A treatment method for anti-corrosion of metal materials in the equipment cooling water system of a nuclear power plant

Abstract

The present invention belongs to the technical field of anti-corrosion for chemical water treatment, and particularly relates to a treatment method for anti-corrosion of metal materials in the equipment cooling water system of a nuclear power plant. When using a purification bed of a mixed resin of lithium type and hydroxide type, the method of slowly releasing lithium is adopted to control the pH value between 9.0 and 10.5. The purification bed of resin is relied on to remove impurities to control that the impurity ions of chlorine, fluorine and sulfate radicals are all less than 50 μg / kg. The method of covering nitrogen gas on the upper part of the elevated water tank and slowly oxidizing and consuming oxygen by the metal materials during the operation process is adopted to control the dissolved oxygen to be less than 50 μg / kg. The present invention eliminates or simplifies the problems existing in the treatment of the system medium water discharge, and at the same time solves the problems that the high-concentration corrosion inhibitor leaks into other systems in the current corrosion inhibition treatment, resulting in unqualified water quality and corrosion problems, and reduces the risk of microbial corrosion during the treatment of the corrosion inhibitor.

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 lubricating oil coolers, stator water heat exchangers, rotor hydrogen cooling heat exchangers, excitation coolers, etc. The pipes of such systems are mainly made of carbon steel, and the heat exchanger materials are mainly stainless steel, copper, iron-nickel alloy, titanium, etc., and the makeup water is demineralized water. The scaling and corrosion perforation of pipes and heat exchangers, resulting in the 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, the anti-corrosion of the equipment cooling water system is crucial for the nuclear power plant. In order to slow down the corrosion of the relevant system and improve the safety of the system, generally, high-concentration chemical drugs are added to the system to achieve the corrosion control of the materials in the equipment cooling water system.

[0003] At present, most of the chemical corrosion inhibitors used in the closed equipment cooling water of domestic nuclear power plants are chromate, phosphate, nitrite, silicate or molybdate formulations. When the system medium water treated by 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 local standard DB33 / 2169-2018 in Zhejiang Province). Among them, for the medium water discharged from the formulations of chromate, phosphate and nitrite, in addition to the pH value not meeting the national standard requirements, 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 by the silicate and molybdate formulations only has an unqualified pH value, the anti-corrosion effect is poor and it is easy to scale during silicate treatment, while the molybdate has a high usage cost. In addition, the usage concentration of these formulated drugs is 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 occur. Moreover, bacterial and microbial corrosion problems have been found in the systems using the above corrosion inhibitor formulations. At present, domestic nuclear power plants are all increasing waste liquid treatment facilities or researching new corrosion inhibitor 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 a nuclear power plant, 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 high-concentration corrosion inhibitors leaking 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:

[0006] A treatment method for preventing corrosion of metal materials in the equipment cooling water system of a nuclear power plant. 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 adopted to control the pH value of the cooling water between 9.0 and 10.5. Relying on the resin purification bed to remove impurities to control the contents of chlorine, fluorine, and sulfate impurity ions in the cooling water to be less than 50 μg / kg, and 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 adopted to control the dissolved oxygen content in the cooling water to be less than 50 μg / kg.

[0007] The resin purification bed and the 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.

[0008] Technical conditions for the water quality of the equipment cooling water system in 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 ion is ≤50 μg / kg; the control range of fluoride ion is ≤50 μg / kg; the control range of sulfate ion is ≤50 μg / kg.

[0009] If the lithium-type cation resin in the purification bed is changed to hydrogen-type, the method of regularly adding lithium hydroxide is adopted to maintain the pH value requirement of the equipment cooling water system.

[0010] The added lithium hydroxide is lithium hydroxide monohydrate LiOH·H2O, and the quality requirements of its components reach the technical indicators of commercially available chemical reagent analytical purity, that is, the purity is greater than 99.7%.

[0011] 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.

[0012] Lithium hydroxide is used as a corrosion inhibitor in the demineralized water medium.

[0013] The beneficial effects obtained by the present invention are as follows:

[0014] Since the medium water treated by the corrosion inhibition 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 needs 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] No phenomenon of microbial corrosion has been found in the system using the corrosion inhibitor of the present invention. Description of the Drawings

[0019] Figure 1 It is a schematic diagram of a typical equipment cooling water system for a nuclear power plant.

[0020] In the figure: 1, high-level water tank; 2, chemical dosing tank; 3, user heat exchanger; 4, circulating water pump; 5, resin purification bed; 6, heat sink heat exchanger; 7, discharge port. Detailed Embodiments

[0021] The present invention will be described in detail below in conjunction with the drawings and specific embodiments.

[0022] A typical equipment cooling water system for a nuclear power plant is as Figure 1 shown. 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 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 circulating water pump 4. The system pressure is maintained by the high-level water tank 1, and the loss of a small amount of leaked filling is maintained by replenishing desalted 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. The heat sink heat exchanger 6 is provided with a discharge port 7.

[0023] 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 in the energy industry. A method of covering the high-level water tank 1 with nitrogen and slowly oxidizing the metal material during operation to consume oxygen is adopted to control the dissolved oxygen content in the cooling water to be less than 50 μg / kg. For the specific water quality technical conditions requirements, see Table 1.

[0024] Table 1 Water Quality Technical Conditions of the Equipment Cooling Water System with Corrosion Inhibition Treatment of the Present Invention

[0025]

[0026] The technical verification schemes for implementing the present invention are as follows:

[0027] Laboratory tests confirm 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. The total iron, an index for measuring the degree of corrosion in the actual system operation, is only a few μg / kg (see Table 5), which is better than the levels of dozens or hundreds of μg / kg when other corrosion inhibition formulations are used in domestic power plants.

[0028] Table 2 Corrosion Rates of Carbon Steel and Cupronickel under Different pH Values (25 °C)

[0029]

[0030] Laboratory tests confirmed the discharge indicators of the system medium water treated with corrosion inhibitor: Under the conditions that the pH values of the medium water treated with corrosion inhibitor 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 make the medium water 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 being directly qualified, other indicators meet the discharge requirements of the national standard and the local standard of Zhejiang Province.

[0031] 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

[0032]

[0033] Table 4 Contents of various pollutants in the direct discharge of the system water during the corrosion inhibitor treatment of the present invention

[0034]

[0035] On-site operation tests showed that the indicators such as chlorine, fluorine, and sulfate in the system water quality 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.

[0036] Table 5 Detection results of the on-site operation system water quality (the statistical period is 2 years)

[0037]

[0038] The key points and points to be protected in the present invention are in the equipment cooling water system of nuclear power plants:

[0039] 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 and 10.5. The performance of the lithium-type mixed resin used meets the requirements of Table 6.

[0040] 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%.

[0041] The method of using nitrogen gas coverage and the slow reaction of the metal in the system pipeline with oxygen is adopted to reduce the dissolved oxygen content in the equipment cooling water to less than 50 μg / kg.

[0042] 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. Ions such as chlorine, fluorine, and sulfate ions are far better than the limit value of 150 μg / kg of the industry standard.

[0043] Or change the lithium-type resin in the purification bed to hydrogen-type and then transform it into a lithium-type resin purification bed 5 for operation on-site by adding lithium hydroxide to the system medium. The performance of the adopted hydrogen-type mixed resin meets the requirements of 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%.

[0044] Other treatment methods using lithium hydroxide as a corrosion inhibitor in the demineralized water medium all fall within the scope of the present invention.

[0045] Table 6 Performance requirements for the mixed resin of lithium-type and hydrogen-type purification beds

[0046]

[0047] The lithium-type cation resin includes ZGCNR507Li lithium-type cation resin, and the hydroxide-type anion resin includes ZGANR170 hydroxide-type anion resin.

[0048] Typical equipment cooling water systems in nuclear power plants are 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 pipelines. 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 of the installation due to a small amount of leakage is maintained by supplementing 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 gas replenishment / exhaust pipeline is connected to the high-level water tank 1, which are all shown by dotted lines in the figure.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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 the corrosion prevention of metal materials in the equipment cooling water system of a nuclear power plant, characterized in that: 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 is adopted to control the pH value of the cooling water between 9.0 and 10.

5. At the same time, impurities are removed by relying on 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. A method of covering the high-level water tank with nitrogen and slowly oxidizing and consuming oxygen by metal materials during the operation process is used to control the dissolved oxygen content in the cooling water to be less than 50 μg / kg.

2. The treatment method for preventing corrosion of metal materials in the equipment cooling water system of a nuclear power plant according to claim 1, characterized in that: 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 gas space of the high-level water tank.

3. The treatment method for preventing corrosion of metal materials in the equipment cooling water system of a nuclear power plant according to claim 1, characterized in that: 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 ion is ≤50 μg / kg; the control range of fluoride ion is ≤50 μg / kg; the control range of sulfate ion is ≤50 μg / kg.

4. The treatment method for preventing corrosion of metal materials in the equipment cooling water system of a nuclear power plant according to claim 1, characterized in that: If the lithium-type cation resin in the resin purification bed is changed to hydrogen-type, the pH value requirement of the equipment cooling water system is maintained by adding lithium hydroxide in the initial stage.

5. The method for anti-corrosion treatment of metal materials in the equipment cooling water system of a nuclear power plant according to claim 4, characterized in that: A hydrogen-type mixed resin purification bed is adopted and it is transformed into a lithium-type mixed resin purification bed for operation by adding lithium hydroxide to the system medium in the initial stage.

6. The method for preventing corrosion of metal materials in the equipment cooling water system of a nuclear power plant 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 anti-corrosion treatment of metal materials in the equipment cooling water system of a nuclear power plant according to claim 1, characterized in that: Lithium hydroxide is used as a corrosion inhibitor in the demineralized water medium.

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