A control method for removing boron from the primary coolant of a nuclear power plant at the end of its life

Abstract

The present invention belongs to the field of primary circuit water quality control in nuclear power plants, and specifically relates to a control method for end-of-life boron removal of the primary circuit coolant in nuclear power plants. When the resin in the anion resin exchanger is used at the end of its first fuel cycle, the boric acid concentration is reduced to within the range of 150mg / L to 60mg / L, and the anion exchanger is put into operation alone. When the resin in the anion resin exchanger is used at the end of its second fuel cycle, the boric acid concentration is reduced to within the range of 120mg / L to 60mg / L, and the anion exchanger is put into operation alone. When the boric acid concentration is reduced to less than 60mg / L, the cation exchanger and the anion exchanger are put into operation together. When the total alkali metal or dissolved hydrogen concentration in the primary circuit is at the lower limit of the control value, the cation exchanger is temporarily taken out of operation and resumed after the water quality stabilizes. The present invention can effectively prevent water quality deviations and reduce the amount of radioactive resin solid waste in the primary circuit.

Description

Technical Field

[0001] The invention belongs to the field of primary circuit water quality control in nuclear power plants, and in particular relates to a control method for end-of-life boron removal of primary circuit coolant in nuclear power plants. Background Art

[0002] During normal operation, the pressurized water reactor nuclear power plant uses boric acid to control the core reactivity. 10 The abundance of B is 19.78%, and it has a large neutron absorption cross section (3837dB), which reacts with neutrons. 10 B(n,α) 7 The Li reaction is distributed throughout the primary circuit as the main pump circulates. During the early and middle stages of the fuel cycle, the coolant's boric acid concentration is high. To compensate for burnup, the coolant's boron concentration is typically reduced by filling and draining the coolant. However, at the end of the fuel cycle, this adjustment method produces large amounts of wastewater, and the boron concentration decreases slowly, insufficient to maintain core reactivity and causing fluctuations in reactor power. Nuclear power plants use an OH-type (i.e., negative) ion exchange resin bed to remove boron from the primary coolant. When the boric acid concentration in the coolant meets reactor operating requirements, the bed is deactivated.

[0003] In actual operation, it was found that there were problems when the anion exchange resin bed was put into operation at the end of its service life to remove the boric acid in the primary circuit:

[0004] 1. The boric acid concentration does not decrease in accordance with the theoretical value, which can easily lead to fluctuations in the core reactivity and increase the amount of water exchange in the primary circuit, thus generating more radioactive wastewater.

[0005] 2. The resin exchange capacity in the anion resin exchanger has not been exhausted. Premature replacement will bring economic losses and increase radioactive solid waste.

[0006] In order to improve the boron removal efficiency of anion resin exchangers during operation and reduce the generation of radioactive wastewater and radioactive solid waste, a control method for end-of-life boron removal of the primary coolant in nuclear power plants was proposed. Summary of the Invention

[0007] The present invention aims to provide a control method for end-of-life boron removal from the primary coolant circuit of a nuclear power plant. The method, by providing two ion exchangers, a cation exchanger and an anion exchanger, safely and stably controls the reactivity of the primary coolant circuit, avoids water quality deviation, reduces the amount of radioactive resin solid waste in the primary coolant circuit, and improves the water chemistry control level and economic benefits of the power plant.

[0008] The technical solution for achieving the purpose of the present invention is as follows:

[0009] A control method for removing boron from a primary coolant in a nuclear power plant at the end of its life cycle, the method comprising:

[0010] Step 1: Load cation exchange resins and anion exchange resins in the cation exchanger and anion exchanger respectively. After the resins are loaded, rinse them with deionized water and make sure they meet the requirements. Then close the inlet and outlet valves and put them in standby mode.

[0011] Step 2: When the resin in the anion exchanger has reached the end of its first fuel cycle and the boric acid concentration in the primary coolant has dropped to between 150 mg / L and 60 mg / L, the anion exchanger is flushed and qualified, and then the anion exchanger is put into operation alone to remove the boric acid in the primary circuit. The operating flow rate is calculated according to the operating flow rate formula.

[0012] Step 3: When the resin in the anion exchanger reaches the end of its second fuel cycle, the boric acid concentration in the primary coolant drops to between 120 mg / L and 60 mg / L, and the anion exchanger passes flushing, the anion exchanger is put into operation separately, with the operating flow rate calculated according to the operating flow rate formula.

[0013] Step 4: When the boric acid concentration in the primary coolant drops to less than 60 mg / L, flush the cation exchanger and put it into operation together with the anion exchanger. The operation flow rate is calculated according to the operation flow rate formula. When the total alkali metal or dissolved hydrogen concentration in the primary circuit is at the lower limit of the control value, temporarily shut down the cation exchanger and resume operation after the water quality stabilizes.

[0014] The operational flow formula is:

[0015]

[0016] Where ρ1 is the boric acid concentration and X is the operating flow rate.

[0017] The method further comprises:

[0018] Step 5: When the cation exchanger and anion exchanger are put into operation together and the reactor power still cannot be stabilized, reduce the ammonia concentration while meeting the dissolved hydrogen control requirements; reduce the flow rate through the bed of the primary coolant purification system KBE while meeting the purification requirements until the reactor power is stabilized.

[0019] The method further comprises:

[0020] Step 6: At the end of the second fuel cycle, after the anion exchanger is put into operation, the resin is unloaded. After the cation exchanger's exchange capacity is exhausted, the resin is unloaded and reloaded. After the deionized water is rinsed and qualified, it is placed in standby mode.

[0021] The flushing method of the anion exchanger in step 2 and step 3 is: a boric acid solution with the same concentration as the boric acid in the main circuit is prepared in the anion exchanger, and after the anion exchanger is flushed and qualified, the anion exchanger is put into operation alone.

[0022] The cation exchanger is flushed in step 4 as follows: a boric acid solution having the same concentration as that of the main circuit is prepared in the cation exchanger. After the cation exchanger is flushed and qualified, the cation exchanger and the anion exchanger are put into operation together.

[0023] The beneficial technical effects of the present invention are:

[0024] 1. The present invention proposes and solves the problem that the boric acid concentration in the primary circuit of a nuclear power plant at the end of its service life does not conform to the theoretical decrease through theoretical analysis and experimental verification.

[0025] 2. The present invention optimizes the method of controlling the boric acid concentration at the end of life, while maintaining the original system design functional requirements, it also improves the economy of system operation.

[0026] 3. The present invention improves the boron removal efficiency of anion resin and reduces the amount of radioactive solid waste and radioactive effluent emissions.

[0027] 4. The combined operation of anionic and cationic resins of the present invention to remove boric acid in the primary circuit can reduce the opening of the downstream flow, reduce the loss of dissolved hydrogen in the primary circuit, and ensure the safety of the water quality in the primary circuit.

[0028] 5. The present invention proposes the timing of commissioning an anion exchanger to remove boric acid at the end of two fuel cycle lives. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a control process flow chart for end-of-life boron removal of a primary-loop coolant in a nuclear power plant provided by the present invention;

[0030] In the figure: 1-reactor pressure vessel; 2-steam generator; 3-main pump; 4-primary circuit purification system KBE; 5-degasser; 6-coolant storage system cation exchange bed KBB10AT001; 7-coolant storage system anion exchange bed KBB10AT002. DETAILED DESCRIPTION

[0031] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0032] In a control method for end-of-life boron removal of a primary coolant in a nuclear power plant provided by the present invention, the coolant storage system includes a cation exchanger and an anion exchanger, specifically as follows:

[0033] The coolant storage system (KBB) consists of two ion exchangers. The cation exchanger (KBB10AT001) is loaded with H-type cation resin. When the total alkali metal concentration (Li + / Na + / K +) exceeds the standard, part of the primary circuit coolant is introduced into the cation exchanger, and the H groups in the cationic resin exchange reaction with the alkali metal ions in the primary circuit coolant. The primary circuit coolant with the alkali metal ions removed is returned to the primary circuit by the charging pump, so as to maintain the total alkali metal concentration of the primary circuit and maintain the water chemical condition of the primary circuit.

[0034] The anion exchanger (KBB10AT002) is loaded with OH-type anion resin, which can exchange with borate ions in the primary coolant and is used to remove boron at the end of the fuel life. When the boric acid concentration in the primary coolant is low, diluting the boric acid by exchanging water is not very effective in reducing the boron concentration, and it will produce too much waste liquid, increasing the burden on the three-waste system. In order to maintain the reactor power, ion exchange is used to remove boron. Part of the primary coolant is introduced into the anion exchanger, and after the borate ions are removed, it is returned to the primary coolant by the charging pump to maintain the stability of the reactor power.

[0035] Both ion exchangers are equipped with drain lines, exhaust lines, compressed air and chemical demineralized water supply lines, hydraulic flushing drain lines, chemical sampling lines, and hydraulic unloading drain lines.

[0036] In the control method for removing boron from the primary coolant of a nuclear power plant at the end of its life provided by the present invention, the boron removal analysis is specifically as follows:

[0037] Boric acid is a monoprotic weak acid with an ionization constant K a =5.8×10 -10 In aqueous solution, it mainly contains orthoboric acid H3BO3, monoborate B(OH)4 - , triborate B3O3(OH)4 - , tetraborate B4O3(OH)4 2- The composition is relatively complex and is closely related to the concentration of boric acid. When a nuclear power plant reactor in China is operating at full power, the pH of the primary coolant is adjusted by adding KOH, and the added ammonia is decomposed by irradiation to produce hydrogen, maintaining the primary coolant in a weakly alkaline reducing water chemical condition. 300℃ Maintained within the range of 7.0-7.2. An alkaline environment will inevitably affect the ionization balance of boric acid, especially at the end of the fuel cycle, when the anion exchanger is put into operation. As the pH increases, the resin's absorption efficiency of boric acid will decrease. In order to improve the efficiency of anion resin in removing boric acid, it is necessary to appropriately adjust the pH value of the purified coolant.

[0038] (1) The anion exchanger KBB10AT002 in the coolant storage system has low boron removal efficiency

[0039] During normal reactor operation, according to theoretical calculations, the boric acid concentration in the primary circuit decreases at a rate of 22 mg / L per day. At the end of its life, to maintain a stable reactor power, the anion exchanger KBB10AT002 needs to be put into the primary circuit at a flow rate of X kg / s to remove boron. Assuming the current boric acid concentration is ρ1, the downstream flow rate is 0.6 kg / s, and the main pump leakage monitoring flow rate is 0.8 kg / s according to operating procedures, the calculation formula for the operation flow rate X of the anion exchanger KBB10AT002 is:

[0040] Right now Formula (1)

[0041] At the end of a certain fuel cycle, the boric acid concentration in the primary loop was 150 mg / L. To maintain reactor power, the anion exchanger KBB10AT002 was connected to the primary loop for boron removal at a flow rate of 1.5 kg / s according to the calculation formula. During actual operation, the flow rate of the anion exchanger KBB10AT002 was finally adjusted to 4 kg / s to maintain power. The next morning, the flow rate of the anion exchanger KBB10AT002 was adjusted to 5.2 kg / s, and the downstream flow rate was adjusted to 1.5 kg / s to maintain power. Both the boron removal flow rate and the downstream flow rate were greater than the theoretically calculated values.

[0042] Moreover, due to the requirement of hydrogen reduction at the end of service life, the dissolved hydrogen in the first circuit is only 2.3 mg / L at this time. Continuously opening the downstream flow can easily cause the dissolved hydrogen to fall below the control value of 2.2 mg / L. The downstream flow should be reduced as much as possible.

[0043] In order to find out the reason for the low boron removal efficiency of the anion exchanger KBB10AT002, the boric acid concentrations at the primary circuit and the outlet of the anion exchanger KBB10AT002 were statistically analyzed. The data are as follows:

[0044] Table 1 Boric acid concentration at the outlet of the primary circuit and KBB10AT002 anion bed and pH of the primary circuit 25℃

[0045]

[0046] As can be seen from Table 1, when the OH-type anion resin bed is put into operation to remove boric acid in the primary circuit, boric acid is present at the bed outlet, and the ratio of boric acid at the bed outlet to boric acid in the primary circuit becomes higher and higher. This may be related to the gradual increase in pH in the primary circuit. The increase in pH reduces the capacity of the anion resin to exchange boric acid, resulting in a decrease in the efficiency of the anion resin in removing boric acid. In addition, since the boric acid value in the primary circuit is very low, the boric acid at the bed outlet cannot be ignored when calculating purification at the end of life.

[0047] In order to improve the removal efficiency of boric acid by the anion resin bed, a cation resin exchanger was selected for operation at the same time. The boric acid concentration at the outlet of the anion resin bed at the end of a certain life span was tracked. The results are shown in Table 2.

[0048] Table 2 Boric acid concentration at the outlet of the anion exchange bed when the anion and cation resin beds are put into operation at the same time, mg / L

[0049]

[0050] As can be seen from Table 2, when the anion and cation resin exchangers are put into operation together, the boric acid in the primary circuit is removed while the total alkali metal in the primary circuit is removed, that is, the pH value is lowered, and the boric acid removal efficiency of the anion resin for the primary circuit is effectively improved, especially after the boric acid concentration in the primary circuit coolant is less than 60 mg / L.

[0051] (2) KBE anionic resin boric acid stripping belt of primary coolant purification system

[0052] The primary coolant purification system (KBE) continuously purifies the impurity ions in the primary coolant at a flow rate of 8.3 kg / s. The OH type anion resin in the purification system is saturated with boric acid at the initial startup and becomes a boric acid saturated resin.

[0053] In an alkaline environment, boric acid reacts to form a complex B(OH)4 - 、B3(OH) 10 - 、B2(OH)7 - 、B4(OH) 14 2- Etc., the reaction formula is as follows:

[0054] B(OH)3+OH - =B(OH)4 -

[0055] 2B(OH)3+OH - =B2(OH)7 -

[0056] 3B(OH)3+OH - =B3(OH) 10 -

[0057] 4B(OH)3+OH - =B2(OH) 14 2-

[0058] As the reactor operates, the boric acid concentration in the primary circuit gradually decreases, and when the boron removal anion resin bed is put into operation, the boric acid concentration in the primary circuit further decreases. The above reaction proceeds in the direction of producing orthoboric acid (B(OH)3), and the boric acid adsorbed in the anion resin bed of the primary coolant purification system (KBE) is all polyborate ions. When the coolant passes through the anion resin bed, the resin bed will release boric acid into the primary circuit, that is, boric acid is discharged.

[0059] To explain the phenomenon of boric acid discharge from boric acid-saturated anion resins, samples were taken from the primary circuit and the outlet and inlet of the coolant storage system anion exchanger KBB10AT002 when the coolant storage system anion exchanger KBB10AT002 was put into operation for analysis of boric acid concentrations. The boric acid concentration at the inlet of the coolant storage system anion exchanger KBB10AT002 was calculated according to formula (1). The results are as follows:

[0060] Table 3 Boric acid concentrations at the inlet and outlet of the primary circuit and anionizer (unit: mg / L)

[0061]

[0062] Table 3 shows that the actual boric acid concentration at the inlet of the coolant storage system anion exchanger KBB10AT002 is higher than the theoretical concentration at the inlet, indicating that a source of boric acid is continuously being released. The inlet of the coolant storage system anion exchanger KBB10AT002 originates from the outlet of the primary coolant purification system KBE. This indicates that when the boric acid concentration in the coolant is lower than the saturated boric acid concentration of the anion exchange resin in the KBE system, boric acid carryover occurs from the boric acid-saturated anion resin. The carryover rate is correlated with the primary boric acid concentration and the boric acid concentration entering the anion resin exchanger, showing a positive correlation with the difference between the two. It is also related to the flow rate through the resin bed. Reducing the flow rate through the bed can also reduce boric acid carryover.

[0063] In the control method for end-of-life boron removal of the primary coolant in a nuclear power plant provided by the present invention, the timing of adding the coolant to the bed is selected as follows:

[0064] 1) The anion resin exchanger KBB10AT002 is filled with 2.4 m3 of nuclear-grade anion resin. The purchase price per cubic meter is approximately RMB 100,000, and the cost of treating each cubic meter of radioactive solid waste is approximately RMB 40,000. If the service life of the anion resin is extended to two fuel cycles, the cost of purchasing and treating the resin can be saved by approximately RMB 168,000 (10 × 1.2 + 4 × 1.2 = 168,000) per fuel cycle.

[0065] 2) After reviewing the operating curve and historical data of the reactor power, when the boric acid concentration in the primary coolant loop is greater than 100 mg / L, water replacement can still maintain reactivity and ensure power stability. Based on statistical historical data and the working exchange capacity of anion exchange resin for boric acid, when the anion exchanger is used for the first fuel cycle, the anion exchanger KBB10AT002 is put into operation to remove boron when the primary boric acid concentration is less than 150 mg / L. When the anion exchanger is used for the second fuel cycle, the primary boric acid concentration is less than 120 mg / L to remove boron, which can maintain core reactivity and ensure that the reactor power does not fluctuate.

[0066] The present invention provides a control method for removing boron from a primary circuit coolant in a nuclear power plant at the end of its life, which specifically comprises the following steps:

[0067] Step 1: Load cation exchange resins and anion exchange resins into the cation exchanger KBB10AT001 and the anion exchanger KBB10AT002 respectively. After the resins are loaded, rinse them with deionized water until they pass the test, close the inlet and outlet valves, and place them in standby mode.

[0068] Step 2: When the resin in the anion exchanger has reached the end of its first fuel cycle life and the boric acid concentration in the primary coolant has dropped to a range of 150 mg / L to 60 mg / L, a boric acid solution with the same concentration as that in the primary circuit is prepared in the anion exchanger KBB10AT002. After the anion exchanger has been flushed and qualified, the anion exchanger KBB10AT002 is put into operation alone to remove the boric acid in the primary circuit. The operating flow rate is calculated according to the operating flow rate formula (Formula (1)).

[0069] Step 3: When the resin in the anion exchanger is used at the end of its second fuel cycle, the boric acid concentration in the primary coolant is reduced to a range of 120 mg / L to 60 mg / L. A boric acid solution with the same concentration as that in the primary circuit is prepared in the anion exchanger KBB10AT002. After the anion exchanger is flushed and qualified, the anion exchanger KBB10AT002 is put into operation alone. The operation flow rate is calculated according to the operation flow rate formula (Formula (1)).

[0070] Step 4: When the boric acid concentration in the primary coolant drops to less than 60 mg / L, a boric acid solution with the same boric acid concentration as that in the main circuit is prepared in the cation exchanger KBB10AT001. After the cation exchanger KBB10AT001 is flushed and qualified, it is put into operation together with the anion exchanger (KBB10AT002). The operation flow rate is calculated according to the operation flow rate formula (Formula (1)). When the total alkali metal or dissolved hydrogen concentration in the primary circuit is at the lower limit of the control value, the cation exchanger KBB10AT001 is temporarily shut down and resumed after the water quality stabilizes.

[0071] Step 5: When the cation exchanger (KBB10AT001) and the anion exchanger (KBB10AT002) are put into operation together and the reactor power still cannot be stabilized, reduce the ammonia concentration while meeting the dissolved hydrogen control requirements; and reduce the flow rate through the bed of the primary coolant purification system KBE while meeting the purification requirements until the reactor power is stabilized.

[0072] Step 6: At the end of the second fuel cycle, after the anion exchanger KBB10AT002 is operational, unload the resin. Once the cation exchanger KBB10AT001 reaches its full capacity, unload the resin, reload the resin, rinse with deionized water, and place it in standby mode. Repeat Steps 2 to 6.

[0073] The method of the present invention is used to optimize the boric acid concentration of the primary circuit of the anion exchanger when it is put into operation, thereby reducing the use of KBB10AT002 resin, increasing the replacement frequency of the anion resin exchanger, and extending the service life of KBB10AT002 to twice its original value.

[0074] Reduced the amount of radioactive solid waste generated by nuclear power plants, saving approximately RMB 120,000 in resin procurement costs and RMB 48,000 in radioactive solid waste disposal costs per fuel cycle;

[0075] The boron removal efficiency of the anion exchange resin at the end of its life cycle is improved, the amount of water exchange in the first circuit is reduced, and thus the generation of radioactive effluents is reduced, providing a method for safely and stably controlling the core reactivity at the end of its life cycle.

[0076] Table 1 Comparison of the amount of resin used for boron removal, radioactive wastewater and solid waste generated per fuel cycle

[0077]

[0078] The present invention has been described in detail above with reference to the accompanying drawings and embodiments. However, the present invention is not limited to the above embodiments. Various modifications can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention. Any content not described in detail in the present invention may be adapted from existing technologies.

Claims

1. A control method for removing boron from the primary coolant of a nuclear power plant at the end of its life, characterized in that: The method comprises: Step 1: Load cation exchange resins and anion exchange resins in the cation exchanger and anion exchanger respectively. After the resins are loaded, rinse them with deionized water and make sure they meet the requirements. Then close the inlet and outlet valves and put them in standby mode. Step 2: When the resin in the anion exchanger has reached the end of its first fuel cycle and the boric acid concentration in the primary coolant has dropped to between 150 mg / L and 60 mg / L, the anion exchanger is flushed and qualified, and then the anion exchanger is put into operation alone to remove the boric acid in the primary circuit. The operating flow rate is calculated according to the operating flow rate formula. Step 3: When the resin in the anion exchanger reaches the end of its second fuel cycle, the boric acid concentration in the primary coolant drops to between 120 mg / L and 60 mg / L, and the anion exchanger passes flushing, the anion exchanger is put into operation separately, with the operating flow rate calculated according to the operating flow rate formula. Step 4: When the boric acid concentration in the primary coolant drops to less than 60 mg / L, flush the cation exchanger and put it into operation together with the anion exchanger. The operation flow rate is calculated according to the operation flow rate formula. When the total alkali metal or dissolved hydrogen concentration in the primary circuit is at the lower limit of the control value, temporarily shut down the cation exchanger and resume operation after the water quality stabilizes. The operational flow formula is: , where ρ1 is the boric acid concentration and X is the operating flow rate.

2. A control method for removing boron from the primary coolant of a nuclear power plant at the end of its life according to claim 1, characterized in that: The method further comprises: Step 5: When the cation exchanger and anion exchanger are put into operation together and the reactor power still cannot be stabilized, reduce the ammonia concentration while meeting the dissolved hydrogen control requirements; reduce the flow rate through the bed of the primary coolant purification system KBE while meeting the purification requirements until the reactor power is stabilized.

3. A control method for removing boron from the primary coolant of a nuclear power plant at the end of its life according to claim 2, characterized in that: The method further comprises: Step 6: At the end of the second fuel cycle, after the anion exchanger is put into operation, the resin is unloaded. After the cation exchanger's exchange capacity is exhausted, the resin is unloaded and reloaded. After the deionized water is rinsed and qualified, it is placed in standby mode.

4. A control method for removing boron from the primary coolant of a nuclear power plant at the end of its life according to claim 3, characterized in that: The flushing method of the anion exchanger in step 2 and step 3 is: a boric acid solution with the same concentration as the boric acid in the main circuit is prepared in the anion exchanger, and after the anion exchanger is flushed and qualified, the anion exchanger is put into operation alone.

5. A control method for removing boron from the primary coolant of a nuclear power plant at the end of its life according to claim 4, characterized in that: The cation exchanger is flushed in step 4 as follows: a boric acid solution having the same concentration as that of the main circuit is prepared in the cation exchanger, and after the cation exchanger is flushed and qualified, the cation exchanger and the anion exchanger are put into operation together.

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