron content in the solution and the corrosion rate of the test pieces are measured. The results are shown in Table 1.

[0038] Table 1. Scale dissolution effect and corrosion rate of various materials in the examples.

[0039]

[0040] As can be seen from the data in Table 1, the cleaning agent of the present invention has excellent performance in dissolving Fe3O4 scale and has a low corrosion rate on common metal materials in the secondary loop of nuclear power plants. It is an excellent cleaning agent for the secondary loop and can be widely used in the cleaning and descaling of the secondary loop of nuclear power plants.

Claims

1. A chemical cleaning agent for the secondary loop system of a nuclear power plant, characterized in that: The cleaning agent is formulated from the following components: cleaning agent, cleaning aid, corrosion inhibitor, pH adjuster, reducing agent, defoamer, and solvent; the addition amount of each component per liter of solvent is: 50-100g of cleaning agent, 10-25g of cleaning aid, 2-5g of corrosion inhibitor, 1-2g of reducing agent, 0.1-0.2g of defoamer, and the pH adjuster controls the pH of the cleaning agent to 5.5-6.5; The main cleaning agent is ethylenediaminetetraacetic acid; The cleaning aid is one or a mixture of two of citric acid and glycolic acid; The pH adjuster is ammonia water; The reducing agent is hydrazine hydrate or sodium D-isoascorbate.

2. The chemical cleaning agent for the secondary loop system of a nuclear power plant according to claim 1, characterized in that: The corrosion inhibitor is corrosion inhibitor N-108.

3. The chemical cleaning agent for the secondary loop system of a nuclear power plant according to claim 1, characterized in that: The defoamer is an organosilicone defoamer.

4. The chemical cleaning agent for the secondary loop system of a nuclear power plant according to claim 1, characterized in that: The cleaning agent is prepared by heating the solvent to 60-70°C, first adding the corrosion inhibitor and stirring to dissolve it; then adding the main cleaning agent and part of the pH adjuster and stirring until completely dissolved; then adding the cleaning aid, reducing agent and defoamer in sequence; finally adding the pH adjuster to adjust the pH value of the solution to 5.5-6.5 and continuing to stir for 30-60 minutes to obtain a uniform and stable cleaning agent.

5. A chemical cleaning method for the secondary loop system of a nuclear power plant, characterized in that: Using the cleaning agent according to any one of claims 1 to 3, the process includes the following steps: (1) Cleaning stage: Inject the cleaning agent into the system, control the temperature at 110-130℃, and perform cyclic cleaning; (2) Passivation stage: After cleaning, add ammonia water as a pH adjuster to the system to adjust the pH value of the liquid in the system to 8.5-9.5 and the temperature to 80-95℃ for passivation treatment; (3) Flushing stage: After passivation is completed, drain the liquid in the system and use demineralized water to thoroughly flush the system.

6. The chemical cleaning method for the secondary loop system of a nuclear power plant according to claim 5, characterized in that: In the cleaning stage (1), if the pH value of the cleaning solution is higher than 6.5, the main cleaning agent ethylenediaminetetraacetic acid is added to adjust the pH.

7. The chemical cleaning method for the secondary loop system of a nuclear power plant according to claim 5, characterized in that: In the passivation stage (2), if the pH value of the liquid in the system is lower than 8.5, ammonia water is added as a pH adjuster to adjust the pH.

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