Anti-corrosion treatment method for metal material of nuclear power plant equipment cooling water system

Abstract

The invention belongs to the technical field of chemical water treatment corrosion prevention, and particularly relates to a corrosion prevention treatment method for a nuclear power plant equipment cooling water system metal material. The pH value is controlled to be 9.0-10.5 by adopting a method of slowly releasing lithium when a lithium type and oxyhydrogen type mixed resin purification bed runs, and impurities are removed by virtue of the resin purification bed so as to control chlorine, fluorine and sulfate radical impurity ions to be less than 50mu g / kg; the dissolved oxygen is controlled to be less than 50mu g / kg by adopting a method of nitrogen coverage at the upper part of the high-level water tank and slow oxidation and oxygen consumption of a metal material in an operation process. The problems existing in the aspect of system medium water discharge treatment are eliminated or simplified, meanwhile, the problems of unqualified water quality and corrosion caused by the fact that a high-concentration corrosion inhibitor leaks into other systems in current corrosion inhibition treatment are solved, and the microbial corrosion risk during corrosion inhibitor treatment is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical water treatment anti-corrosion, and particularly relates to a treatment method for anti-corrosion of metal materials in the equipment cooling water system of a nuclear power plant. Background Art

[0002] In a nuclear power plant, the main users of the equipment cooling water system are the shutdown cooling heat exchanger, moderator heat exchanger, spent fuel pool heat exchanger, main pump shaft seal cooler, emergency chiller, air compressor, etc. in the nuclear island, and the users in the conventional island include the lubricating oil cooler, stator water heat exchanger, rotor hydrogen cooling heat exchanger, excitation cooler, etc. The pipes of such systems are mainly made of carbon steel, and the materials of the heat exchangers are mainly stainless steel, copper, iron-nickel alloy, titanium, etc., and the makeup water is demineralized water. Scaling and corrosion perforation of the pipes and heat exchangers, resulting in leakage of equipment cooling water, will affect the safety of the system, especially the equipment related to the safety of the nuclear power plant, such as the shutdown cooling heat exchanger, moderator heat exchanger, and main pump shaft seal heat exchanger. Therefore, anti-corrosion of the equipment cooling water system is crucial for a nuclear power plant. In order to slow down the corrosion of related systems and improve the safety of the system, generally, high-concentration chemical drugs are added into the system to achieve corrosion control of the materials in the equipment cooling water system.

[0003] Currently, most of the chemical corrosion inhibitors for the closed equipment cooling water in domestic nuclear power plants use formulations such as chromate, phosphate, nitrite, silicate or molybdate. When the medium water of the systems treated with these formulations is directly discharged due to maintenance, the pollutant indicators cannot meet the requirements of the National Comprehensive Wastewater Discharge Standard GB 8978-1996 or more stringent local standards (such as the Zhejiang Provincial Local Standard DB33 / 2169-2018). In addition to the pH value not meeting the national standard requirements for the discharged medium water treated with the formulations of chromate, phosphate and nitrite, the total chromium, total phosphorus, total nitrogen and ammonia nitrogen indicators also cannot meet the requirements, and the treatment cost of the waste liquid with these 4 unqualified indicators is relatively high. Although the discharged medium water treated with the two formulations of silicate and molybdate only has an unqualified pH value, the anti-corrosion effect is poor and it is easy to scale during silicate treatment, while molybdate has an expensive use cost. In addition, the use concentrations of these formulated drugs are generally very high. If they leak into other systems with high water quality requirements, secondary problems such as unqualified water quality or corrosion problems in other systems will be caused. Moreover, in the systems using the above corrosion inhibition formulations, problems of bacterial and microbial corrosion have been found. Currently, each domestic nuclear power plant is increasing waste liquid treatment facilities or researching new corrosion inhibition formulations to meet the requirements of corrosion inhibition and discharge. The purpose of the present invention is to find a chemical drug formulation with a corrosion effect meeting the requirements, being green and environmentally friendly, and having a relatively low cost, so as to eliminate or alleviate the above existing problems. Summary of the Invention

[0004] The object of the present invention is to provide a treatment method for preventing corrosion of metal materials in the equipment cooling water system of nuclear power plants, eliminating or simplifying the problems existing in the treatment of system medium water discharge, and at the same time solving the problems of unqualified water quality and corrosion caused by the leakage of high-concentration corrosion inhibitors into other systems in the current corrosion inhibition treatment, and reducing the risk of microbial corrosion during the treatment with corrosion inhibitors.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows: A treatment method for preventing corrosion of metal materials in the equipment cooling water system of nuclear power plants. When the purification bed filled with a mixture of lithium-type cation resin and hydroxide-type anion resin operates, the method of slowly releasing lithium is used to control the pH value of the cooling water between 9.0 and 10.5. The resin purification bed is relied on to remove impurities to control the contents of impurity ions such as chlorine, fluorine, and sulfate ions in the cooling water to be less than 50 μg / kg. The method of covering the upper part of the high-level water tank with nitrogen and the slow oxidation of metal materials during operation to consume oxygen is used to control the dissolved oxygen content in the cooling water to be less than 50 μg / kg.

[0006] The resin purification bed and the chemical dosing tank are connected to the inlet and outlet of the circulating water pump, and the nitrogen supplement / exhaust pipeline is connected to the upper space of the high-level water tank.

[0007] The water quality technical conditions of the equipment cooling water system for corrosion inhibition treatment: the control range of pH value at 25 °C is 9.0 - 10.5; the control range of lithium concentration is 0.07 - 2.2 mg / kg; the control range of dissolved oxygen is ≤50 μg / kg; the control range of total iron is ≤500 μg / kg; the control range of chloride ions is ≤50 μg / kg; the control range of fluoride ions is ≤50 μg / kg; the control range of sulfate ions is ≤50 μg / kg.

[0008] If the lithium-type cation resin in the purification bed is changed to hydrogen-type, the method of regularly adding lithium hydroxide is used to maintain the pH value requirement of the equipment cooling water system.

[0009] The added lithium hydroxide is lithium hydroxide monohydrate LiOH·H2O, and the mass requirements for its components reach the technical indicators of commercially available chemical reagent analytical purity, that is, the purity is greater than 99.7%.

[0010] A hydrogen-type mixed resin purification bed is adopted and then transformed into a lithium-type resin purification bed for operation by adding lithium hydroxide to the system medium on site.

[0011] Lithium hydroxide is used as a corrosion inhibitor in the demineralized water medium.

[0012] The beneficial effects obtained by the present invention are as follows: Since the medium water for the corrosion inhibition treatment of the present invention is high-purity water containing only a small amount of lithium, the drainage of this medium can generally be recycled or only requires air bubbling to meet the direct discharge requirements for various pollutant indicators in national or local standards. It is a chemical corrosion inhibition treatment method with environmental protection and green characteristics in the current corrosion inhibition formulation treatment of nuclear power plants.

[0013] Compared with similar environmentally friendly molybdate corrosion inhibition treatments (where only the pH value of the direct drainage does not meet the national standard requirements), the operating cost of the corrosion inhibition treatment method of the present invention is less than half of that of the molybdate corrosion inhibition treatment.

[0014] During the corrosion inhibition treatment of the present invention, except for adding a small amount of lithium hydroxide and hydrazine during the initial system water filling, no corrosion inhibitor needs to be added during the normal operation of the system, reducing the operating workload.

[0015] During the actual system operation of the corrosion inhibition treatment of the present invention, the impurity ions fluorine, chlorine, and sulfate are all maintained at less than 10 μg / kg or around this value, far better than the water quality of other corrosion inhibition formulations and also better than the less than 150 μg / kg specified in the energy industry standard.

[0016] No phenomenon of microbial corrosion has been found in the system using the corrosion inhibitor of the present invention. Description of the Drawings

[0017] Figure 1 It is a schematic diagram of a typical equipment cooling water system for a nuclear power plant.

[0018] In the figure: 1, elevated water tank; 2, chemical dosing tank; 3, user heat exchanger; 4, circulation pump; 5, resin purification bed; 6, heat sink heat exchanger; 7, discharge port. Detailed Embodiments

[0019] The present invention will be described in detail below with reference to the drawings and specific embodiments.

[0020] A typical equipment cooling water system for a nuclear power plant is as Figure 1 shown. The system includes a circulation pump 4, a user heat exchanger 3, a heat sink heat exchanger 6, an elevated water tank 1, and related metal pipelines. The circulation pump 4 transports desalted cooling water to each user heat exchanger 3. The user medium on the shell side of the heat exchanger is cooled, and the desalted cooling water is heated. The heated desalted cooling water is cooled by seawater or river water through the heat sink heat exchanger 6 and then returns to the inlet of the circulation pump 4. The system pressure is maintained by the elevated water tank 1, and the loss due to a small amount of leakage is maintained by supplementing desalted water. The resin purification bed 5 and the chemical dosing tank 2 specifically related to the present invention are connected to the inlet and outlet of the circulation pump 4. The heat sink heat exchanger 6 is provided with a discharge port 7.

[0021] The theory and operation practice have proved that for a system with metal material for equipment pipelines and demineralized water as the working medium, metal corrosion can be prevented under appropriate alkaline and anaerobic environments. After comprehensive consideration and tests, especially considering that the system water discharge under the corrosion inhibition treatment of the present invention should preferably meet the requirements of the national Comprehensive Wastewater Discharge Standard GB8978-1996 and the local standard DB33 / 2169-2018 of Zhejiang Province, the present invention adopts a method of slowly releasing lithium during the operation of a purification bed filled with a mixture of lithium-type cation resin and hydroxide-type anion resin to control the pH value of the cooling water between 9.0 and 10.5, and relies on the purification bed to remove impurities to control the content of impurity ions such as chlorine, fluorine, and sulfate in the cooling water to be less than 50 μg / kg, meeting the relevant water quality requirements in the Chemical Control of Pressurized Water Reactor Nuclear Power Plants NB / T 20436-2017 of the energy industry. The method of covering the high-level water tank 1 with nitrogen and the slow oxidation of the metal material during operation to consume oxygen is used to control the dissolved oxygen content in the cooling water to be less than 50 μg / kg. The specific water quality technical conditions are shown in Table 1.

[0022] Table 1 Water Quality Technical Conditions of the Equipment Cooling Water System with Corrosion Inhibition Treatment of the Present Invention The technical verification schemes for implementing the present invention are as follows: Laboratory tests confirmed that the anti-corrosion effect under the technical conditions of this corrosion inhibition treatment meets the requirements of the national standard GB 50050-2017 Design Code for Industrial Recirculating Cooling Water Treatment, which stipulates that the steel corrosion rate is less than 0.075 mm / a and the copper corrosion rate is less than 0.005 mm / a. Table 2 shows the test results under three conditions of pH values of 9, 10, and 11. The results show that the corrosion rates of the typical materials of equipment cooling water, carbon steel A106B and copper B10, meet the above national standard requirements under the water quality technical conditions in Table 1, and the observed corrosion behavior and the formed oxide film are normal. In the actual system operation, the total iron, which is an indicator for measuring the degree of corrosion, is only a few μg / kg (see Table 5), which is better than the level of dozens or hundreds of μg / kg when other corrosion inhibition formulations are used in domestic power plants.

[0023] Table 2 Corrosion Rates of Carbon Steel and Cupronickel under Different pH Values (25°C) Laboratory tests confirmed the discharge indicators of the system medium water treated with corrosion inhibitors: Under the conditions that the pH values of the medium water treated with corrosion inhibitors were 9.5, 10, and 10.5, 100 mL of the medium water was placed in a 250 mL open conical flask, and the time required for the pH value to drop below the national standard of 9.0 under the condition of manual shaking (see Table 3). The results showed that the air dissolved in the medium water could quickly meet the discharge requirements. If the air bubbling method was used for the medium water, the required time would be shorter. For other possible pollutants in the system medium water treated with this corrosion inhibitor on-site, such as total nitrogen, ammonia nitrogen, and chemical oxygen demand, etc., the indicators were also close to or lower than the lower limit of the national standard detection method (see Table 4), meeting the direct discharge requirements of the national standard GB 8978-1996 and the local standard DB33 / 2169-2018 of Zhejiang Province for total nitrogen, ammonia nitrogen, and COD Cr and other indicators. Therefore, when using this corrosion inhibitor treatment, except for the need for simple air bubbling treatment due to the pH value not meeting the standard directly, other indicators meet the discharge requirements of the national standard and the local standard of Zhejiang Province.

[0024] Table 3 Time required for the medium water with different pH values to drop to 9 during the corrosion inhibitor treatment of the present invention Table Contents of various pollutants in the system water during direct discharge during the corrosion inhibitor treatment of the present invention On-site operation tests showed that the indicators such as chlorine, fluorine, and sulfate in the system water treated with this corrosion inhibitor met the water quality index requirements for the equipment cooling water in nuclear power plants in the energy industry standard NB / T 20436-2017. See Table 5 for details.

[0025] Table 5 Detection results of the water quality of the on-site operation system (the statistical period is 2 years) The key points and points to be protected in the present invention are in the equipment cooling water system of nuclear power plants: Utilize the characteristic that the lithium-type and hydroxide-type mixed resin purification bed 5 slowly releases a small amount of lithium to maintain the alkalinity (pH value) of the equipment cooling water system between 9.0 - 10.5. The performance of the lithium-type mixed resin used meets the requirements of Table 6.

[0026] The added lithium hydroxide is actually lithium hydroxide monohydrate (LiOH·H2O), and its component mass requirements reach the technical indicators of commercially available chemical reagent analytical pure (AR), that is, the purity is greater than 99.7%.

[0027] Adopt the method of nitrogen gas covering and slow reaction of the metal in the system pipeline with oxygen to reduce the dissolved oxygen content in the equipment cooling water to less than 50 μg / kg.

[0028] A lithium-type mixed resin purification bed 5 is adopted and used during operation to remove impurity ions in the equipment cooling water to maintain excellent water quality. The limits of ions such as chlorine, fluorine, and sulfate are far better than the industry standard limit of 150 μg / kg.

[0029] Or change the lithium-type resin in the purification bed to hydrogen-type and then convert it to a lithium-type resin purification bed 5 by adding lithium hydroxide to the system medium on-site. The performance of the adopted hydrogen-type mixed resin meets the requirements in Table 6. The technical conditions of the added lithium hydroxide are the same as those in the above Article 2, that is, the actually added lithium hydroxide is lithium hydroxide monohydrate (LiOH·H₂O), and the component mass requirements reach the technical indicators of commercially available chemical reagent analytical pure (AR), that is, the purity is greater than 99.7%.

[0030] Other treatment methods using lithium hydroxide as a corrosion inhibitor in the demineralized water medium belong to the scope of the present invention.

[0031] Table 6 Performance requirements of lithium-type and hydrogen-type purification bed mixed resins The lithium-type cation resin includes ZGCNR507Li lithium-type cation resin, and the hydroxide-type anion resin includes ZGANR170 hydroxide-type anion resin.

[0032] A typical nuclear power plant equipment cooling water system is as Figure 1 shown by the solid line in the figure. The system includes a circulating water pump 4, a user heat exchanger 3, a heat sink heat exchanger 6, a high-level water tank 1, and related metal pipes. The circulating water pump 4 conveys demineralized cooling water to each user heat exchanger 3. The user medium on the shell side of the heat exchanger is cooled, and the demineralized cooling water is heated. The heated demineralized cooling water is cooled by seawater or river water through the heat sink heat exchanger 6 and then returns to the inlet of the circulating water pump 4. The system pressure is maintained by the high-level water tank 1, and the loss due to a small amount of leakage is maintained by replenishing demineralized water. The resin purification bed 5 and the chemical dosing tank 2 specifically involved in the present invention are connected to the inlet and outlet of the circulating water pump 4, and the nitrogen supplement / exhaust pipeline is connected to the high-level water tank 1, which are all shown by dotted lines in the figure.

[0033] The sizes of the resin purification bed 5 and the chemical dosing tank 2 vary with the size of the equipment cooling water system (system water filling volume). For a general system with a water filling volume of 500 M 3 , the resin purification bed 5 needs to be filled with 200 L of resin, the purification flow rate is 30 M 3 / h, and the volume of the chemical dosing tank 2 is 1 L.

[0034] The pressure of nitrogen covering the upper part of the high-level water tank 1 is controlled at 10-16KPa. Nitrogen can be drawn from the nitrogen supply system near the inside of the power plant. The nitrogen supply valve and exhaust valve are designed to be automatically controlled. When the pressure of the covering gas is lower than 10 KPa, the supply valve opens, and when the pressure is greater than 16KPa, the exhaust valve opens. When the pressure is between 10-16KPa, both valves are closed.

[0035] After the cooling water system of the equipment is debugged and flushed and the purification bed resin is filled, if the resin bed is filled with lithium mixed resin, the initial lithium hydroxide dosage can be calculated as per M 3 0.5 g Li (equivalent to 3 g LiOH·H2O) is added to the system water. For a water loading of 500 M 3 For the system, the dosage of LiOH·H2O is 1500 g.

[0036] If the resin bed is filled with hydrogen-type mixed resin, the initial lithium hydroxide dosage is 4 grams of Li per liter of resin (equivalent to 24 LiOH·H2O) plus 10 grams of Li per liter of resin. 3 0.5 g Li (equivalent to 3 g LiOH·H2O) is added to the system water. For a water loading of 500 M 3 For the system and the system with a 200 liter resin purification bed installed, the dosage of LiOH·H2O is 3420 grams.

[0037] In the early stage of system operation, nitrogen scavenging may not be able to reduce the dissolved oxygen to below the target of 50μg / kg. At this time, the purification bed can be isolated for a period of time and commercially available chemical reagents and pure hydrazine can be added to remove oxygen. The amount of added water is calculated by multiplying the dissolved oxygen content in the water by the system water load. For a water load of 500 M 3 For a system with a dissolved oxygen content of 8 mg / kg, the amount of hydrazine added is 4000 grams.

[0038] During the operation of the system, the pH value, lithium concentration and dissolved oxygen content listed in Table 1 need to be measured once a week, and other parameters should be measured once a month. If the pH value or lithium concentration is unqualified, the lithium concentration can be controlled between 0.07-2.2 mg / kg by adjusting the purification bed flow, that is, the pH value is between 9.0-10.5. If the dissolved oxygen is unqualified, hydrazine can be added to solve it.

Claims

1. A method for treating metal materials for corrosion protection of cooling water system of nuclear power plant equipment, characterized by: The pH value of cooling water is controlled between 9.0 and 10.5 by slowly releasing lithium during operation of a purification bed filled with a mixture of lithium-type cation resin and hydroxide-type anion resin. At the same time, impurities are removed by the resin purification bed to control the contents of chlorine, fluorine and sulfate impurity ions in the cooling water to be less than 50 μg / kg. The dissolved oxygen content in the cooling water is controlled to be less than 50 μg / kg by covering the high-level water tank with nitrogen and slowly oxidizing and consuming oxygen by metal materials during operation.

2. The method for treating metal materials for corrosion protection of cooling water system of nuclear power plant equipment according to claim 1, characterized in that: The resin purification bed and the dosing box are connected to the inlet and outlet of the circulating water pump, and the nitrogen supplement / exhaust pipeline is connected to the upper air space of the high-level water tank.

3. The method for treating metal materials for corrosion protection of cooling water system of nuclear power plant equipment according to claim 1, characterized in that: Technical conditions for water quality of cooling water system of equipment subjected to corrosion inhibition treatment: pH value control range at 25°C is 9.0-10.5; lithium concentration control range is 0.07-2.2 mg / kg; dissolved oxygen control range is ≤50μg / kg; total iron control range is ≤500μg / kg; chloride ion control range is ≤50μg / kg; fluoride ion control range is ≤50μg / kg; sulfate ion control range is ≤50μg / kg.

4. The method for treating metal materials for corrosion protection of cooling water system of nuclear power plant equipment according to claim 1, characterized in that: If the lithium-type cationic resin in the resin purification bed is changed to hydrogen-type, lithium hydroxide is initially added to maintain the pH value requirements of the equipment cooling water system.

5. The method for treating metal materials for corrosion protection of cooling water system of nuclear power plant equipment according to claim 4, characterized in that: A hydrogen-type mixed resin purification bed is used and converted to a lithium-type mixed resin purification bed by initially adding lithium hydroxide to the system medium.

6. The method for treating metal materials for corrosion protection of cooling water system of nuclear power plant equipment according to claim 5, characterized in that: The added lithium hydroxide is lithium hydroxide monohydrate LiOH·H2O, and its purity is greater than 99.7%.

7. The method for treating metal materials for corrosion protection of cooling water system of nuclear power plant equipment according to claim 1, characterized in that: Lithium hydroxide is used as a corrosion inhibitor in desalted water medium.

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