Chemical control method for primary circuit deaeration during overhaul start-up of nuclear power plant

Abstract

The present application relates to the technical field of chemical technology of pressurized water reactor nuclear power plant, and particularly relates to a chemical control method for primary loop deoxygenation during the overhaul start of a nuclear power plant, which comprises the following steps: step one, confirming that the primary loop meets the conditions for adding hydrazine; step two, adding hydrazine solution to the primary loop reactor coolant for the first time; step three, raising the temperature of the main system, and after sufficient reaction, checking whether the liquid-phase dissolved oxygen content of the main system loop and the pressurizer is qualified for deoxygenation, if not, executing step four of adding hydrazine; step four, adding hydrazine solution to the primary loop reactor coolant for the second time; step five, after sufficient reaction, checking again whether the liquid-phase dissolved oxygen content of the main system loop and the pressurizer is qualified for deoxygenation, if not, repeating step four until the liquid-phase dissolved oxygen content is qualified; and step six, performing sweep blowing on the main system and putting into operation the ion-exchange bed. The method optimizes the chemical control method of the deoxygenation platform, effectively improves the deoxygenation effect, and proposes a control formula for the amount of hydrazine addition, thereby reducing the number of hydrazine addition.

Description

Technical Field

[0001] This invention relates to the field of chemical technology for pressurized water reactor nuclear power plants, and in particular to a chemical control method for deoxygenation of the primary loop during the start-up of a nuclear power plant overhaul. Background Technology

[0002] During nuclear power plant overhauls, the reactor coolant contains high levels of dissolved oxygen due to the openness of the main system to the outside environment and extensive contact with air. Dissolved oxygen, in high-temperature environments, severely corrodes equipment and shortens its lifespan. Therefore, deoxygenation of the primary coolant is necessary during reactor startup. Hydrazine (chemical formula N₂H₄) has strong reducing properties; it reacts with oxygen in water to produce water and nitrogen without increasing the salinity of the water. It is used in pressurized water reactor nuclear power plants for chemical deoxygenation of the primary coolant. Therefore, during nuclear power plant overhauls, after the reactor coolant reaches a certain temperature, a certain amount of hydrazine is added to achieve primary coolant deoxygenation through the reaction of hydrazine with oxygen.

[0003] During the chemical deoxygenation platform phase of a nuclear power plant overhaul, if too little hydrazine is added, it needs to be added multiple times, and after each addition, the oxygen concentration in the main system must be stabilized before deciding on the amount of hydrazine to add. If too much is added, the oxygen concentration in the main system can be quickly reduced to a qualified level, but the reactor coolant temperature will rise, causing hydrazine to decompose into ammonia (chemical formula NH3). This results in a large amount of ammonia and hydrazine residue in the main system. Operators need to continuously purge the pressurizer vapor phase and the volumetric control box (equipment number RCV002BA) with nitrogen to purge the ammonia and hydrazine to the ventilation system of the nuclear auxiliary building to reduce the concentration of ammonia and hydrazine until the sum of the ammonia and hydrazine concentrations is less than or equal to 1.0 mg / kg. Only then can the chemical and volumetric control system (referred to as RCV) purification and desalination bed be put into operation. Therefore, adding too much or too little hydrazine greatly affects the progress of the overhaul. In existing technologies, the reaction between hydrazine and oxygen is very complex. The reaction rate depends on factors such as temperature, pH, hydrazine concentration, catalyst, and reaction time. In addition, the temperature control rate, purging control, and chemical measurements will inevitably vary each time oxygenation is performed. Therefore, it is difficult to achieve successful oxygenation in one go by simply relying on formula calculations. There is no truly suitable hydrazine addition scheme yet. Summary of the Invention

[0004] This invention provides a chemical control method for primary loop deoxygenation during the startup of a nuclear power plant overhaul, which solves the problems in the prior art of difficult-to-control hydrazine addition dosage in the primary loop deoxygenation scheme during the startup of a nuclear power plant overhaul, excessive hydrazine addition, and long time and high labor costs for subsequent cleaning of ammonia and hydrazine residues.

[0005] The technical solution of the present invention is as follows:

[0006] This invention proposes a chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant overhaul, the method comprising:

[0007] Step 1: Confirm that the primary circuit meets the conditions for adding hydrazine;

[0008] Step 2: Add hydrazine solution to the primary coolant for the first time. The calculation method for the volume of the first addition of hydrazine solution is as shown in formula (1).

[0009] V1 = 1.9 × M (1)

[0010] Where V1 is the volume of hydrazine to be added at a concentration of 50%; M is the dissolved oxygen concentration of the main system before addition;

[0011] Step 3: Increase the temperature of the main system. After the reaction is complete, check whether the dissolved oxygen content in the liquid phase of the main system loop and the voltage regulator is qualified for deoxygenation. If it is not qualified, proceed to step 4 to add hydrazine.

[0012] Step 4: Add hydrazine solution to the primary coolant circuit again. The volume of hydrazine solution added is calculated using formula (2).

[0013] V2=A×Y (2)

[0014] Where V2 is the volume of hydrazine to be added (50%), Y is the dissolved oxygen concentration of the main system before addition, and A is the addition coefficient; if the dissolved oxygen content in the liquid phase of step three is less than 0.4 mg / kg, A = 3.2; if the dissolved oxygen content in the liquid phase of step three is greater than or equal to 0.4 mg / kg, the value of A should be reduced.

[0015] Step 5: After the reaction is complete, check again whether the dissolved oxygen content in the liquid phase of the main system circuit and the voltage regulator is qualified for deoxygenation. If it is not qualified, repeat step 4 until the dissolved oxygen content in the liquid phase is qualified.

[0016] Step 6: Sweep and purge the main system and put the desalination bed into operation.

[0017] In some embodiments, confirming that the primary loop meets the conditions for hydrazine charging in step one specifically means that: the reactor coolant system is in a cold shutdown state, and the coolant temperature is 80°C to 120°C; the control box is sealed with nitrogen; the chemical and volume control system No. 1 and No. 2 mixed bed desalination devices and the boron recovery system mixed bed desalination device purification bed bypass are in operation; the main system pressurizer heater and main pump are in operation; and the pressurizer spray is fully activated.

[0018] In some embodiments, the standard for determining whether the dissolved oxygen content in the main system circuit and the voltage regulator is qualified in step two is specifically whether the dissolved oxygen content in the liquid phase is less than 100 μg / kg; if the dissolved oxygen content in the liquid phase is less than 100 μg / kg, then the chemical deoxygenation is qualified; if the dissolved oxygen content in the liquid phase is greater than or equal to 100 μg / kg, then hydrazine needs to be added again.

[0019] In some embodiments, step three raises the main system temperature to 110 degrees Celsius.

[0020] In some embodiments, if the dissolved oxygen content in the liquid phase in step three is greater than or equal to 0.4 mg / kg, then the value of A is 2.5.

[0021] In some embodiments, step five involves checking whether the dissolved oxygen content in the main system circuit and the voltage regulator is qualified for deoxygenation, specifically whether the dissolved oxygen content in the liquid phase is less than 100 μg / kg.

[0022] In some embodiments, step six involves purging the main system and putting the desalination bed into operation. Specifically, after the dissolved oxygen content in step five is qualified, the mixed bed desalination device of the boron recovery system is put into operation directly. After the concentrations of ammonia and hydrazine meet the requirements, the mixed bed desalination device of the chemical and volumetric control system is put into operation.

[0023] In some embodiments, the concentrations of ammonia and hydrazine meet the requirements, specifically, the sum of the concentrations of ammonia and hydrazine does not exceed 1.0 mg / kg.

[0024] In some embodiments, the chemical and volumetric control system mixed bed desalination unit is put into operation, specifically including the operation of chemical and volumetric control system No. 1 mixed bed desalination unit or chemical and volumetric control system No. 2 mixed bed desalination unit.

[0025] The implementation of this invention has the following beneficial effects:

[0026] The advantages of this invention compared to the prior art are:

[0027] 1) This invention proposes a chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant overhaul. This method optimizes the chemical control method of the deoxygenation platform and effectively improves the deoxygenation effect. This method also proposes a control formula for the amount of hydrazine added, which reduces the number of times hydrazine is added.

[0028] 2) This invention proposes a chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant overhaul. This method optimizes the desalination bed commissioning control strategy after hydrazine addition, significantly shortens the residence time of the chemical deoxygenation process, and extends the service life of the RCV mixed bed resin, resulting in extremely high economic benefits. This invention is simple to operate, highly applicable, and can be widely promoted and used. Attached Figure Description

[0029] Figure 1 This is a flowchart of a chemical control method for primary loop deoxygenation during the startup of a nuclear power plant overhaul, as proposed in an embodiment of the present invention. Detailed Implementation

[0030] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] like Figure 1 As shown, this invention proposes a chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant overhaul, the method comprising:

[0032] Step 1: Determine if the primary circuit meets the conditions for hydrazine injection, including:

[0033] 1. The reactor coolant system (RCP) is in a cold shutdown state, with the coolant temperature ranging from 80°C to 120°C;

[0034] 2. The capacity control box RCV002BA is sealed with nitrogen.

[0035] 3. The No. 1 and No. 2 mixed bed desalination units (RCV001 and RCV002DE) of the chemical and volume control system have been bypassed, and the purified bed of the mixed bed desalination unit (TEP006DE) of the boron recovery system has been bypassed.

[0036] 4. The main system voltage regulator heater is in operation;

[0037] 5. Operators should increase the pressure stabilizer spray as much as possible to ensure the uniformity of water in the pressure stabilizer and the main circuit.

[0038] 6. The main pump has been put into operation.

[0039] Step 2: Add hydrazine to the primary coolant circuit for the first time. The added hydrazine solution is a commercially available 50% hydrazine solution. For the first addition, add the 50% hydrazine solution to the chemical mixing tank REA006BA, and inject it into the primary main system after meeting the conditions of Step 1. The calculation method for the first hydrazine addition is as shown in formula (1).

[0040] V1 = 1.9 × M (1)

[0041] Where V1 is the volume of hydrazine with a concentration of 50% to be added, in L; M is the dissolved oxygen concentration of the main system before addition, in mg / kg.

[0042] Step 3: After the first addition of hydrazine, the temperature of the main system directly rises to nearly 110°C. After the reaction of the added hydrazine is sufficient, check the dissolved oxygen content in the liquid phase of the main system loop and the pressurizer. If the content is less than 100 μg / kg, the chemical deoxygenation is qualified. If the content is greater than or equal to 100 μg / kg, hydrazine needs to be added again.

[0043] Step 4: After the reaction of the added hydrazine is sufficient, according to the dissolved oxygen content in the liquid phase of the main system loop and the pressurizer in Step 3, add hydrazine again. If the decrease in oxygen content is slow and reaches a basic stability and Y exceeds 0.4 mg / kg, the calculation method for the second addition of hydrazine is as follows:

[0044] V2 = 2.5 × Y (2)

[0045] Where, V2 is the volume of hydrazine with a concentration of 50% to be added, in L; Y is the dissolved oxygen concentration of the main system before addition, in mg / kg.

[0046] If the decrease in oxygen content is slow and reaches a basic stability and is in the range of 100 μg / kg < Y < 0.4 mg / kg, the calculation method for the second addition of hydrazine is as follows:

[0047] V2 = 3.2 × Y (3)

[0048] Where, V2 is the volume of hydrazine with a concentration of 50% to be added, in L; Y is the dissolved oxygen concentration of the main system before addition, in mg / kg.

[0049] Step 5: After adding hydrazine again, check the dissolved oxygen content in the liquid phase of the main system loop and the pressurizer. If the oxygen content still does not meet the control specification of less than 100 μg / kg, repeat the addition amount of hydrazine in formula (2) or formula (3) until it is qualified.

[0050] Step 6: Blow the main system and put the demineralization bed into operation. After the oxygen content is qualified, purging and putting the demineralization bed into operation are required. In the prior art, after the dissolved oxygen in the liquid phase of the main system loop and the pressurizer is qualified, detect and purge until the concentration of [NH3] + [N2H4] ≤ 1.0 mg / kg, and then put the No. 1 or No. 2 mixed bed demineralizer (RCV001 or 002DE demineralization bed) of the chemical and volume control system into operation. The demineralizers used are shown in Table 1.

[0051] Table 1 Demineralizer Information Table

[0052]

[0053] This invention employs a method where, after the dissolved oxygen level meets the requirements, the TEP006DE mixed-bed desalination unit of the boron recovery system is directly activated. Once the [NH3]+[N2H4] concentration meets the requirements, either the No. 1 or No. 2 mixed-bed desalination unit (RCV001 or 002DE desalination bed) of the chemical and volumetric control system is activated. This control method allows the TEP006DE mixed-bed desalination unit of the boron recovery system to purify excess ammonia and hydrazine quickly, meeting the requirements. Simultaneously, the TEP006DE unit can continue to purify the main system without affecting any major overhaul process, effectively saving time during critical overhaul paths. Furthermore, the hydrogen-form resin used in TEP is inexpensive, costing only a fraction of the price of the lithium-form resin used in RCV. This method significantly extends the service life of the RCV mixed-bed resin, resulting in substantial economic benefits.

[0054] The method proposed in this invention is simple to operate, highly applicable, and can effectively improve deoxygenation efficiency, reduce the number of times hydrazine is added, and significantly shorten the residence time of the chemical deoxygenation process. Table 2 below shows the deoxygenation time of a nuclear power plant unit during various major overhauls.

[0055] Table 2. Time consumed during deoxygenation process during each major overhaul of a certain nuclear power unit.

[0056]

[0057] As can be seen from the table, after adopting the hydrazine addition control formula of the present invention during the overhaul of code 106, the number of times hydrazine was added was effectively reduced, and it can now be stably controlled to within 2 times. After adopting the method of combining the hydrazine addition control formula of the present invention with the modified desalination bed operation strategy during the overhaul of code 107, compared with the previous method of adding based solely on experience and the traditional bed operation strategy, the deoxygenation time was shortened by more than 90%, which greatly reduced the time occupied by the critical deoxygenation path.

[0058] In the existing technology, operators add hydrazine based on experience (which takes a long time and involves many cycles). After the deoxygenation reaches the standard, the system is purged until the ammonia and hydrazine residue are ≤1.0mg / kg (which takes a long time, and the main system cannot be purified because the desalination device cannot be put into the main system during the purging process). Finally, the RCV001 / 002DE purification bed is put into operation.

[0059] This invention employs a hydrazine dosing formula to introduce hydrazine for deoxygenation. After deoxygenation reaches the standard, a purification method combining the operation of a boron recovery system mixed-bed desalination unit TEP006DE and purging is used to remove ammonia and hydrazine residue until (short time, while simultaneously purifying the primary loop) ≤1.0 mg / kg. Finally, the RCV001 / 002DE purification bed is activated. This invention reduces the number of hydrazine injections, reduces the time spent on deoxygenation and ammonia / hydrazine residue removal, and lowers manpower, material, and time costs, resulting in better economic benefits.

[0060] The above embodiments merely illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant during a major overhaul, characterized in that, The method includes: Step 1: Confirm that the primary circuit meets the conditions for adding hydrazine; Step 2: Add hydrazine solution to the primary coolant for the first time. The calculation method for the volume of the first addition of hydrazine solution is as shown in formula (1). V1 = 1.9 × M (1) Where V1 is the volume of hydrazine to be added at a concentration of 50%; M is the dissolved oxygen concentration of the main system before addition; Step 3: Increase the temperature of the main system. After the reaction is complete, check whether the dissolved oxygen content in the liquid phase of the main system loop and the voltage regulator is qualified for deoxygenation. If it is not qualified, proceed to step 4 to add hydrazine. Step 4: Add hydrazine solution to the primary coolant circuit again. The volume of hydrazine solution added is calculated using formula (2). V2=A×Y (2) Where V2 is the volume of hydrazine to be added (50%), Y is the dissolved oxygen concentration of the main system before addition, and A is the addition coefficient; if the dissolved oxygen content in the liquid phase of step three is less than 0.4 mg / kg, A = 3.2; if the dissolved oxygen content in the liquid phase of step three is greater than or equal to 0.4 mg / kg, the value of A should be reduced. Step 5: After the reaction is complete, check again whether the dissolved oxygen content in the main system circuit and the voltage regulator is qualified for deoxygenation. If it is not qualified, repeat step 4 until the dissolved oxygen content in the liquid phase is qualified. Step 6: Sweep and purge the main system and put the desalination bed into operation.

2. The chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant overhaul, as described in claim 1, is characterized in that... In step one, confirming that the primary loop meets the conditions for hydrazine refueling specifically means that: the reactor coolant system is in a cold shutdown state, and the coolant temperature is between 80°C and 120°C; the capacity control box is sealed with nitrogen; the chemical and volume control system No. 1 and No. 2 mixed bed desalination devices and the boron recovery system mixed bed desalination device are bypassed; the main system pressurizer heater and main pump are in operation; and all pressurizer sprayers are turned on.

3. The chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant during a major overhaul, as described in claim 2, is characterized in that... In step two, the standard for determining whether the dissolved oxygen content in the main system circuit and the voltage regulator is qualified for deoxygenation is specifically whether the dissolved oxygen content in the liquid phase is less than 100 μg / kg. If the dissolved oxygen content in the liquid phase is less than 100 μg / kg, the chemical deoxygenation is qualified. If the dissolved oxygen content in the liquid phase is greater than or equal to 100 μg / kg, hydrazine needs to be added again.

4. The chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant overhaul, as described in claim 1, is characterized in that... In step three, the temperature of the main system is increased to 110 degrees Celsius.

5. The chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant overhaul according to claim 1, characterized in that, In step four, if the dissolved oxygen content in the liquid phase of step three is greater than or equal to 0.4 mg / kg, the value of A is 2.

5.

6. The chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant overhaul according to claim 1, characterized in that, In step five, the secondary check is performed to determine whether the dissolved oxygen content in the main system circuit and the voltage regulator is qualified for deoxygenation, specifically whether the dissolved oxygen content in the liquid phase is less than 100 μg / kg.

7. The chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant overhaul according to claim 1, characterized in that, Step six involves purging the main system and putting the desalination bed into operation. Specifically, after the dissolved oxygen content in step five is qualified, the mixed bed desalination device of the boron recovery system is put into operation directly. After the ammonia and hydrazine concentrations meet the requirements, the mixed bed desalination device of the chemical and volumetric control system is put into operation.

8. The chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant during a major overhaul, as described in claim 7, is characterized in that... The ammonia and hydrazine concentrations must meet the requirements, specifically, the sum of the ammonia and hydrazine concentrations must not exceed 1.0 mg / kg.

9. A chemical control method for primary loop deoxygenation during the start-up of a nuclear power plant during a major overhaul, as described in claim 7, is characterized in that... Put into operation the mixed-bed desalination unit of the chemical and volumetric control system, specifically including putting into operation the No. 1 mixed-bed desalination unit of the chemical and volumetric control system or the No. 2 mixed-bed desalination unit of the chemical and volumetric control system.

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