Method for controlling upper and lower temperature difference of voltage stabilizer during cooling period of VVER nuclear power unit

Abstract

The invention relates to the field of nuclear power operation, in particular to an upper and lower temperature difference control method for a voltage stabilizer during the cooling period of a VVER nuclear power unit. The control method comprises the following steps that S1, before nitrogen is charged into the voltage stabilizer, the liquid level of the voltage stabilizer is increased to 11.6-11.7 m, and the temperature difference between the upper portion and the lower portion of the voltage stabilizer is controlled to be smaller than 10 DEG C; the liquid level of the voltage stabilizer is lowered to the L2 liquid level line 11.1 m; reducing the cooling speed of a primary loop; s2, after the temperature of the primary loop is lower than 146 DEG C, a voltage stabilizer starts to charge nitrogen; after nitrogen charging, the following operations are carried out: a spraying adjusting valve of the pressure stabilizer is cut off and the current opening degree is kept automatically; part of the electric heater is cut off at a time, and the remaining electric heater is gradually cut off; after all the electric heaters are switched off, the pressure stabilizer spraying adjusting valve is continuously opened until the pressure stabilizer spraying adjusting valve is fully opened; and when the temperature difference between the coolant in the pressure stabilizer and the primary loop coolant is less than 5 DEG C, the pressure stabilizer spraying stop valve is opened. According to the invention, the temperature difference between the voltage stabilizer and the primary loop can be controlled to be within a limit value, a spray pipe and a surge pipe connected with the voltage stabilizer and the primary loop are prevented from being damaged by thermal stress, the nuclear risk and the operation risk in the operation process can be reduced, and the protection action of the primary loop overpressure safety system is avoided.

Description

Technical Field

[0001] The invention relates to the field of nuclear power operation, and in particular to a method for controlling the upper and lower temperature differences of a voltage stabilizer during the cooling period of a VVER nuclear power unit. Background Art

[0002] In the later stage of cooling of the VVER nuclear power unit, in order to ensure the normal operation of the main coolant pump, it is necessary to establish a nitrogen cushion in the pressurizer. However, the thermal conductivity of nitrogen is poor, resulting in a large temperature difference between the upper and lower parts of the pressurizer. Once the temperature difference exceeds the limit, the pressurizer may be deformed and damaged.

[0003] During the cooling period of the VVER nuclear power unit, the conventional measures to control the upper and lower temperature differences of the pressurizer are: first, to strengthen the metal cooling of the upper part of the pressurizer by the high liquid level L1 (11.4-11.6m) of the pressurizer, and second, to slow down the metal cooling of the lower part of the pressurizer by operating the electric heater and reducing the spray flow of the pressurizer. Disadvantages of the above conventional measures to control the upper and lower temperature differences of the pressurizer:

[0004] (1) The nitrogen space in the regulator is small and the compressibility of nitrogen is poor. The risk of overpressure in the primary circuit is extremely high. Once the safety valve is activated, the main coolant pump will be shut down for protection.

[0005] (2) It is easy to cause a large temperature difference (greater than 70°C) between the stabilizer and the primary circuit, which may cause the spray pipe and surge pipe connecting the stabilizer and the primary circuit to be damaged by thermal stress. Summary of the invention

[0006] The technical problem to be solved by the present invention is: to provide a method for controlling the upper and lower temperature differences of a pressurizer during the cooling period of a VVER nuclear power unit. On the basis of controlling the upper and lower temperature differences of the pressurizer within a limit and reducing the risk of damage to the pressurizer, the method can control the temperature difference between the pressurizer and a primary circuit within a limit and avoid thermal stress damage to a spray pipe and a surge pipe connected to the pressurizer and the primary circuit, and can also reduce the nuclear risk and operation risk during operation and avoid the protection action of the primary circuit overpressure safety system.

[0007] The present invention provides a method for controlling the upper and lower temperature difference of a pressurizer during cooling of a VVER nuclear power unit, comprising the following steps:

[0008] Step S1: before the pressurizer is filled with nitrogen, the pressurizer liquid level is raised to 11.6-11.7m, and the upper and lower temperature difference of the pressurizer is controlled to be less than 10°C; the pressurizer liquid level is lowered to 11.1m of the L2 liquid level line; and the primary circuit cooling speed is reduced;

[0009] Step S2: After the primary circuit temperature is less than 146°C, the regulator starts to charge nitrogen;

[0010] After nitrogen filling, perform the following operations:

[0011] Cut off the automatic opening of the spray regulating valve of the pressure stabilizer and keep the current opening;

[0012] First, cut off some of the electric heaters at one time, and then gradually cut off the remaining electric heaters;

[0013] After all the electric heaters have been cut off, continue to open the large stabilizer spray regulating valve to full open;

[0014] When the temperature difference between the coolant in the stabilizer and the coolant in the first circuit is less than 5°C, open the stabilizer spray stop valve.

[0015] The number of electric heaters removed at one time accounts for half of the total number of electric heaters.

[0016] When the remaining electric heaters are gradually cut off, the cooling speed limit of the voltage regulator is controlled at 30° C. / h.

[0017] After all the electric heaters are cut off, the spray regulating valve of the pressurizer is opened to full opening, and the cooling speed limit of the pressurizer is controlled at 20°C / h.

[0018] The cooling rate of the first circuit is reduced to no more than 8°C / h.

[0019] Compared with the prior art, the method for controlling the upper and lower temperature difference of the pressurizer during the cooling period of the VVER nuclear power unit of the present invention has the following beneficial effects:

[0020] (1) Reduce nuclear and operational risks during operation, avoid primary circuit overpressure, and increase the economic benefits of the generator set;

[0021] (2) Effectively control the temperature difference between the upper and lower parts of the voltage stabilizer to prevent its deformation and damage;

[0022] (3) Effectively control the temperature difference between the voltage stabilizer and the primary circuit to prevent the spray pipe and surge pipe connecting the voltage stabilizer and the primary circuit from being damaged by thermal stress. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram showing a pressurizer spray system;

[0024] Figure 2 A simplified diagram showing the control process of the upper and lower temperature difference of the voltage stabilizer;

[0025] In the figure, 1-stabilizer spray valve, 2-stabilizer spray stop valve, 3-stabilizer, 4-stabilizer heater, 5-L1 liquid level line, 6-L2 liquid level line, 7-stabilizer spray pipe, 8-stabilizer fluctuation pipe. DETAILED DESCRIPTION

[0026] In order to further understand the present invention, the embodiments of the present invention are described below in conjunction with examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the present invention.

[0027] Stabilizer spray system such as Figure 1 As shown, according to Figure 1 It can be seen that during the period when the nitrogen cushion is established in the pressurizer of the VVER nuclear power unit, the heat of the upper metal of the pressurizer is discharged through two pathways. The first is: the upper metal of the pressurizer → nitrogen → the liquid medium in the pressurizer (the primary coolant); the second is: the upper metal of the pressurizer → the metal shell of the pressurizer → the liquid medium in the pressurizer (the primary coolant). The final heat sink of the two heat discharge pathways is the liquid medium (the primary coolant) in the pressurizer. The main technical solution of the patent of this invention is to increase the flow rate of the liquid medium (the primary coolant) in the pressurizer.

[0028] The embodiment of the present invention discloses a method for controlling the upper and lower temperature difference of a pressurizer during cooling of a VVER nuclear power unit. Figure 2 As shown, the following steps are included:

[0029] Step S1: before the pressurizer is filled with nitrogen, the pressurizer liquid level is raised to 11.6-11.7m, and the upper and lower temperature difference of the pressurizer is controlled to be less than 10°C; the pressurizer liquid level is lowered to 11.1m of the L2 liquid level line to reduce the risk of overpressure in the primary circuit; the primary circuit cooling rate is reduced, and the primary circuit cooling rate is reduced to no more than 8°C / h;

[0030] Step S2: After the primary circuit temperature is less than 146°C, the regulator starts to charge nitrogen;

[0031] After nitrogen filling, perform the following operations:

[0032] Cut off the automatic opening of the spray regulating valve of the pressure stabilizer and keep the current opening;

[0033] First, some of the electric heaters are cut off at one time, and the number of electric heaters cut off at one time accounts for half of the total number of electric heaters;

[0034] Gradually cutting off the remaining electric heaters; when gradually cutting off the remaining electric heaters, the cooling speed limit of the voltage regulator is controlled at 30°C / h;

[0035] After all the electric heaters are cut off, the spray regulating valve of the pressurizer is opened to full opening, and the cooling speed limit of the pressurizer is controlled at 20°C / h.

[0036] When the temperature difference between the coolant in the stabilizer and the coolant in the first circuit is less than 5°C, open the stabilizer spray stop valve.

[0037] The present invention reduces the cooling speed of a primary circuit before the pressurizer is filled with nitrogen and increases the cooling speed of the pressurizer after the pressurizer is filled with nitrogen, thereby avoiding an excessive temperature difference of more than 70° C. between the pressurizer and the primary circuit.

[0038] In order to effectively control the upper and lower temperature difference of the pressurizer, this scheme reduces the initial upper and lower temperature difference of the pressurizer to within 10°C by raising the pressurizer liquid level before the pressurizer is filled with nitrogen. When the temperature difference between the coolant in the pressurizer and the primary circuit coolant is less than 5°C (to avoid exceeding the cooling speed of the pressurizer), the pressurizer spray stop valve is opened, and the spray water flow rate is greatly increased to enhance the metal cooling of the upper part of the pressurizer and reduce the upper and lower temperature difference of the pressurizer.

[0039] In order to further understand the present invention, the method for controlling the upper and lower temperature difference of the pressurizer during the cooling period of the VVER nuclear power unit provided by the present invention is described in detail below in conjunction with the embodiments. The protection scope of the present invention is not limited by the following embodiments.

[0040] Example 1

[0041] Step 1: Perform the following operations in sequence before the voltage regulator 3 is filled with nitrogen:

[0042] 1. Raise the liquid level of the stabilizer 3 to 11.6-11.7m, and control the temperature difference between the upper and lower parts of the stabilizer to be less than 10℃;

[0043] 2. Reduce the liquid level of the stabilizer 3 to 11.1m of the L1 liquid level line, and start the operation at 148℃ in the first circuit;

[0044] 3. Reduce the cooling rate of the primary circuit to no more than 8°C / h.

[0045] Step 2: After the primary circuit temperature is less than 146°C, the pressurizer 3 starts to charge with nitrogen. After the pressurizer 3 starts to charge with nitrogen, the following operations are performed in sequence:

[0046] 1. Cut off the automatic function of the spray regulating valve 1 of the pressure stabilizer and keep the current opening;

[0047] 2. Cut off 14 electric heaters 4 at one time, a total of 28, cut off half, do not cut off all, otherwise the cooling speed of the voltage stabilizer will exceed the limit value of 30℃ / h;

[0048] 3. Gradually cut off the remaining 14 electric heaters 4. At this stage, the gas space in the pressurizer is mainly steam, and the cooling speed limit of the pressurizer is controlled at 30℃ / h;

[0049] 4. After all the electric heaters 4 are cut off, continue to open the large stabilizer spray regulating valve 1 to full open; in this stage, the gas space in the stabilizer is mainly nitrogen, and the stabilizer cooling speed limit is controlled at 20℃ / h;

[0050] 5. When the temperature difference between the coolant in the regulator and the coolant in the primary circuit is less than 5°C, open the two regulator spray stop valves 2.

[0051] The above embodiments are only used to help understand the method and core idea of ​​the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

[0052] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for controlling the upper and lower temperature difference of a pressurizer during cooling of a VVER nuclear power unit, characterized in that: The following steps are involved: Step S1: before the pressurizer is filled with nitrogen, the pressurizer liquid level is raised to 11.6-11.7m, and the upper and lower temperature difference of the pressurizer is controlled to be less than 10°C; the pressurizer liquid level is lowered to 11.1m of the L2 liquid level line; and the primary circuit cooling speed is reduced; Step S2: After the primary circuit temperature is less than 146°C, the regulator starts to charge nitrogen; After nitrogen filling, perform the following operations: Cut off the automatic opening of the spray regulating valve of the pressure stabilizer and keep the current opening; First, cut off some of the electric heaters at one time, and then gradually cut off the remaining electric heaters; After all the electric heaters have been cut off, continue to open the large stabilizer spray regulating valve to full open; When the temperature difference between the coolant in the stabilizer and the coolant in the first circuit is less than 5°C, open the stabilizer spray stop valve.

2. The method for controlling the upper and lower temperature difference of a pressurizer during cooling of a VVER nuclear power unit according to claim 1, characterized in that: The number of electric heaters removed at one time accounts for half of the total number of electric heaters.

3. The method for controlling the upper and lower temperature difference of a pressurizer during cooling of a VVER nuclear power unit according to claim 1, characterized in that: When the remaining electric heaters are gradually cut off, the cooling speed limit of the voltage regulator is controlled at 30° C. / h.

4. The method for controlling the upper and lower temperature difference of a pressurizer during cooling of a VVER nuclear power unit according to claim 1, characterized in that: After all the electric heaters are cut off, the spray regulating valve of the pressurizer is opened to full opening, and the cooling speed limit of the pressurizer is controlled at 20°C / h.

5. The method for controlling the upper and lower temperature difference of a pressurizer during cooling of a VVER nuclear power unit according to claim 1, characterized in that: The cooling rate of the first circuit is reduced to no more than 8°C / h.

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