A thermal performance supervision method for steam generators in nuclear power plants

Abstract

The present invention discloses a thermal performance supervision method for a steam generator in a nuclear power plant. The supervision method includes obtaining the current actual steam pressure value and other current relevant thermal parameters of the steam generator, and calculating the current theoretical optimal steam pressure value of the steam generator in combination with the heat transfer mathematical model of the steam generator; calculating the current state factor of the steam generator according to the current theoretical optimal steam pressure value, the current actual steam pressure value, and the maximum allowable steam pressure drop of the steam generator; comparing the current state factor with a preset state factor threshold to obtain the current thermal performance state of the steam generator; and taking corresponding treatment measures according to the current thermal performance state. The present invention uses the state factor of the steam generator to monitor the change of the thermal performance of the steam generator and makes corresponding treatment measures, which are efficient and accurate.

Description

Technical Field

[0001] The present invention relates to the field of nuclear power, and in particular to a method for monitoring the thermal performance of a steam generator in a nuclear power plant. Background Art

[0002] From the 1980s to the early 1990s, the design margin of the natural circulation steam generator (SG) of PWR nuclear power plants was reduced; coupled with the continuous accumulation of fouling during the operation of the steam generator, the secondary side heat transfer efficiency was reduced, which further reduced the steam pressure generated by the steam generator. The steam pressure loss can usually reach 0.7bar-2.1bar, and in some cases even exceeds 3.4bar. The reduction in steam generator pressure will cause multiple nuclear power plants to be unable to produce rated unit output. The accumulation of fouling in steam generators not only poses safety risks such as corrosion, but also has a serious impact on the economic efficiency of nuclear power plants.

[0003] At present, the fouling coefficient method is usually used to evaluate the thermal performance of steam generators, but the variation patterns of the fouling index vary greatly between different units, which is not convenient for horizontal comparison between units. Based on comprehensive considerations of heat transfer resistance, video inspection and chemical parameters, the French Electricity Group (EDF) proposed a steam generator evaluation toolkit, the fouling index, to guide the chemical cleaning and evaluation of steam generators. The basic premise of this method is that the thermal resistance is directly proportional to the content of corrosion products in the steam generator. However, the latest research results show that in addition to being related to the thickness of the corrosion products, the thermal resistance also depends on the structure and morphology of the corrosion products.

[0004] As my country's second-generation nuclear power plants are put into commercial operation in large numbers, some nuclear power plants have also experienced a decrease in the steam pressure at the secondary outlet of the steam generator. For example, in the first fuel cycle of a nuclear power plant unit, the steam pressure decreased by about 1 bar, and in the 2nd to 5th fuel cycles, the steam pressure continued to be at a low level, sometimes decreasing to below the design guarantee value of 67.1 bar. The steam pressure was reduced by about 2 bar at most compared to the initial commercial operation value, but after the fourth cycle, the steam pressure rebounded, and the maximum rebound was about 1.5 bar compared to the lowest value. After five fuel cycles, it was still lower than the steam pressure at the beginning of commercial operation. With the increase in service time, the heat transfer thermal resistance of the steam generator gradually becomes greater than the design fouling thermal resistance of 8.8×10 -6 m 2 ℃ ·W -1 In the fifth fuel cycle, as the steam pressure slowly rises, the heat transfer resistance slowly decreases to 4×10 -6 m 2 ℃ ·W -1Around. Whether it is the change trend of steam pressure or the change trend of heat transfer thermal resistance, it is just a kind of phenomenon, which cannot reveal the root cause of the decrease in the secondary side steam pressure of the steam generator, nor can it evaluate or supervise the degree of influence of the steam pressure decrease.

[0005] In addition, the throttle control method is mostly adopted for the steam turbines in the conventional island of nuclear power units. There is a certain margin at the initial design of the steam generator and the steam turbine, that is, the maximum allowable steam pressure drop of the steam generator. When the decrease value of the steam pressure is less than the margin, the decrease in the steam pressure has a small impact on the unit output. That is, the loss caused by the decrease in the Carnot cycle efficiency can be compensated by reducing the throttle loss of the valve. Once the decrease value of the steam pressure is too large, after the main regulating valve of the steam turbine is fully opened (VWO), the loss caused by the decrease in the steam pressure will have a greater impact on the unit. Taking a 1000MW pressurized water reactor nuclear power unit as an example, when the (main) steam pressure is higher than the threshold pressure, when the steam pressure changes by 1 bar, the unit electric power changes by about 0.1MW; when the (main) steam pressure is lower than the threshold pressure, when the steam pressure changes by 1 bar, the unit electric power changes by about 17MW. The influence relationship curve is as Figure 1 shown. It can be seen that when the (main) steam pressure is lower than the threshold point, it will have a significant impact on the economy of the unit. It should be noted that at this time, the large change in the electric power is not caused by the change in the thermal efficiency, but is caused by the fact that after the regulating valve is fully opened, the full power generation of the unit reactor power is restricted.

[0006] Therefore, for operating nuclear power plants, it is necessary to not only consider the fouling problem of the steam generator, but also pay attention to the matching margin between the steam generator and the steam turbine. Summary of the Invention

[0007] In view of the above, it is necessary to provide a thermal performance supervision method for the steam generator of a nuclear power plant, which uses the state factor of the steam generator to monitor the change of the thermal performance of the steam generator and make corresponding treatment measures. The technical solution of the present invention is as follows:

[0008] The present invention provides a thermal performance supervision method for the steam generator of a nuclear power plant, which monitors and manages the thermal performance of the steam generator in the natural circulation state through the state factor of the steam generator. The supervision method includes the following steps:

[0009] S1. Obtain the current actual steam pressure value of the steam generator and other current relevant thermal parameters, and calculate the current theoretical optimal steam pressure value of the steam generator in combination with the heat transfer mathematical model of the steam generator;

[0010] S2. Calculate the current state factor of the steam generator through the following formula:

[0011]

[0012] In the formula, η f is the current state factor of the steam generator, P max is the current theoretical optimal steam pressure value of the steam generator, P is the current actual steam pressure value of the steam generator, and ΔP is the maximum allowable steam pressure drop of the steam generator;

[0013] S3. Compare the current state factor of the steam generator obtained in S2 with a preset state factor threshold to obtain the current thermal performance state of the steam generator;

[0014] S4. According to the current thermal performance state in S3, take measures corresponding to the current thermal performance state for the steam generator.

[0015] Furthermore, the thermal performance states in S3 include excellent state, good state, and poor state.

[0016] Furthermore, S3 includes: if the current state factor is not higher than a preset first state factor threshold, the current thermal performance state of the steam generator is an excellent state; if the current state factor is not lower than a preset second state factor threshold, the current thermal performance state of the steam generator is a poor state; if the current state factor is higher than the first state factor threshold and lower than the second state factor threshold, the current thermal performance state of the steam generator is a good state; where the first state factor threshold is less than the second state factor threshold.

[0017] Preferably, the first state factor threshold is 40%, and the second state factor threshold is 70%.

[0018] Preferably, if the current thermal performance state is an excellent state, maintain the current operating state of the steam generator; if the current thermal performance state is a good state, use the dispersant PAA for the steam generator; if the current thermal performance state is a poor state, use soft chemical cleaning methods such as ASCA for the steam generator.

[0019] Furthermore, the heat transfer mathematical model includes a primary side thermal resistance model, a secondary side thermal resistance model, a tube wall heat conduction thermal resistance model, and a fouling thermal resistance model of the steam generator.

[0020] Preferably, the supervision method further includes obtaining relevant thermal parameters through systems such as the power station KDO.

[0021] Furthermore, the other relevant parameters include the secondary side thermal power, cold leg temperature, hot leg temperature, wide range water level of the steam generator, and primary loop flow rate.

[0022] Furthermore, the maximum allowable steam pressure drop of the steam generator is obtained according to the design conditions of the steam turbine cooperating with the steam generator.

[0023] Further, the supervision method further includes storing the state factor and / or the thermal performance state of the steam generator and / or the measures taken.

[0024] Meanwhile, the present invention provides a thermal performance evaluation model for a natural circulation steam generator of a pressurized water reactor nuclear power plant, that is, combining the heat transfer calculation of the steam generator with the margin of the steam turbine design to establish a comprehensive evaluation index, namely the state factor.

[0025] The state factor is calculated through a four-layer heat transfer calculation model of the steam generator, a theoretical optimal heat transfer calculation model of the steam generator, and a matching margin calculation model of the steam generator and the steam turbine. That is, using a four-layer heat transfer mathematical model of the primary side thermal resistance, secondary side thermal resistance, tube wall heat conduction thermal resistance, and fouling thermal resistance models of the steam generator, combined with the design parameters and actual operating parameters of the nuclear power plant, including (main) steam pressure, secondary side thermal power, hot and cold leg temperatures, steam generator water level, and primary loop flow rate, to predict the theoretical steam pressure at the secondary side outlet of the steam generator, which represents the best heat transfer performance of the steam generator without fouling; the difference between the theoretical steam pressure and the actual operating steam pressure reflects the steam pressure loss caused by fouling in the steam generator. According to the design conditions of the conventional island steam turbine, the maximum allowable steam pressure drop is calculated. The proportional fraction of the current actual pressure loss (i.e., the steam pressure loss) and the maximum allowable pressure loss (i.e., the maximum steam pressure drop) of the steam generator is used as a quantitative index to evaluate the impact of the reduction of the thermal performance of the steam generator on the power generation capacity of the nuclear power plant, and is called the steam generator state factor. This calculation model, on the one hand, links the thermal performance evaluation of the steam generator with the power generation capacity of the secondary loop conventional island; on the other hand, presents the degree of reduction of the thermal performance of the steam generator in a unified manner with a quantitative score.

[0026] The advantages of the present invention are as follows: Using the ratio of the current actual pressure loss and the maximum design allowable pressure loss of the steam generator, that is, the state factor, as an index to evaluate the thermal performance of the steam generator, not only links the thermal performance evaluation of the steam generator with the power generation capacity of the secondary loop conventional island steam turbine, but also presents the degree of reduction of the thermal performance of the steam generator in a unified manner with a quantitative score, and proposes different intervals to monitor and evaluate the operating state of the steam generator, and finally proposes corresponding fouling control measures, which are accurate and efficient. Description of the Drawings

[0027] Figure 1 Schematic diagram of the correction curve of the (main) steam pressure of the 1000MW nuclear power plant unit in the background technology with respect to the unit electric power;

[0028] Figure 2 Calculation logic diagram of the state factor of the thermal performance supervision method provided by the embodiment of the present invention;

[0029] Figure 3 Schematic diagram for monitoring the state factors of the thermal performance supervision method provided by the embodiment of the present invention. Detailed implementation manners

[0030] In order to enable the personnel in the technical field to better understand the solution of the present invention, and to more clearly understand the purpose, technical solution and its advantages of the present invention, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be noted that the implementation manners not illustrated or described in the drawings are in the forms known to those of ordinary skill in the technical field. Additionally, although this document may provide examples including specific values of parameters, it should be understood that the parameters need not exactly equal the corresponding values, but may approximate the corresponding values within an acceptable tolerance of error or design constraints. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the scope of protection of the present invention. In addition, the terms "including" and "having" and any variations thereof in the specification and claims of the present invention are intended to cover non-exclusive inclusion. For example, a process, method, apparatus, product or device that includes a series of steps or units need not be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0031] In one embodiment of the present invention, a thermal performance supervision method for a nuclear power plant steam generator is provided. Aiming at the reduction of thermal performance caused by fouling on the secondary side of a natural circulation steam generator in a nuclear power plant, the supervision method can perform diagnosis and evaluation, and provide corresponding fouling treatment measures and suggestions.

[0032] The supervision method includes the following steps:

[0033] First step, obtain the current actual steam pressure value P of the steam generator and other current relevant thermal parameters, and combine with the heat transfer mathematical model of the steam generator to calculate the current theoretical optimal steam pressure value P max . Among them, the heat transfer mathematical model includes the primary side thermal resistance model, secondary side thermal resistance model, tube wall heat conduction thermal resistance model and fouling thermal resistance model of the steam generator, and the other relevant parameters include the secondary side thermal power, cold leg temperature, hot leg temperature and primary loop flow rate of the steam generator. The relevant thermal parameters are obtained through systems such as the power plant KDO, and the protection scope of the present invention is not limited thereby.

[0034] Specifically, in this embodiment, as Figure 2As shown, based on the designed performance data and structural parameters of the steam generator under various operating conditions, it can be known that the heat transfer capacity of the steam generator, namely the heat transfer coefficient inside the tubes, the heat transfer coefficient outside the tubes, the thermal resistance of the tube wall conduction, and the fouling thermal resistance. Then, combined with the cold leg temperature, hot leg temperature, thermal power, steam pressure, and wide-range water level of the steam generator obtained from actual measurements, the optimal steam pressure value P of the steam generator under the current primary loop flow rate is obtained. max 。

[0035] Second step, calculate the current state factor of the steam generator through the following formula:

[0036]

[0037] In the formula, η f is the current state factor of the steam generator; P max is the current theoretical optimal steam pressure value of the steam generator; P is the current actual steam pressure value of the steam generator; ΔP is the maximum allowable steam pressure drop of the steam generator, that is, the matching margin between the steam generator and the steam turbine, which is obtained according to the design conditions of the steam turbine cooperating with the steam generator.

[0038] Third step, compare the current state factor of the steam generator obtained in S2 with the preset state factor threshold to obtain the current thermal performance state of the steam generator:

[0039] If the current state factor is not higher than the preset first state factor threshold, the current thermal performance state of the steam generator is an excellent state; if the current state factor is not lower than the preset second state factor threshold, the current thermal performance state of the steam generator is a poor state; if the current state factor is higher than the first state factor threshold and lower than the second state factor threshold, the current thermal performance state of the steam generator is a good state; among them, the first state factor threshold is less than the second state factor threshold, and the threshold can be selected according to the specific operation conditions of the nuclear power plant, without limiting the protection scope of the present invention hereby.

[0040] Fourth step, according to the current thermal performance state in S3, take measures corresponding to the current thermal performance state for the steam generator.

[0041] In this embodiment, according to the regulating characteristic curve of the (main) steam control valve of the conventional island steam turbine, three thermal performance states of excellent, good, and poor are set to monitor and evaluate the operating state of the steam generator, and corresponding fouling control measures are provided according to the thermal performance state. Specifically, such as Figure 2As shown, if the status factor is less than or equal to 40%, it indicates that the thermal performance status of the steam generator is excellent, and the current operating state of the steam generator can be maintained; when the status factor is between 40% - 70%, it means that the thermal performance status of the steam generator is good, but enhanced attention is required. At this time, the dispersant PAA can be used for the steam generator; if the status factor is greater than or equal to 70%, it indicates that the thermal performance status of the steam generator has deteriorated (poor), and corrective measures need to be taken immediately, such as soft chemical cleaning methods like ASCA, to avoid affecting the output of the steam turbine.

[0042] In addition, the supervision method further includes obtaining relevant thermal parameters from systems such as KDO every week and calculating the status factor, and storing the status factor and / or thermal performance status and / or measures taken of the steam generator in a database for convenient retrieval and viewing.

[0043] In an embodiment of the present invention, a thermal performance supervision method for a steam generator in a nuclear power plant is provided, which is used to monitor and diagnose the state where the fouling on the secondary side of the steam generator causes a reduction in steam heat transfer performance, and evaluate the impact on the output of the conventional island unit; combined with the operating state of the main steam regulating valve, different monitoring intervals (i.e., excellent, good, and poor) are established, and corresponding fouling control measures and suggestions are proposed according to the different intervals (i.e., excellent, good, and poor) where the status factor is located.

[0044] For a certain unit in a certain power plant, the thermal performance parameters of the steam generator in six cycles ( Figure 3 among which are 101, 102, 103, 104, 105, 106) since commercial operation are calculated, and the trend of the status factor of the steam generator is as Figure 3 shown. The status factor was in the excellent region in the first cycle 101 but increased rapidly; however, it still gradually increased in the subsequent cycles 102 and 103 and had entered the good monitoring area (enhanced attention). In the fourth fuel cycle 104, it was close to the steam flow alarm line, and further reduction would affect the unit's electric power. In the sixth fuel cycle 106, the status factor recovered to a certain extent. During the fifth refueling outage, i.e., 105, the power plant did not perform additional treatment work, which was the result of the natural operation of the steam generator. This is because the positive effect of the particulate matter accumulated in the steam generator gradually became greater than the negative effect of the dissolved substances, resulting in an increase in the nucleate boiling heat transfer coefficient on the secondary side, thereby increasing the steam pressure at the outlet of the steam generator and causing the status factor of the steam generator to return to the excellent region again. Given that the status factor of this unit has a slow downward trend, the power plant can adopt the method of maintaining the existing chemical technical specifications, but the long-term online use of the dispersant PAA (polyacrylic acid dispersant) is recommended. Under normal circumstances, the immediate (and continuous) beneficial change in the heat transfer thermal resistance caused by the use of the dispersant is equal to -1.94×10 -6 m2 ·°C·W -1 (Equivalent to an increase in steam pressure of approximately 0.21 bar - 0.28 bar).

[0045] Combined with the monitoring range of the steam generator status factor (i.e., excellent, good, and poor), once the steam generator status factor enters the good monitoring area (enhanced attention), it is recommended to immediately implement measures such as the dispersant PAA to improve the blowdown efficiency of the steam generator and alleviate the deteriorating trend of the steam generator's thermal performance; after the steam generator status factor enters the poor (affecting output) area, it will have a certain impact on the power generation capacity of the conventional island unit. It is recommended to implement soft chemical cleaning methods such as ASCA (Advanced Scale Conditioning Agents) to remove part of the accumulated dirt in the steam generator and improve the accumulated dirt structure in the tube bundle area to slightly stagewise improve the heat transfer performance. After one application of ASCA, the average heat transfer resistance is reduced by -2.99×10 - 6 m 2 ·°C·W -1 (Equivalent to an increase in pressure of 0.37 bar). In the case of multiple applications, the heat transfer resistance can still be further reduced. For example, after three applications, the cumulative reduction in heat transfer resistance is -7.39×10 -6 m 2 ·°C·W -1 to -8.8×10 -6 m 2 ·°C·W -1 (Equivalent to an increase in pressure of 0.91 bar - 1.08 bar). If multiple applications are continuously made, the heat transfer resistance will eventually return to the clean state at the initial stage of commercial operation (the change in thermal resistance is 0).

[0046] The present invention utilizes the mathematical model of the four-layer heat transfer of the steam generator and combines the actual operating parameters of the nuclear power plant to propose the concept of the status factor, revealing the change law of the thermal performance of the steam generator and its impact on the output of the conventional island steam turbine unit with a normalized quantitative score; and according to the regulation characteristic curve and operation status of the (main) steam regulating valve of the conventional island steam turbine, three different operating intervals of excellent, good, and poor are set to monitor and evaluate the operating status of the steam generator (i.e., the thermal performance status), and at the same time, corresponding suggestions for treating the accumulated dirt in the steam generator are proposed according to the different intervals where the steam generator status factor is located.

[0047] The above is only the preferred embodiment of the present invention, and it does not limit its patent scope accordingly. Any equivalent structure or equivalent process transformation made by using the content of the specification and drawings of the present invention, directly or indirectly applied in other related technical fields, is equally included in the patent protection scope of the present invention.

Claims

1. A thermal performance supervision method for a steam generator in a nuclear power plant, characterized in that, Supervise and manage the thermal performance of a steam generator in the natural circulation state through the state factor of the steam generator. The supervision method includes the following steps: S1. Obtain the current actual steam pressure value of the steam generator and other current relevant thermal parameters, and calculate the current theoretical optimal steam pressure value of the steam generator in combination with the heat transfer mathematical model of the steam generator; S2. Calculate the current state factor of the steam generator through the following formula: where η f is the current state factor of the steam generator, P max is the current theoretically optimal steam pressure value of the steam generator, P is the current actual steam pressure value of the steam generator, and ΔP is the maximum allowable steam pressure drop of the steam generator; S3. Compare the current state factor of the steam generator obtained in S2 with a preset state factor threshold to obtain the current thermal performance state of the steam generator; S4. According to the current thermal performance state in S3, take governance measures corresponding to the current thermal performance state for the steam generator.

2. The thermal performance supervision method according to claim 1, wherein The thermal performance state in S3 includes an excellent state, a good state, and a poor state.

3. The thermal performance monitoring method according to claim 2, characterized in that, S3 Including: If the current state factor is not higher than the preset first state factor threshold, the current thermal performance state of the steam generator is an excellent state; If the current state factor is not lower than the preset second state factor threshold, the current thermal performance state of the steam generator is a poor state; if the current state factor is higher than the first state factor threshold and lower than the second state factor threshold, the current thermal performance state of the steam generator is a good state; where the first state factor threshold is less than the second state factor threshold.

4. The thermal performance supervision method according to claim 3, characterized in that, The first state factor threshold is 40%, and the second state factor threshold is 70%.

5. The thermal performance monitoring method according to claim 2, characterized in that S4 Including: If the current thermal performance state is an excellent state, maintain the current operating state of the steam generator; if the current thermal performance state is a good state, use the dispersant PAA for the steam generator; if the current thermal performance state is a poor state, use the soft chemical cleaning method for the steam generator.

6. The thermal performance supervision method according to claim 1, characterized in that The heat transfer mathematical model includes a primary side thermal resistance model, a secondary side thermal resistance model, a tube wall heat conduction thermal resistance model, and a fouling thermal resistance model of the steam generator.

7. The thermal performance supervision method according to claim 1, characterized in that The other relevant parameters include the secondary side thermal power, cold leg temperature, hot leg temperature, wide range water level of the steam generator, and primary loop flow rate.

8. The thermal performance monitoring method according to claim 1, characterized in that The maximum allowable steam pressure drop of the steam generator is obtained according to the design conditions of the steam turbine cooperating with the steam generator.

9. The thermal performance monitoring method according to claim 6, characterized in that, The supervision method further includes obtaining relevant thermal parameters through the KDO system of the nuclear power plant.

10. The thermal performance supervision method according to claim 1, characterized in that, The supervision method further includes storing the state factor and / or thermal performance state and / or measures taken of the steam generator.

Images