Anti-corrosion process of closed cooling water system in radioactive environment

Abstract

The invention provides an anti-corrosion process of a closed cooling water system in a radioactive environment, which comprises the following steps: step 1, adding lithium hydroxide into a primary loop closed cooling water system of a nuclear power plant, and adjusting the pH value of the primary loop closed cooling water system of the nuclear power plant to 9.5-10.5 through the lithium hydroxide; 2, hydrazine is added into the primary loop closed cooling water system of the nuclear power plant, and the dissolved oxygen concentration of the primary loop closed cooling water system of the nuclear power plant is controlled to be smaller than 50 micrograms per kilogram through hydrazine. By adding lithium hydroxide to adjust the pH value of the system and adding hydrazine to adjust the dissolved oxygen of the system, the problems that the influence of a radioactive environment on a corrosion inhibitor anti-corrosion process is hardly considered at present, so that when a part of newly-built nuclear power plants are designed in a primary loop closed type cooling water anti-corrosion process, the corrosion resistance of the system is influenced are solved; and neglecting the negative influence of the activation of the corrosion inhibitor on the personnel environment.

Description

Technical Field

[0001] This application belongs to the field of chemical water treatment corrosion prevention technology, specifically relating to a corrosion prevention process for a closed cooling water system under radioactive conditions. Background Technology

[0002] Closed-loop cooling water systems in radioactive environments primarily include the primary loop equipment cooling water system and the end-shield cooling water system in nuclear power plants. The primary loop equipment closed-loop cooling water system provides a cooling source for equipment such as reactor coolant pumps, heat exchangers in the reactor refueling pool and spent fuel pool coolant handling systems, condensers in the nuclear island chilled water system, non-regenerative heat exchangers in the steam generator blowdown system, heat exchangers in the spray system, and chilled water condensers in the electrical building. It continuously removes heat from these devices, ensuring that their operating conditions always meet the requirements of nuclear power plant specifications.

[0003] The end-shield cooling water system in a nuclear power plant provides a cooling source for equipment such as the reactor cavity, end shield, and shielding rings. Since the reactor cavity surrounding the piping container and the shielding rings supporting the shield are primarily made of concrete, which is easily damaged by heat and becomes brittle at temperatures exceeding 60°C, it must be cooled with circulating water not exceeding 50°C. Simultaneously, the end-shield cooling water system is in contact with the moderator water, experiencing extremely strong neutron radiation, necessitating the minimization of neutron activation products.

[0004] These equipment pipelines are primarily constructed of carbon steel and stainless steel, while heat exchangers are mainly made of stainless steel and nickel-based alloys. Both the equipment cooling water and the end-shield cooling water makeup water are demineralized water. If these pipelines or heat exchangers leak due to scaling, corrosion, or perforation, leading to leaks in the closed cooling water systems such as the equipment cooling water and end-shield cooling water, it will severely impact the safety of the system equipment, especially systems related to nuclear power plant safety, such as spray system heat exchangers, shutdown or exhaust heat exchangers, etc. Therefore, corrosion inhibitors must be added to the system to control system corrosion and ensure the safety and reliability of the system and equipment.

[0005] The corrosion inhibitors and anti-corrosion processes for primary closed-loop cooling water mainly include two types of corrosion inhibitor formulations: one for deoxygenation and one for non-deoxygenation.

[0006] Non-oxygen-removing corrosion inhibitor formulations mainly include molybdate corrosion inhibitors, phosphate corrosion inhibitors, and nitrite corrosion inhibitors. Molybdate slow-release formulations are expensive and less economical; phosphate and nitrite corrosion inhibitor concentrations are generally controlled at 200-1000 mg / kg, putting significant pressure on phosphorus and nitrogen emissions from power plants. Nitrite corrosion inhibitor formulations also commonly face the problem of bacterial and microbial growth within the system during operation.

[0007] The formulation of oxygen scavenging and corrosion inhibitors mainly includes oxygen scavengers and alkalizing agents. Hydrazine, commonly used in power plants, is generally chosen as the oxygen scavenger, while sodium hydroxide is commonly used as the alkalizing agent. Sodium hydroxide (Na₂₃) readily absorbs neutrons and is activated to form Na₂₄ in a radioactive environment, resulting in significant additional radiation exposure for operating and maintenance personnel.

[0008] Currently, phosphate-based and nitrite-based corrosion inhibitors are mainly used in China, and the impact of radioactive environment on corrosion inhibitor anti-corrosion process is hardly considered. As a result, some newly built nuclear power plants have neglected the negative impact of corrosion inhibitor activation on personnel and environment when designing the corrosion prevention process of primary loop closed cooling water. Summary of the Invention

[0009] In view of this, this application provides a corrosion prevention process for a closed cooling water system under radioactive conditions. By adding lithium hydroxide to adjust the pH value of the system and adding hydrazine to adjust the dissolved oxygen, this process addresses the technical problem that existing corrosion prevention processes, which have almost never considered the impact of radioactive environments on corrosion inhibitors, have led to some newly built nuclear power plants neglecting the negative impact of corrosion inhibitor activation on personnel and the environment when designing the primary loop closed cooling water corrosion prevention process.

[0010] This application provides a corrosion protection process for a closed cooling water system under radioactive conditions, the corrosion protection process including: Step 1: Add lithium hydroxide to the primary loop closed-loop cooling water system of the nuclear power plant to adjust the pH value of the primary loop closed-loop cooling water system to 9.5-10.5. Step 2: Add hydrazine to the primary loop closed cooling water system of the nuclear power plant to control the dissolved oxygen concentration of the primary loop closed cooling water system to <50μg / kg.

[0011] In one specific embodiment of this application, the concentration of lithium hydroxide is 0.2-2.2 mg / kg, and the concentration of hydrazine is 0.5-30 mg / kg.

[0012] In one specific embodiment of this application, the lithium hydroxide is a commercially available industrial-grade product with a LiOH·H2O content ≥95.0 wt%, a lithium carbonate content ≤2 wt%, and passes the clarity test. The hydrochloric acid insoluble matter is ≤0.005 wt%, chloride is ≤0.005 wt%, sulfate is ≤0.02 wt%, sodium is ≤0.05 wt%, potassium is ≤0.05 wt%, magnesium is ≤0.01 wt%, calcium is ≤0.01 wt%, and iron is ≤0.002 wt%.

[0013] In one specific embodiment of this application, hydrazine is a commercially available industrial-grade product with a hydrated hydrazine (N2H4·H2O) content ≥35.0 wt%, ignition residue ≤0.002 wt%, pH value (3% aqueous solution) of 9.9, chloride ≤0.0001 wt%, fluoride ≤0.0001 wt%, sulfate ≤0.0001 wt%, sodium ≤0.0001 wt%, iron ≤0.001 wt%, and heavy metals (as Pb) ≤0.0005 wt%.

[0014] In one specific embodiment of this application, step 1 includes steps 11 and 12.

[0015] Step 11: Calculate the required weight of lithium hydroxide based on the water volume of the primary loop closed cooling water system of the nuclear power plant and the target concentration of the chemical dosing.

[0016] Step 12: Then, add the required weight of lithium hydroxide to the primary loop closed cooling water system of the nuclear power plant through the lithium hydroxide dosing tank.

[0017] In one specific embodiment of this application, step 2 includes steps 21 and 22.

[0018] Step 21: Calculate the required weight of hydrazine based on the water volume of the primary loop closed cooling water system of the nuclear power plant and the target concentration of the chemical dosing.

[0019] Step 22: Then, add the required weight of hydrazine to the primary loop closed cooling water system of the nuclear power plant through the hydrazine dosing tank.

[0020] In one specific embodiment of this application, when system water needs to be discharged during equipment maintenance, the dosage of chemicals is reduced in a planned manner before the maintenance time of the equipment requiring drainage, so that the lithium hydroxide concentration in the closed cooling water system is maintained at 0.2-0.5 mg / kg and the hydrazine concentration is maintained at 0.5-1.0 mg / kg. After the closed cooling water system is drained into the factory wastewater collection tank, an appropriate amount of strong acid is added according to the analysis results to make the wastewater meet the requirements of the national "Integrated Wastewater Discharge" standard.

[0021] The beneficial effects of this technical solution are as follows: By controlling dissolved oxygen in the primary loop closed-loop cooling water system of a nuclear power plant using hydrazine, the corrosion of the system is controlled. This not only provides excellent corrosion protection for carbon steel but also significantly reduces the discharge pressure and treatment process of maintenance wastewater, avoiding the problem of increased environmental radiation doses to personnel caused by the introduction of activation products in some corrosion inhibitor formulations. Furthermore, by using lithium hydroxide and hydrazine as corrosion inhibitors, the neutron activation of chemical additives in the closed-loop cooling water system under radioactive conditions can be minimized, thereby reducing the negative impact of neutron activation products on humans and the environment. Attached Figure Description

[0022] Figure 1 The diagram shown is a schematic flow chart of an anti-corrosion process for a closed cooling water system in a radioactive environment, according to an embodiment of this application.

[0023] Figure 2 The diagram shown is a structural schematic of a closed cooling water system in a radioactive environment according to an embodiment of this application.

[0024] Figure 3 The figure shown is a trend diagram of total iron content in the cooling water of equipment in a nuclear power plant.

[0025] Figure 4 The figure shown is a trend chart of total copper in the cooling water of equipment in a nuclear power plant.

[0026] In the diagram, 1 is the elevated water tank; 2 is the dosing system; 3 is the agitator; 4 is the circulating water pump; 5 is the user heat exchanger; and 6 is the heat trap heat exchanger. Detailed Implementation

[0027] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] At least one embodiment of this application provides a corrosion protection process for a closed-loop cooling water system in a radioactive environment, applicable to chemical corrosion protection of closed-loop cooling water systems in radioactive environments. (Reference) Figure 1 The corrosion prevention process for the closed cooling water system in this radioactive environment includes the following steps.

[0029] Step 1: Add lithium hydroxide to the primary loop closed-loop cooling water system of the nuclear power plant to adjust the pH value of the primary loop closed-loop cooling water system to 9.5-10.5.

[0030] Step 2: Add hydrazine to the primary loop closed cooling water system of the nuclear power plant to control the dissolved oxygen concentration of the primary loop closed cooling water system to <50μg / kg.

[0031] It should be noted that both lithium hydroxide and hydrazine are corrosion inhibitors.

[0032] Working Principle: Since the makeup water for the cooling water system of nuclear power plant equipment is demineralized water with extremely low impurity content, the materials used for equipment pipes include carbon steel, stainless steel, and nickel alloys. Stainless steel and nickel alloys exhibit excellent corrosion resistance in demineralized water-based media; therefore, the formulation primarily focuses on the corrosion resistance of carbon steel. In this embodiment, lithium hydroxide is used to adjust the pH value of the primary loop closed-loop cooling water system of the nuclear power plant, and hydrazine is used to control the dissolved oxygen in the primary loop closed-loop cooling water system, thereby achieving the goal of controlling corrosion in the primary loop closed-loop cooling water system of the nuclear power plant. This not only provides excellent corrosion resistance for carbon steel but also significantly reduces the discharge pressure and treatment process of maintenance wastewater, avoiding the problem of increased environmental radiation dose to personnel caused by the introduction of activation products in some corrosion inhibitor formulations. Furthermore, by using lithium hydroxide and hydrazine as corrosion inhibitors, the neutron activation of chemical additives in the closed-loop cooling water system under radioactive conditions can be minimized, thereby reducing the negative impact of neutron activation products on humans and the environment.

[0033] In at least one embodiment of this application, the concentration of lithium hydroxide is 0.2 mg / kg to 2.2 mg / kg (calculated as lithium), and the concentration of hydrazine is 0.5 mg / kg to 30 mg / kg.

[0034] In at least one embodiment of this application, the lithium hydroxide is a commercially available industrial-grade product with a LiOH·H2O content ≥95.0 wt%, a lithium carbonate content ≤2 wt%, a clearness test qualification, and hydrochloric acid insoluble matter ≤0.005 wt%, chloride ≤0.005 wt%, sulfate ≤0.02 wt%, sodium ≤0.05 wt%, potassium ≤0.05 wt%, magnesium ≤0.01 wt%, calcium ≤0.01 wt%, and iron ≤0.002 wt%.

[0035] In at least one embodiment of this application, hydrazine is a commercially available industrial-grade product with a hydrated hydrazine (N2H4·H2O) content ≥35.0 wt%, a residue on ignition ≤0.002 wt%, a pH value (3% aqueous solution) of 9.9, chloride ≤0.0001 wt%, fluoride ≤0.0001 wt%, sulfate ≤0.0001 wt%, sodium ≤0.0001 wt%, iron ≤0.001 wt%, and heavy metals (as Pb) ≤0.0005 wt%.

[0036] In at least one embodiment of this application, step 1 includes steps 11 and 12.

[0037] Step 11: Calculate the required weight of lithium hydroxide based on the water volume of the primary loop closed cooling water system of the nuclear power plant and the target concentration of the chemical dosing.

[0038] Step 12: Then, add the required weight of lithium hydroxide to the primary loop closed cooling water system of the nuclear power plant through the lithium hydroxide dosing tank.

[0039] In at least one embodiment of this application, step 2 includes steps 21 and 22.

[0040] Step 21: Calculate the required weight of hydrazine based on the water volume of the primary loop closed cooling water system of the nuclear power plant and the target concentration of the chemical dosing.

[0041] Step 22: Then, add the required weight of hydrazine to the primary loop closed cooling water system of the nuclear power plant through the hydrazine dosing tank.

[0042] In at least one embodiment of this application, when system water needs to be discharged during equipment maintenance, the dosage of chemicals is reduced in a planned manner before the maintenance time of the equipment that needs to be drained, so that the lithium hydroxide concentration in the closed cooling water system is maintained at 0.2-0.5 mg / kg and the hydrazine concentration is maintained at 0.5-1.0 mg / kg. After the closed cooling water system is drained into the factory wastewater collection tank, an appropriate amount of strong acid is added according to the analysis results to make the wastewater meet the requirements of the national "Integrated Wastewater Discharge" standard.

[0043] The corrosion prevention process provided in the above embodiments not only provides satisfactory corrosion prevention effects, but also reduces wastewater discharge pressure, avoids additional activation products, and prevents the introduction of bacteria / microorganisms.

[0044] Example 1: This invention provides a corrosion prevention process for a closed-loop cooling water system under radioactive conditions, applied to the corrosion prevention of the primary loop equipment cooling water system of a nuclear power plant. The process includes the following steps: adding lithium hydroxide to the primary loop closed-loop cooling water system of the nuclear power plant to adjust the pH value of the cooling water system. The lithium hydroxide concentration is controlled within the range of 0.2-2.2 mg / kg (calculated as lithium), resulting in a pH value of 9.5-10.5 for the closed-loop cooling water system; adding hydrazine to control the dissolved oxygen in the closed-loop cooling water system. The hydrazine concentration is controlled within the range of 0.5-30 mg / kg, resulting in a dissolved oxygen concentration <50 μg / kg for the system.

[0045] See Figure 2 The primary loop closed-loop cooling water system of a nuclear power plant includes an elevated water tank 1, a chemical dosing system 2, a stirrer 3, a circulating water pump 4, user heat exchangers 5, heat sink heat exchangers 6, and related equipment and piping. The circulating water pump 4 delivers cooling water to each user heat exchanger 5. The user medium on the shell side of the user heat exchanger 5 is cooled, and the cooling water is heated. The heated cooling water is then cooled by seawater in the heat sink heat exchanger 6 and returned to the inlet of the circulating water pump 4. The pressure of the closed-loop cooling water system is maintained by the elevated water tank 1, and any minor leakage loss is compensated for by replenishing with demineralized water.

[0046] To control the lithium hydroxide concentration (calculated as lithium) and hydrazine concentration (calculated as 0.5-30 mg / kg) in the primary loop closed-loop cooling water system of a nuclear power plant, the following measures were taken: Figure 2The chemical dosing system 2 adds lithium hydroxide and hydrazine hydrate. During the initial filling of the primary loop closed-loop cooling water system of the nuclear power plant with oxygenated demineralized water, the dosage and amount of chemicals added are calculated based on the upper limit of the specifications.

[0047] Dosing method: Calculate the required weight of lithium hydroxide and / or hydrazine hydrate based on the water volume of the primary loop closed cooling water system of the nuclear power plant and the target dosing concentration. Then, add the lithium hydroxide and / or hydrazine hydrate to the cooling water system of the nuclear power plant equipment through the dosing tank. This method requires raising the target dosing concentration to near the upper limit of lithium (2.2 mg / kg) or / and the upper limit of hydrazine (30 mg / kg). After a period of operation, when the concentration drops to near the lower limit of pH (9.5) or / and the lower limit of hydrazine (0.5 mg / kg), the chemicals are added again.

[0048] Supervision Requirements: To ensure the concentration of corrosion inhibitor formulations is controlled within the specified range, regular testing of the corrosion inhibitors (lithium hydroxide, hydrazine) and their control effects, including pH, dissolved oxygen, iron, and copper, is required. Monitoring of lithium hydroxide, hydrazine, pH, and dissolved oxygen should be conducted weekly; if levels are exceeded, reagents must be added promptly. Testing of iron and copper, reflecting corrosion control effectiveness, should be conducted monthly.

[0049] Furthermore, when system water needs to be discharged during equipment maintenance, the dosage of chemicals should be reduced in a planned manner before the maintenance time of the equipment that needs to be drained, so that the concentration of lithium hydroxide and hydrazine in the system water is maintained at slightly higher than the lower limit of the index. After the system is drained into the wastewater collection tank of the plant, an appropriate amount of strong acid is added according to the analysis results to make the wastewater meet the requirements of the national "Integrated Wastewater Discharge" standard.

[0050] Using carbon steel 106B, a representative material with poor corrosion resistance in the equipment cooling water system, static and dynamic coating tests, corrosion behavior evaluations, and oxide film assessments were conducted in two water quality environments ("0.2ppm lithium hydroxide + 0.5ppm hydrazine" and "2.2ppm lithium hydroxide + 5ppm hydrazine") and two system temperatures ("20℃" and "30℃"), according to national standards such as GB / T 18175-2000 "Determination of Corrosion Inhibiting Properties of Water Treatment Agents - Rotary Coating Method" and GB / T24196-2009 "Electrochemical Test Methods for Corrosion of Metals and Alloys". The results are shown in Table 1.

[0051] Table 1 Corrosion rate of carbon steel in different water quality environments The test results show that the corrosion prevention results fully meet the requirements of the national standard GB 50050-2017 "Design Code for Industrial Circulating Cooling Treatment" which requires the steel corrosion rate to be less than 0.075 mm / a, and the observed corrosion behavior and the formed oxide film are normal.

[0052] The corrosion inhibitor and corrosion prevention process for a closed-loop cooling water system under radioactive conditions, as described in this invention, were applied to the corrosion prevention of the cooling water system of the primary loop equipment in a nuclear power plant. By adding lithium hydroxide and hydrazine to adjust the system's pH and dissolved oxygen, the iron content in the water, a characterizing factor for the degree of corrosion, was maintained below 100 μg / kg, and the copper content below 20 μg / kg, indicating excellent corrosion control of the system. Analysis data are shown below. Figure 3 , Figure 4 Internal inspections revealed no signs of microbial corrosion, and environmental measurements showed no abnormalities. During maintenance, the system can be neutralized by adding a small amount of strong acid after draining into the wastewater storage system; the discharged water quality meets the discharge requirements stipulated in national standard GB8978-1996.

[0053] The corrosion prevention process for a closed cooling water system in a radioactive environment provided in this application embodiment fully meets the requirement of steel corrosion rate being less than 0.075 mm / a as required by the national standard GB 50050-2017 "Design Code for Industrial Circulating Cooling Water Treatment". It significantly reduces the generation of high-radiation-energy neutron activation products, reduces the risk of personnel and environmental irradiation, and basically eliminates the problem of microbial growth. The discharged water can meet the national standard discharge requirements after simple treatment.

[0054] It should be noted that the combination of the technical features in the embodiments of this application is not limited to the combination methods described in the embodiments of this application or the combination methods described in specific embodiments. All technical features described in this application can be freely combined or combined in any way, unless they contradict each other.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications to the technical solutions described in the foregoing embodiments, or equivalent substitutions for some or all of the technical features, do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A corrosion prevention process for a closed-loop cooling water system under radioactive conditions, characterized in that, include: Step 1: Add lithium hydroxide to the primary loop closed-loop cooling water system of the nuclear power plant to adjust the pH value of the primary loop closed-loop cooling water system to 9.5-10.

5. Step 2: Add hydrazine to the primary loop closed cooling water system of the nuclear power plant to control the dissolved oxygen concentration of the primary loop closed cooling water system to <50μg / kg.

2. The corrosion prevention process for a closed cooling water system under radioactive conditions according to claim 1, characterized in that, The concentration of lithium hydroxide is 0.2-2.2 mg / kg, and the concentration of hydrazine is 0.5-30 mg / kg.

3. The corrosion prevention process for a closed cooling water system under radioactive conditions according to claim 1, characterized in that, Step 1 includes: Step 11: Calculate the required weight of lithium hydroxide based on the water volume of the primary loop closed cooling water system of the nuclear power plant and the target concentration of the chemical dosing. Step 12: Then, add the required weight of lithium hydroxide to the primary loop closed cooling water system of the nuclear power plant through the lithium hydroxide dosing tank.

4. The corrosion prevention process for a closed cooling water system under radioactive conditions according to claim 1, characterized in that, Step 2 includes: Step 21: Calculate the required weight of hydrazine based on the water volume of the primary loop closed cooling water system of the nuclear power plant and the target concentration of the chemical dosing. Step 22: Then, add the required weight of hydrazine to the primary loop closed cooling water system of the nuclear power plant through the hydrazine dosing tank.

5. The corrosion prevention process for a closed cooling water system under radioactive conditions according to claim 1, characterized in that, When system water needs to be discharged during equipment maintenance, the dosage of chemicals should be reduced in a planned manner before the maintenance time of the equipment that needs to be drained, so that the lithium hydroxide concentration in the closed cooling water system is maintained at 0.2-0.5 mg / kg and the hydrazine concentration is maintained at 0.5-1.0 mg / kg. After the closed cooling water system is drained into the plant wastewater collection tank, an appropriate amount of strong acid should be added according to the analysis results to make the wastewater meet the requirements of the national "Integrated Wastewater Discharge" standard.

6. A corrosion prevention process for a closed cooling water system under radioactive conditions according to any one of claims 1 to 5, characterized in that, The lithium hydroxide is a commercially available industrial-grade product with a LiOH·H2O content ≥95.0wt%, a lithium carbonate content ≤2wt%, and a clearness test result that meets the following requirements: hydrochloric acid insoluble matter ≤0.005wt%, chloride ≤0.005wt%, sulfate ≤0.02wt%, sodium ≤0.05wt%, potassium ≤0.05wt%, magnesium ≤0.01wt%, calcium ≤0.01wt%, and iron ≤0.002wt%.

7. A corrosion prevention process for a closed cooling water system under radioactive conditions according to any one of claims 1 to 5, characterized in that, Hydrazine is a commercially available industrial-grade product with a hydrated hydrazine (N2H4·H2O) content ≥35.0 wt%, ignition residue ≤0.002 wt%, pH value (3% aqueous solution) 9.9, chloride ≤0.0001 wt%, fluoride ≤0.0001 wt%, sulfate ≤0.0001 wt%, sodium ≤0.0001 wt%, iron ≤0.001 wt%, and heavy metals (as Pb) ≤0.0005 wt%.

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