Method for zinc addition to primary coolant of pressurized water reactor nuclear power plant

Abstract

The application discloses a zinc adding method for a pressurized water reactor nuclear power unit one-loop coolant, which comprises the following steps: during the zinc adding process, the zinc adding concentration is gradually increased while the zinc adding flow is kept constant. According to the zinc concentration monitoring result of the one-loop, the zinc adding method for the pressurized water reactor nuclear power unit one-loop coolant gradually increases the zinc adding amount in three stages; compared with the method of adding zinc at a high flow for the first time, the method can avoid the phenomenon that the zinc concentration of the one-loop instantaneously increases and the safety risk of fuel-related CIPS and CILC. On the premise that the zinc adding flow of the zinc injection metering pump is kept constant, the zinc adding concentration is gradually increased, and the method has the advantages of stable zinc adding flow and convenience for on-site implementation. The method can reduce the core radiation field of the unit in operation, relieve the stress corrosion cracking of the one-loop, ensure the fuel cladding integrity and avoid the safety risk of fuel-related CIPS and CILC.

Description

Technical Field

[0001] This invention belongs to the field of water chemistry technology for pressurized water reactor nuclear power plants, and relates to a method for adding zinc acetate depleted in the primary loop system of an operating pressurized water reactor nuclear power unit. Background Technology

[0002] Corrosion of primary coolant materials in pressurized water reactor (PWR) nuclear power units threatens the safe operation of the unit. The corrosion products generated during the corrosion process, upon activation and deposition, can lead to an increase in external radiation dose to the reactor core. Zinc injection technology for the primary coolant is an internationally adopted water chemistry control technology used to mitigate primary side stress corrosion cracking (PWSCC) of alloy 600 in PWR units and to reduce external radiation dose to the reactor core by replacing activated corrosion products in the metal oxide film. The principle is that after the implementation of zinc injection technology in the primary coolant of a PWR unit, zinc can replace iron, cobalt, nickel, and other elements from the original oxides and prevent cobalt from re-entering the lattice positions in the oxide film. The newly formed oxide film has a higher surface density and better corrosion resistance. Therefore, the implementation of primary coolant zinc injection technology can alleviate PWSCC of alloy 600 and reduce the amount of activated corrosion products, thereby reducing the source term dose.

[0003] The benefits generated by zinc injection in the primary loop system are related to the zinc concentration, the control method of the amount of chemical added during the zinc injection process, and the control of the zinc injection process during start-up and shutdown. Therefore, in order to ensure the operation of the units, especially those that have not implemented primary loop zinc injection technology during hot functional testing and the initial stage of commercial operation, it is necessary to determine the above technical parameters.

[0004] Chinese invention patent application number 202010548155.4 discloses a method for controlling zinc concentration in the primary loop of a nuclear power plant. The method describes the following: "During the fuel loading operation, the zinc concentration is controlled as follows: when the zinc concentration is less than the detection limit, the zinc addition rate in the primary loop is 0.001–0.002 g / (day·m² primary loop surface area); when the detection limit is less than the zinc concentration and less than 5 μg / L, the zinc addition rate in the primary loop is 0.001–0.0015 g / (day·m² primary loop surface area); when 5 μg / L ≤ zinc concentration ≤ 15 μg / L, the zinc addition rate in the primary loop is 0.0002–0.001 g / (day·m² primary loop surface area)." The zinc concentration change is divided into three stages, with the zinc addition rate gradually decreasing in each stage. For example, Chinese invention patent application number 2009801282394 discloses a method for operating a nuclear energy device, involving a method for injecting zinc into the cooling material present in the primary cooling system of a nuclear reactor. Specifically, "during the warm-state functional test after the construction of the nuclear reactor, when the concentration of zinc contained in the cooling material is lower than a predetermined value, the zinc injection rate is increased compared to before, and the increase in the zinc injection rate in the early stage of zinc injection is greater than the increase in the zinc injection rate in the later stage of zinc injection." However, the zinc concentration control method of gradually reducing the zinc injection rate in stages during the fuel loading operation in the above two patents is applicable to pressurized water reactor units that start injecting zinc into the primary circuit from the hot functional test. For operating units in China that did not undergo primary circuit zinc injection during the hot functional test before fuel loading, this method may lead to an increase in the content of corrosion products deposited on the surface of the fuel cladding and the nickel content in the primary coolant after primary circuit zinc injection, thereby leading to the risk of fuel axial power offset (CIPS) and fuel scale-induced localized corrosion (CILC). Furthermore, the flow rate and reagent concentration parameters for zinc addition during the zinc injection process of the operating unit were not disclosed.

[0005] Existing literature discloses methods for controlling the primary circuit zinc concentration during hot functional testing and fuel loading operations of newly constructed pressurized water reactor (PWR) nuclear power units, as well as the prerequisites and procedures for zinc injection. However, these methods are not applicable to the implementation of primary circuit zinc injection technology in operating PWR nuclear power units after multiple fuel cycles. The methods disclosed in existing literature do not address the issues of zinc injection operations during start-up and shutdown, or the restoration of zinc injection conditions during unit startup after shutdown or temporary shutdown of an operating unit. Summary of the Invention

[0006] In view of this, in order to overcome the shortcomings of the prior art, the object of the present invention is to provide an improved method for adding zinc to the primary coolant of a pressurized water reactor nuclear power unit.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for adding zinc to the primary coolant of a pressurized water reactor nuclear power unit includes the following steps: during the zinc addition process, the dosing flow rate is kept constant while the zinc concentration is gradually increased.

[0009] According to some preferred embodiments of the invention, the zinc addition process is divided into multiple stages, with the concentration of the zinc added remaining constant in a single stage, and the concentration of the zinc added gradually increasing from the early stage to the later stage in the multiple stages.

[0010] According to some preferred embodiments of the invention, the target concentration of zinc in the primary loop corresponding to each stage gradually increases from the early stage to the later stage.

[0011] According to some preferred embodiments of the present invention, the actual concentration of zinc in the primary circuit is monitored, and when the actual concentration of zinc is greater than the target concentration of zinc corresponding to the current stage, the next stage is initiated and the dosing concentration corresponding to the next stage is used for dosing.

[0012] According to some preferred embodiments of the invention, the dosing process is divided into three stages: the first stage corresponds to a dosing concentration of 0.605–0.610 g / kg; the second stage corresponds to a dosing concentration of 1.51–1.53 g / kg; and the third stage corresponds to a dosing concentration of 3.00–3.08 g / kg. Preferably, the first stage corresponds to a dosing concentration of 0.608 g / kg; the second stage corresponds to a dosing concentration of 1.52 g / kg; and the third stage corresponds to a dosing concentration of 3.04 g / kg. After zinc injection into the generator unit, zinc can displace iron, cobalt, and nickel from the original oxide film on the surface of the primary circuit equipment and prevent cobalt from re-entering its lattice position. The replacement of cobalt and nickel in the primary circuit oxide film by zinc may lead to an increase in the content of corrosion products deposited on the fuel cladding surface. The main risks of increased primary circuit corrosion products are the potential increase in fuel axial power offset (CIPS) and fuel scale-induced localized corrosion (CILC) risks, the increase in CIPS and CILC risks being related to the primary circuit zinc concentration. Furthermore, gradually increasing the dosage after zinc injection in operating units can avoid a sudden increase in the primary circuit zinc concentration. Therefore, the possibility of CIPS and CILC risks caused by nickel elevation following a low zinc injection concentration in operating units is lower. During primary circuit zinc injection in operating units, the zinc concentration in the primary circuit is gradually increased in stages to avoid potential CIPS and CILC risks during the zinc injection period. Based on industry experience with zinc-injected units, combined with the range of local boiling rates in domestic operating units, and the goal of reducing core radiation field dose during primary circuit zinc injection in domestic operating units, a zinc concentration of 5-15 μg / kg (zinc concentration in the primary circuit) is selected, and the dosage increase is divided into three stages based on the primary circuit zinc concentration monitoring results.

[0013] According to some preferred embodiments of the present invention, the target zinc concentration in the primary circuit corresponding to the first stage is 1.5–2.5 μg / kg; the target zinc concentration in the primary circuit corresponding to the second stage is 4.5–5.5 μg / kg; and the target zinc concentration in the primary circuit corresponding to the third stage is 9.5–10.5 μg / kg. Preferably, the target zinc concentration in the primary circuit corresponding to the first stage is 2 μg / kg; the target zinc concentration in the primary circuit corresponding to the second stage is 5 μg / kg; and the target zinc concentration in the primary circuit corresponding to the third stage is 10 μg / kg. The present invention confirms that after the primary circuit zinc injection is stabilized, the target zinc concentration in the primary circuit is 5–15 μg / kg, and the zinc balance after stabilization is calculated using a primary circuit zinc concentration of 10 μg / kg. The method of gradually increasing the dosage can avoid instantaneous increases in the primary circuit zinc concentration and fuel-related safety risks. The zinc addition rate in the primary loop is related to the adsorption rate of zinc in the primary loop oxide film, the zinc removal rate of the purification system, and the deposition rate of zinc on the fuel surface. For low-concentration zinc-injection units, the amount of zinc deposited on the fuel surface is negligible after zinc injection. In the short period after zinc injection, zinc adsorption in the primary loop oxide film dominates, and the zinc addition rate equals the adsorption rate of zinc in the oxide film. As the zinc concentration in the oxide film gradually increases, a more complex diffusion process occurs in the multi-layer oxide film, and the zinc adsorption rate is lower than the zinc addition rate. When zinc adsorption in the oxide film reaches saturation, the zinc addition rate equals the amount of zinc removed by the purification system. During the zinc adsorption stage on the oxide film surface, zinc ions are undetectable in the primary loop. In the initial stage of zinc injection, monitoring of primary loop zinc is strengthened to determine the time point when zinc adsorption in the oxide film reaches saturation. A primary loop zinc concentration of 2 μg / kg is used as the first-stage target concentration. As zinc adsorption reaches equilibrium, the dosage can be further increased, with a target concentration of 5 μg / kg as the lower limit of the zinc concentration target for the second stage. The final target concentration is 10 μg / kg, the equilibrium value of the primary loop zinc concentration after stable dosing.

[0014] According to some preferred embodiments of the present invention, the dosing flow rate is 145-155 mL / h, preferably 150 mL / h. The dosing strategy implemented in this invention is to maintain a constant dosing flow rate while gradually increasing the dosing concentration. To maintain a stable and accurate dosing flow rate during the dosing process, a plunger-type metering pump is selected and equipped with a diaphragm pulse damper to minimize baseline noise and pulsation. Based on the available space and location of the dosing equipment, the solution tank size is set at 25L. Considering the dosing habits and work patterns of the nuclear power plant's chemical personnel, the preparation cycle for depleted zinc acetate is set at one week. Based on the metering pump selection, the preferred dosing flow rate is Q = 25L / (7*24)h = 148.8 mL / h, controlled at 150 mL / h.

[0015] According to some preferred embodiments of the present invention, the dosage per application in each stage is 22-28 L, and the corresponding application time is 165-170 h. Preferably, the dosage per application in each stage is 25 L, and the corresponding application time is 168 h.

[0016] According to some preferred embodiments of the invention, during each dosing process, the amount of chemical added is kept equivalent to the amount of zinc removed by the purification system.

[0017] According to some preferred embodiments of the invention, zinc injection is stopped at least 24 hours before the unit is shut down; zinc released during the shutdown is removed by a purification system and is completely removed before the unit is started up.

[0018] According to some preferred embodiments of the present invention, after the unit is started, the concentrations of silicon, nickel and zinc in the primary circuit are monitored. If the silicon concentration in the primary circuit is less than 600 μg / kg, the nickel concentration is less than 2 μg / kg and the zinc concentration is less than 15 μg / kg, zinc addition is resumed and the zinc concentration is gradually increased in multiple stages.

[0019] Compared to existing technologies, the advantages of this invention, based on the above technical solutions, are as follows: The zinc injection method for the primary coolant in pressurized water reactor nuclear power units, based on industry experience with zinc-injected units, combined with the local boiling rate range of domestic operating units, and the goal of reducing core radiation field measurement in primary coolant injection in domestic operating units, selects a zinc injection concentration of 5-15 μg / kg from the perspective of fuel integrity and avoiding CIPS and CILC risks. A primary coolant concentration balance is calculated using 10 μg / kg during the stabilization phase. Based on the primary coolant concentration monitoring results, the injection rate is gradually increased in three stages. Compared to the method of adding zinc at a higher flow rate initially, this method avoids the phenomenon of instantaneous increase in primary coolant concentration and fuel-related CIPS and CILC safety risks. Gradually increasing the injection concentration while maintaining a constant injection flow rate offers advantages such as stable injection flow and ease of on-site implementation. This method, when implemented in operating units, can reduce the core radiation field, alleviate primary coolant stress corrosion cracking, ensure fuel cladding integrity, and avoid fuel-related safety risks. Detailed Implementation

[0020] To enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. 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 should fall within the scope of protection of the present invention.

[0021] The zinc addition method for the primary coolant of a pressurized water reactor nuclear power unit in this embodiment includes the following steps: During the zinc addition process, the addition flow rate is kept constant while the zinc concentration is gradually increased. The addition concentration is 5–15 μg / kg; the addition flow rate is 145–155 mL / h, preferably 150 mL / h.

[0022] The selection of zinc injection concentration is related to the desired goals of reducing the core radiation field, mitigating PWSCC, and ensuring fuel cladding integrity. The reduction effect of core dose is proportional to the cumulative zinc exposure (μg / kg·month), meaning that higher concentrations are beneficial for dose reduction. However, after zinc injection, cobalt and nickel in the primary circuit oxide film are replaced by zinc, which may lead to an increase in the content of corrosion products deposited on the fuel cladding surface, thereby increasing the risk of fuel-related axial power shift (CIPS). Therefore, lower zinc injection concentrations are less likely to lead to CIPS risk. Considering the local boiling rate and core load of domestic M310 units, a zinc injection concentration of 5–15 μg / kg is selected, which can reduce collective dose, mitigate PWSCC, and avoid fuel-related risks and the risk of zinc deposition on the core surface.

[0023] In this embodiment, the zinc addition process is divided into multiple stages. The concentration of zinc in a single stage remains constant, while the concentration gradually increases from the earlier stages to the later stages. The target concentration of zinc in the primary circuit corresponding to each stage gradually increases from the earlier stages to the later stages. The actual concentration of zinc in the primary circuit is monitored, and when the actual concentration of zinc exceeds the target concentration for that stage, the addition concentration for the next stage is adopted.

[0024] Specifically, in this embodiment, the dosing process is preferably divided into three stages. In the first stage, the target zinc concentration in the primary circuit is 1.5–2.5 μg / kg, and the dosing concentration is 0.605–0.610 g / kg. In the second stage, the target zinc concentration in the primary circuit is 4.5–5.5 μg / kg, and the dosing concentration is 1.51–1.53 g / kg. In the third stage, the target zinc concentration in the primary circuit is 9.5–10.5 μg / kg, and the dosing concentration is 3.00–3.08 g / kg. Preferably, in the first stage, the target zinc concentration in the primary circuit is 2 μg / kg, and the dosing concentration is 0.608 g / kg; in the second stage, the target zinc concentration in the primary circuit is 5 μg / kg, and the dosing concentration is 1.52 g / kg; and in the third stage, the target zinc concentration in the primary circuit is 10 μg / kg, and the dosing concentration is 3.04 g / kg.

[0025] The dosage for each stage is 22–28 L, and the corresponding dosing time is 165–170 h. Preferably, the dosage for each stage is 25 L, and the corresponding dosing time is 168 h.

[0026] The control parameters for the dosing process in this embodiment are shown in Table 1 below:

[0027] Table 1 Control parameters during drug administration

[0028]

[0029]

[0030] Zinc injection during operation: To avoid CIPS risks caused by increased nickel content in the primary circuit and to prevent transient increases in primary circuit zinc concentration after zinc injection into the operating unit, the dosage of zinc in the primary circuit is gradually increased during the injection process. Monitoring of nickel, iron, and zinc in the primary circuit is conducted during the injection period (the expected concentrations of silicon, nickel, and zinc in the primary circuit during zinc injection are controlled at 600 μg / kg, 2 μg / kg, and 15 μg / kg, respectively). Based on the monitoring results of the primary circuit zinc concentration and the amount of zinc lost through the purification system, the dosage is gradually increased in three stages.

[0031] Specifically, during the initial stage of zinc dosing, which is the adsorption phase of zinc on the oxide film, no zinc ions can be detected in the primary loop, and the amount of zinc added is equal to the amount of zinc adsorbed on the oxide film. After the zinc addition stabilizes, the amount of zinc added is equal to the sum of the amount of zinc removed through the primary loop purification system, where the normal operating flow rate of the primary loop purification system is 13.6 m³ / s. 3 / h.

[0032] Taking the first dosing stage as an example, the target concentration of zinc in the primary circuit is 2 μg / kg. The dosing concentration is calculated as follows:

[0033] The zinc removal loss in the primary loop purification system is S1 = 13.6 m³. 3 / h*2μg / kg=0.0272g / h (calculated as zinc ions)

[0034] The drug concentration C = 0.0272 g / h ÷ 0.15 L / h = 0.1813 g / kg (calculated as zinc ions)

[0035] Converted to the concentration of zinc acetate dihydrate:

[0036] C1 = 0.1813 * 219.5 ÷ 65.4 = 0.608 g / kg (calculated as zinc acetate dihydrate).

[0037] The calculation method for the second and third stages is the same. The zinc concentration in the primary circuit is calculated at 5 μg / kg and 10 μg / kg, respectively, resulting in a dosage concentration of 1.52 g / kg and 3.04 g / kg for zinc acetate dihydrate depletion.

[0038] The control strategy involves maintaining a constant flow rate from the dosing pump while gradually increasing the dosing concentration. After each dosing cycle stabilizes, the dosing amount is kept equivalent to the zinc removal rate of the purification system. The primary loop zinc dosing rate is related to the zinc adsorption rate in the primary loop oxide film, the zinc removal rate of the purification system, and the zinc deposition rate on the fuel surface. For low-concentration zinc-injected units, the amount of zinc deposited on the fuel surface is negligible. Therefore, the zinc dosing amount comprises two parts: the amount of zinc ions adsorbed in the oxide film initially and the amount removed by the purification system. In the short period after zinc injection, zinc adsorption in the primary loop oxide film dominates, and the dosing rate equals the adsorption rate. As the zinc concentration in the oxide film gradually increases, once adsorption reaches saturation, the amount adsorbed in the primary loop oxide film stabilizes, and the dosing rate equals the zinc removal rate of the purification system. In other words, at the end of each dosing stage, the zinc concentration in the primary loop reaches the target concentration, and the dosing amount equals the zinc removal rate of the purification system.

[0039] In practical implementation, the method for adding zinc to the primary coolant of a pressurized water reactor nuclear power unit includes the following steps:

[0040] 1. After the unit is started, monitor the silicon, nickel and zinc in the primary circuit. If the silicon in the primary circuit is lower than the expected value of 600 μg / kg, the nickel in the primary circuit is lower than the expected value of 2 μg / kg, and the zinc concentration in the primary circuit is lower than 15 μg / kg, zinc injection can be resumed. The dosage should be gradually increased from a low concentration in multiple stages.

[0041] 2. During unit operation, zinc injection shall be carried out in stages according to the parameters in Table 1. After the actual zinc concentration reaches 10 μg / kg, zinc injection shall continue in the third stage, and the zinc concentration in the primary loop shall be maintained at 5-15 μg / kg. The amount of zinc added during the continuous process shall be equal to the amount of zinc removed by the purification system.

[0042] 3. Zinc injection should be stopped at least one day (24 hours) before unit shutdown. After zinc injection stops, some of the zinc adsorbed in the primary loop oxide film begins to be released. During shutdown and unit cooling, the release of zinc, nickel, and cobalt from the primary loop will increase. Stopping zinc injection in advance can reduce the amount of zinc released after shutdown. The zinc released during shutdown should be removed through the purification system and completely removed before unit startup. If all released zinc is not removed before startup, a momentary increase in zinc concentration will occur after startup.

[0043] The primary loop purification system maintains the primary loop water balance through upflushing and downflushing. Zinc adsorbed in the oxide film on the surface of the primary loop equipment replaces iron, cobalt, and nickel from the original oxides and prevents cobalt from re-entering the crystal lattice. The newly formed spinel oxide has a higher surface density and better corrosion resistance compared to iron / cobalt / nickel oxides. Therefore, it can alleviate stress corrosion cracking of alloy 600 and reduce the amount of activated corrosion products, thereby reducing the source term. The purpose of this invention is to determine the control methods for zinc injection concentration, chemical dosage during zinc injection, and zinc injection process control parameters during start-up and shutdown of operating units, especially those that have not implemented primary loop zinc injection technology during hot functional testing and the initial stage of commercial operation. The determination of the technical parameters for zinc injection in operating pressurized water reactor units, including the control of zinc injection concentration, chemical dosage during zinc injection, and zinc injection during start-up and shutdown, provides technical guidance for achieving the best expected results in zinc injection for operating units.

[0044] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for adding zinc to the primary coolant of a pressurized water reactor nuclear power unit, applicable to operating units that did not undergo primary coolant addition during the pre-loading hot functional test, characterized in that, The steps include: during the zinc addition process, the flow rate is kept constant while the zinc concentration is gradually increased; The zinc addition process is divided into multiple stages. The concentration of zinc in a single stage remains constant, while the concentration of zinc in multiple stages gradually increases from the early stage to the later stage. The target concentration of zinc in the primary circuit corresponding to each stage gradually increases from the early stage to the later stage. The actual concentration of zinc in the primary circuit is monitored. When the actual concentration of zinc is greater than the target concentration of zinc for that stage, the process proceeds to the next stage and the zinc is added at the concentration corresponding to the next stage.

2. The zinc addition method according to claim 1, characterized in that, The dosing process is divided into three stages: the dosing concentration for the first stage is 0.605–0.610 g / kg; the dosing concentration for the second stage is 1.51–1.53 g / kg; and the dosing concentration for the third stage is 3.00–3.08 g / kg.

3. The zinc addition method according to claim 2, characterized in that, The target zinc concentration in the primary loop corresponding to the first stage is 1.5–2.5 μg / kg; the target zinc concentration in the primary loop corresponding to the second stage is 4.5–5.5 μg / kg; and the target zinc concentration in the primary loop corresponding to the third stage is 9.5–10.5 μg / kg.

4. The zinc addition method according to claim 1, characterized in that, The drug dosing flow rate is 145–155 mL / h.

5. The zinc addition method according to any one of claims 1-4, characterized in that, The dosage for each stage is 22-28 L, and the corresponding dosage time is 165-170 h.

6. The zinc addition method according to claim 1, characterized in that, During each dosing process, the amount of chemical added should be kept equivalent to the amount of zinc removed by the purification system.

7. The zinc addition method according to claim 1, characterized in that, Zinc injection must be stopped at least 24 hours before the unit is shut down; zinc released during the shutdown period must be removed by a purification system and completely removed before the unit is started up.

8. The zinc-addition method according to claim 1 or 7, characterized in that, After the unit is started, monitor the concentrations of silicon, nickel and zinc in the primary circuit. If the silicon concentration in the primary circuit is below 600 μg / kg, the nickel concentration is below 2 μg / kg and the zinc concentration is below 15 μg / kg, resume zinc addition and gradually increase the zinc concentration in multiple stages.

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