Method for control of natural oxidation of primary circuit of VVER unit of nuclear power plant

Abstract

The present application relates to the field of nuclear power, and particularly relates to a method for controlling natural oxidation of a primary loop of a nuclear power plant. The method comprises the following steps: hydrogen dissolving control of the primary loop; nitrogen blowing and hydrogen removing of the primary loop; and natural oxidation of the primary loop. The present application is applicable to VVER units, reduces the activity concentration of activated corrosion products in the primary loop main system, reduces the deposition of the activated corrosion products on the surface of main equipment, thereby reducing the radiation dose field level during the maintenance of the unit and reducing the radiation dose of workers in the radioactive environment.

Description

Technical Field

[0001] This invention relates to the field of nuclear power, and more particularly to a method for controlling the natural oxidation of the primary loop in a VVER unit of a nuclear power plant. Background Technology

[0002] During nuclear power plant outages, to reduce the radiation dose rate and minimize worker exposure, oxidation reactions are necessary to rapidly remove activated corrosion products deposited on the surfaces of reactor main system equipment. Most pressurized water reactor (PWR) units (such as M310, AP1000, and Hualong One) have mechanical filters installed in front of the resin purification bed in the primary loop. These filters operate using forced oxidation, rapidly injecting hydrogen peroxide into the main system through a chemical compressive testing system during shutdown and cooling. This causes activated corrosion products to quickly peel off from the pipe surfaces and be absorbed by the filters and purification bed, achieving a reduction in primary loop radioactivity within a relatively short time.

[0003] The VVER unit was designed without a mechanical filter before the primary loop resin purification bed. If forced oxidation is used during overhaul, the following problems may occur:

[0004] 1) Rapidly peeling large particles of activated corrosion products clog the ion exchange resin in the primary loop purification bed, affecting radioactive removal;

[0005] 2) The activated corrosion products that are concentrated and peeled off cannot be removed by the ion exchange resin in a timely and effective manner, and are prone to deposit in low flow rate areas to form radiation hotspots.

[0006] 3) Particles of activated corrosion products that cannot be removed in time will continue to deposit on the surface of the fuel element cladding when the next fuel cycle is started, affecting the integrity of the fuel cladding.

[0007] In addition, after the VVER unit is shut down to a cold state, the primary loop purification system loses power due to the shutdown of the main pumps, making it impossible to remove radioactive activated corrosion products. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a method for controlling the natural oxidation of the primary loop in a nuclear power plant, applicable to VVER units, to reduce the activity concentration of active corrosion products in the main primary loop system, reduce their deposition on the surface of main equipment, thereby reducing the radiation dose field level during unit maintenance and reducing the radiation dose to workers in a radioactive environment.

[0009] This invention provides a control method for natural oxidation in the primary loop of a VVER unit in a nuclear power plant, comprising the following steps:

[0010] Step 1: Primary loop hydrogen dissolution control, specifically including:

[0011] One week before the start of the overhaul, reduce the amount of ammonia added to the primary circuit and control the dissolved hydrogen in the primary circuit at 2.2-2.3 mg / L;

[0012] After the unit begins to reduce power, ammonia is intermittently supplied to the primary circuit;

[0013] After the unit's power output is reduced to ≤40%, stop adding ammonia to the primary circuit;

[0014] When the unit is reduced to zero power level, boron is injected into the primary circuit while hydrazine is stopped being added to the primary circuit.

[0015] Step 2: Primary loop nitrogen purging to remove hydrogen, specifically including:

[0016] After the reactor is cooled to a cold state, the hydrogen in the primary loop is purged.

[0017] After purging for a period of time, first measure the dissolved hydrogen concentration in the primary loop. Once the dissolved hydrogen concentration is within acceptable limits, analyze the hydrogen percentage in the gas overflow area of ​​the control rod drive mechanism sleeve until purging is complete.

[0018] Step 3: Primary circuit natural oxidation;

[0019] After the reactor nitrogen purging is completed, the water in the boric acid solution storage tank is replaced with that in the primary loop. The strong radioactivity in the primary loop reacts with the oxygen in the boric acid solution to generate hydrogen peroxide, which causes the activated corrosion products to peel off quickly and be converted into ionic state.

[0020] The water exchanged in the boric acid solution storage tank is passed through the spent fuel water pool purification system at a preset flow rate to remove corrosion products.

[0021] In one specific embodiment of the present invention, in step 1, the operating mode of the primary ammonia pump is changed from continuous operation to intermittent operation.

[0022] In a specific embodiment of the present invention, in step 1, after the boron injection is completed, the dissolved hydrogen concentration in the primary circuit is maintained at 0.95 to 1.05 mg / L.

[0023] In a specific embodiment of the present invention, in step 1, after the boron injection is completed, the dissolved hydrogen concentration in the primary circuit is maintained at 1 mg / L.

[0024] In one specific embodiment of the present invention, in step 2, hydrogen from the primary loop is purged to the degasser using a nitrogen supply system, and then removed through a hydrogen combustion system.

[0025] In one specific embodiment of the present invention, in step 2, the purging time is not less than 3 hours.

[0026] In a specific embodiment of the present invention, in step 2, after the dissolved hydrogen concentration in the primary loop is less than 0.5 mg / L and the dissolved hydrogen concentration is qualified, the hydrogen concentration in the gas overflow area of ​​the control rod drive mechanism sleeve is analyzed. If the hydrogen percentage in the gas overflow area of ​​the control rod drive mechanism sleeve is less than 0.8%, then the purging is completed.

[0027] In one specific embodiment of the present invention, in step 3, the oxygen content in the boric acid solution storage tank is 5-7 mg / L.

[0028] In one specific embodiment of the present invention, in step 3, the preset flow rate is 8-12 kg / s.

[0029] In one specific embodiment of the present invention, the preset flow rate is 10 kg / s.

[0030] Compared with existing technologies, the control method for the primary loop natural oxidation of a nuclear power plant VVER unit of the present invention is applicable to VVER units. By controlling the dissolved hydrogen in the primary loop, the concentration of dissolved hydrogen in the primary loop is effectively reduced, thereby extending the natural oxidation time of the primary loop while ensuring the water quality in the primary loop meets standards. By optimizing the monitoring sequence of chemical indicators after the primary loop nitrogen purging, the inspection time is shortened and the accuracy of the inspection is improved. During the cold shutdown phase of maintenance, the water exchange between the primary loop and the boric acid storage tank accelerates the natural oxidation process of the primary loop water chemical conditions. During the stage of large-scale release of activated corrosion products, they are introduced into the boric acid solution storage tank, and at the same time, the spent fuel water pool purification system is put into operation to remove the activated corrosion products in the boric acid storage tank, solving the problem that the primary loop radioactivity cannot be purified after the reactor nitrogen purging is completed and the main pump is shut down. Attached Figure Description

[0031] Figure 1 A schematic diagram of the apparatus used for primary loop natural oxidation;

[0032] Figure 2 This graph shows the trend of dissolved hydrogen and hydrogen peroxide concentration changes during the primary circuit's natural oxidation period.

[0033] In the diagram, 1-Reactor pressure vessel; 2-Main pump; 3-Steam generator; 4-Primary loop purification system; 5-Primary loop ammonia tank; 6-Nitrogen supply system; 7-Degasser; 8-Hydrogen combustion system; 9-Primary loop ammonia pump; 10-Primary loop hydrazine tank; 11-Primary loop hydrazine pump; 12-Charging pump; 13-Spent fuel water pool purification system; 14-Borate solution storage tank. Detailed Implementation

[0034] To further understand the present invention, embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the present invention.

[0035] The control method for primary-loop natural oxidation described in this invention is based on Figure 1 The apparatus shown is now complete.

[0036] Unlike other pressurized water reactor units, the VVER (Variable Voltage Refrigerated Reactor) does not directly inject hydrogen into the primary coolant. Instead, it adds ammonia water to the primary coolant, generating hydrogen through ammonia irradiation decomposition. During normal operation, the dissolved hydrogen concentration in the primary coolant is required to be in the range of 2.2-4.5 mg / L, but is actually controlled at 2.8-3.5 mg / L. This effectively inhibits water irradiation decomposition and ensures the primary coolant operates under reducing water chemistry conditions.

[0037] An embodiment of the present invention discloses a control method for natural oxidation in the primary loop of a VVER unit in a nuclear power plant, comprising the following steps:

[0038] Step 1: Primary loop hydrogen dissolution control, specifically including:

[0039] One week before the start of the overhaul, the amount of ammonia water added to the primary circuit was reduced by controlling the stroke frequency of the primary circuit ammonia pump 9 and the liquid level of the primary circuit ammonia tank 5, thereby keeping the dissolved hydrogen in the primary circuit at a low concentration level of 2.2-2.3 mg / L.

[0040] After the unit begins to reduce power, the operating mode of the primary circuit ammonia pump 9 is changed to intermittently supply ammonia to the primary circuit;

[0041] After the unit power is reduced to ≤40%, the ammonia supply to the primary circuit is stopped by stopping the operation of the primary circuit ammonia supply pump 9;

[0042] When the unit is reduced to zero power level, while injecting boron into the primary circuit, the operation of the hydrazine pump 11 in the primary circuit is stopped, thereby stopping the addition of hydrazine into the primary circuit and increasing the consumption of dissolved hydrogen in the primary circuit.

[0043] After boron injection is completed, the dissolved hydrogen concentration in the primary loop is maintained at 0.95–1.05 mg / L, preferably at 1.0 mg / L.

[0044] Step 2: Primary loop nitrogen purging to remove hydrogen, specifically including:

[0045] After the reactor is cooled to a cold state, the hydrogen in the primary loop is purged.

[0046] The purging process is as follows: the hydrogen from the primary loop is purged to the degasser 7 using the nitrogen supply system 6, and then the hydrogen is removed by the hydrogen combustion system 8.

[0047] After the rapid loss of dissolved hydrogen in the primary loop begins, the highly radioactive environment begins to irradiate and decompose water, producing oxygen and hydrogen peroxide. Natural oxidation then begins in the primary loop, as shown in the following reaction:

[0048]

[0049] H₂ + OH⁻ → H₂O

[0050] ·OH + ·OH → H₂O₂

[0051] 2H₂O₂→O₂+2H₂O

[0052] The purging time shall not be less than 3 hours;

[0053] After purging, first measure the dissolved hydrogen concentration in the primary loop. Once the dissolved hydrogen concentration in the primary loop is less than 0.5 mg / L and is considered qualified, analyze the hydrogen concentration in the gas overflow area of ​​the control rod drive mechanism sleeve. If the hydrogen percentage in the gas overflow area of ​​the control rod drive mechanism sleeve is less than 0.8%, then the purging is complete.

[0054] According to the traditional nitrogen purging method, the gas concentration in the primary loop dissolved hydrogen and the gas overflow area of ​​the control rod drive mechanism sleeve on the reactor top cover needs to be analyzed simultaneously. According to historical data, each sampling and analysis takes about 3 hours, and at least 3 analyses are required, taking about 9 hours in total.

[0055] According to the method of the present invention, in order to prolong the natural oxidation time, the hydrogen concentration in the primary loop is measured first. After the hydrogen concentration is <0.5mg / L, the hydrogen in the gas overflow area of ​​the control rod drive mechanism sleeve is sampled and analyzed. One sampling is sufficient to pass the test. According to practical statistics, the total time is about 4.5 hours, which is 50% shorter than before optimization.

[0056] Step 3: Primary circuit natural oxidation;

[0057] After the reactor nitrogen purging is completed, the water in the boric acid solution storage tank 14 is replaced with that in the primary loop. The strong radioactivity of the liquid in the primary loop reacts with the oxygen in the boric acid solution to generate hydrogen peroxide, which causes the activated corrosion products to peel off quickly and be converted into ionic state.

[0058] As the water is changed, some of the activated corrosion products are transferred to the boric acid solution storage tank 14;

[0059] The water exchanged in the boric acid solution storage tank is passed through the spent fuel water pool purification system at a preset flow rate to remove corrosion products.

[0060] The preset flow rate is 8 to 12 kg / s, preferably 10 kg / s.

[0061] By implementing natural oxidation of the primary circuit, the release rate of activated corrosion products is significantly slowed down, greatly reducing the risk of the resin bed in the purification system becoming clogged.

[0062] In implementing the control method, this invention monitors the trends in dissolved hydrogen and hydrogen peroxide concentrations during the primary loop natural oxidation process, such as... Figure 2 As shown. By Figure 2 It is evident that this invention effectively suppresses the amount of dissolved hydrogen in the primary loop, providing an environment for natural oxidation and facilitating the slow shedding of corrosion products. Combined with subsequent water replacement, it can reduce the activity concentration of activated corrosion products in the main primary loop system, reduce their deposition on the surface of main equipment, thereby reducing the radiation dose field level during unit maintenance and reducing the radiation dose received by workers in a radioactive environment.

[0063] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0064] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those 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 invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for controlling natural oxidation of a primary circuit of a VVER unit of a nuclear power plant, characterized in that, The method comprises the following steps: Step 1: One-loop hydrogen dissolving control, specifically comprising: A week before the overhaul starts, the amount of ammonia water added to the one-loop is reduced, and the one-loop hydrogen dissolving is controlled at 2.2-2.3 mg / L; After the unit starts to reduce power, ammonia is intermittently provided into the one-loop; After the unit reduces power to ≤40%, the addition of ammonia into the one-loop is stopped; After the unit reduces to zero power level, the addition of ammonia into the one-loop is stopped while boron is injected into the one-loop; Step 2: One-loop nitrogen purging to remove hydrogen, specifically comprising: After the reactor is cooled to cold state, hydrogen in the one-loop is purged; After purging for a period of time, the one-loop hydrogen dissolving concentration is first measured, and then the hydrogen proportion in the control rod drive mechanism sleeve gas overflow area is analyzed after the one-loop hydrogen dissolving concentration is qualified, until the purging is completed; Step 3: One-loop natural oxidation; After the reactor nitrogen purging is completed, the one-loop is water exchanged with the boric acid solution storage tank, the strong radioactivity in the one-loop and the oxygen in the boric acid solution are irradiated to generate hydrogen peroxide, so that the activated corrosion products are quickly peeled off and converted into ionic state; The exchanged water in the boric acid solution storage tank is passed through the spent fuel pool purification system at a preset flow rate to remove the corrosion products therein.

2. The method for control of natural oxidation of the primary circuit of the VVER unit of the nuclear power plant according to claim 1, characterized by the fact that, In the step 1, after the unit starts to reduce power, the operation mode of the one-loop ammonia adding pump is changed from continuous operation to intermittent operation, so as to provide ammonia into the one-loop.

3. The method for control of natural oxidation of the primary circuit of the VVER unit of the nuclear power plant according to claim 1, characterized by the fact that, In the step 1, after the boron injection is completed, the one-loop hydrogen dissolving concentration is maintained at 0.95-1.05 mg / L.

4. The method for control of natural oxidation of the primary circuit of the VVER unit of the nuclear power plant according to claim 3, characterized by the fact that, In the step 1, after the boron injection is completed, the one-loop hydrogen dissolving concentration is maintained at 1 mg / L.

5. The method for control of natural oxidation of the primary circuit of the VVER unit of the nuclear power plant according to claim 1, characterized by the fact that, In the step 2, the hydrogen in the one-loop is purged to the degasser by the nitrogen supply system, and then the hydrogen is removed by the hydrogen combustion system.

6. The method for control of natural oxidation of the primary circuit of the VVER unit of the nuclear power plant according to claim 1, characterized by the fact that, In the step 2, the purging time is not less than 3 hours.

7. The method for controlling the natural oxidation of the primary circuit of a VVER unit of a nuclear power plant according to claim 1, characterized by the fact that, In the step 2, after the one-loop hydrogen dissolving concentration is less than 0.5 mg / L, the hydrogen dissolving concentration is qualified, and then the hydrogen concentration in the control rod drive mechanism sleeve gas overflow area is analyzed, and the hydrogen proportion in the control rod drive mechanism sleeve gas overflow area is less than 0.8%, and then the purging is completed.

8. The method for controlling the natural oxidation of the primary circuit of a VVER unit of a nuclear power plant according to claim 1, characterized by the fact that, In the step 3, the oxygen content in the boric acid solution storage tank is 5-7 mg / L.

9. The method for controlling the natural oxidation of the primary circuit of a VVER unit of a nuclear power plant according to claim 8, characterized by the fact that, In the step 3, the preset flow rate is 8-12 kg / s.

10. The method for controlling the natural oxidation of the primary circuit of a VVER unit of a nuclear power plant according to claim 9, characterized by the fact that, The preset flow rate is 10 kg / s.

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