Control method for natural oxidation of primary loop of nuclear power plant

Abstract

The invention relates to the field of nuclear power, in particular to a control method for natural oxidation of a primary loop of a nuclear power plant. The method comprises the following steps: carrying out primary loop dissolved hydrogen control; primary loop nitrogen purging hydrogen removal; and the primary loop is naturally oxidized. The method is suitable for a VVER unit, the activity concentration of the activated corrosion product in a primary loop main system is reduced, deposition of the activated corrosion product on the surface of main equipment is reduced, and therefore the radiation dose field level in the unit maintenance period is reduced, and the irradiation dose of workers in the radioactive environment is reduced.

Description

Technical Field

[0001] The invention relates to the field of nuclear power, and in particular to a control method for natural oxidation of a primary loop of a nuclear power plant. Background Art

[0002] During the shutdown of nuclear power plants for overhaul, in order to reduce the radiation field dose rate level during the shutdown period and reduce the radiation dose of workers, it is necessary to release the activated corrosion products deposited on the surface of the reactor main system equipment through oxidation reaction to achieve the purpose of rapid removal. Most pressurized water reactor nuclear power units (such as M310, AP1000, Hualong One, etc.) have mechanical filters installed in front of the resin purification bed in the primary circuit, and adopt the forced oxidation operation mode. During the shutdown and cooling process, hydrogen peroxide is quickly injected into the main system through the chemical volume system, so that the activated corrosion products are quickly peeled off from the pipeline surface and absorbed by the filter and purification bed, achieving the purpose of reducing the radioactivity of the primary circuit in a relatively short time.

[0003] When designing the VVER unit, no mechanical filter was installed in front of the resin purification bed in the primary circuit. If forced oxidation is used during the overhaul, the following problems may occur:

[0004] 1) Rapidly peeling large particles activate corrosion products, blocking the ion exchange resin of the primary purification bed and affecting radioactivity removal;

[0005] 2) The activated corrosion products that are concentratedly peeled off cannot be effectively removed in time by ion exchange resin, and are easily deposited in low flow rate areas to form radiation hot spots;

[0006] 3) Activated corrosion product particles that cannot be removed in time will continue to be deposited 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 pump, and can no longer remove radioactive activated corrosion products. Summary of the invention

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

[0009] The present invention provides a control method for natural oxidation of a primary loop of a nuclear power plant, comprising the following steps:

[0010] Step 1: Primary circuit hydrogen dissolution control, 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 to 2.2-2.3 mg / L;

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

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

[0014] When the unit drops to zero power level, while injecting boron into the primary circuit, stop adding hydrazine into the primary circuit;

[0015] Step 2: Purge the primary loop with nitrogen to remove hydrogen, specifically including:

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

[0017] After purging for a period of time, measure the dissolved hydrogen concentration in the primary circuit first. After the dissolved hydrogen concentration is qualified, analyze the hydrogen proportion in the gas overflow area of ​​the control rod drive mechanism sleeve until the purging is completed;

[0018] Step 3: One-loop natural oxidation;

[0019] After the nitrogen purge of the reactor is completed, the boric acid solution storage tank and the primary circuit are used to replace water. The strong radioactivity in the primary circuit reacts with the oxygen in the boric acid solution to generate hydrogen peroxide, which causes the activated corrosion products to quickly peel off and turn into ionic state.

[0020] 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 corrosion products.

[0021] In a specific embodiment of the present invention, in step 1, the operation mode of ammonia adding pump in a circuit 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 first loop is maintained at 0.95-1.05 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 first loop is maintained at 1 mg / L.

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

[0025] In a 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 first 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 proportion of hydrogen in the gas overflow area of ​​the control rod drive mechanism sleeve is less than 0.8%, the purge is completed.

[0027] 8. The method for controlling natural oxidation of a primary circuit of a nuclear power plant according to claim 1, characterized in that in said step 3, the oxygen content in said boric acid solution storage tank is 5-7 mg / L.

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

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

[0030] Compared with the prior art, the control method for natural oxidation of a primary loop of a nuclear power plant of the present invention is applicable to a VVER unit. Through the control of dissolved hydrogen in the primary loop, the concentration of dissolved hydrogen in the primary loop is effectively reduced, thereby extending the time of natural oxidation of the primary loop and ensuring the qualified water quality in the primary loop. By optimizing the monitoring sequence of chemical indicators after nitrogen purge in the primary loop, the inspection time is shortened and the inspection accuracy is improved. In the maintenance cold shutdown stage, water is exchanged between the primary loop and the boric acid storage tank to accelerate the process of natural oxidation of the chemical working condition of the primary loop water. In the stage where a large amount of activated corrosion products are released, they are introduced into the boric acid solution storage tank and simultaneously put into the spent fuel pool purification system to remove the activated corrosion products in the boric acid storage tank, thereby solving the problem that the radioactivity of the primary loop cannot be purified after the main pump is shut down after the nitrogen purge of the reactor is completed. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 A schematic diagram showing the structure of the device used for natural oxidation in one loop;

[0032] Figure 2 A graph showing the concentration variation trend of dissolved hydrogen and hydrogen peroxide during natural oxidation in one loop;

[0033] In the figure, 1-reactor pressure vessel; 2-main pump; 3-steam generator; 4-primary circuit purification system; 5-primary circuit ammonia tank; 6-nitrogen supply system; 7-degasser; 8-hydrogen combustion system; 9-primary circuit ammonia pump; 10-primary circuit ammonia tank; 11-primary circuit ammonia pump; 12-charging pump; 13-spent fuel pool purification system; 14-boric acid solution storage tank. DETAILED DESCRIPTION

[0034] 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.

[0035] The control method of the primary natural oxidation of the present invention is based on Figure 1 The device shown is complete.

[0036] The difference between VVER and other PWR units is that the primary circuit of the VVER is not to inject hydrogen directly, but to add ammonia water to the primary circuit coolant to produce hydrogen through ammonia irradiation decomposition. In normal operation, the concentration of dissolved hydrogen in the primary circuit coolant is required to be in the range of 2.2-4.5mg / L, and is actually controlled at the level of 2.8-3.5mg / L, which can inhibit the irradiation decomposition of water and ensure that the primary circuit is in a reducing water chemistry condition.

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

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

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

[0040] After the unit starts to reduce power, the operation 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 addition of ammonia to the primary circuit is stopped by stopping the operation of the primary circuit ammonia pump 9;

[0042] When the unit is reduced to zero power level, while injecting boron into the primary circuit, the operation of the primary circuit hydrazine pump 11 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 the boron injection is completed, the dissolved hydrogen concentration in the primary loop is maintained at 0.95-1.05 mg / L, preferably at a level of 1.0 mg / L.

[0044] Step 2: Purge the primary loop with nitrogen to remove hydrogen, specifically including:

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

[0046] The specific purging process is as follows: using the nitrogen supply system 6 to purge the hydrogen in the primary circuit to the degasser 7, and then passing through the hydrogen combustion system 8 to remove the hydrogen;

[0047] After the primary circuit begins to lose dissolved hydrogen rapidly, the highly radioactive environment begins to irradiate and decompose water, producing oxygen and hydrogen peroxide. Natural oxidation begins in the primary circuit, and the reaction formula is as follows:

[0048]

[0049] H2+·OH→H2O

[0050] ·OH+·OH→H2O2

[0051] 2H2O2→O2+2H2O

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

[0053] After the purge is completed, first measure the dissolved hydrogen concentration in the first circuit. When the dissolved hydrogen concentration in the first circuit is less than 0.5 mg / L and the dissolved hydrogen concentration is qualified, analyze the hydrogen concentration in the gas overflow area of ​​the control rod drive mechanism sleeve. When the hydrogen proportion in the gas overflow area of ​​the control rod drive mechanism sleeve is less than 0.8%, the purge is completed.

[0054] According to the traditional nitrogen purge method, it is necessary to simultaneously analyze the gas concentration of the primary loop dissolved hydrogen and the gas overflow area of ​​the control rod drive mechanism sleeve on the reactor top cover. According to historical data statistics, each sampling and analysis takes about 3 hours, and at least 3 analyses are required, which takes about 9 hours.

[0055] According to the method of the present invention, in order to prolong the time of natural oxidation, the dissolved hydrogen concentration of the first circuit is measured first. After the dissolved hydrogen concentration is less than 0.5 mg / L, the hydrogen in the gas overflow area of ​​the control rod drive mechanism sleeve is sampled and analyzed. One sampling can be qualified. According to practical statistics, the total time consumption is about 4.5 hours, which is 50% shorter than that before optimization.

[0056] Step 3: One-loop natural oxidation;

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

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

[0059] 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 corrosion products.

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

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

[0062] When implementing the control method, the present invention monitors the concentration trend of dissolved hydrogen and hydrogen peroxide during the natural oxidation of a loop, such as Figure 2 As shown. Figure 2 It can be seen that the present invention effectively suppresses the amount of dissolved hydrogen in the primary circuit, provides an environment for natural oxidation, and is conducive to the slow peeling of corrosion products; combined with subsequent water changes, it can reduce the activity concentration of activated corrosion products in the main system of the primary circuit and reduce their deposition on the surface of major equipment, thereby reducing the radiation dose field level during unit maintenance and reducing the exposure dose of workers in a radioactive environment.

[0063] 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.

[0064] 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 natural oxidation of a primary loop of a nuclear power plant, characterized in that: The following steps are involved: Step 1: Primary circuit hydrogen dissolution control, including: 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 to 2.2-2.3 mg / L; After the unit starts to reduce power, ammonia is intermittently supplied to the primary circuit; After the unit power is reduced to ≤40%, stop adding ammonia to the primary circuit; When the unit drops to zero power level, while injecting boron into the primary circuit, stop adding hydrazine into the primary circuit; Step 2: Purge the primary loop with nitrogen to remove hydrogen, specifically including: After the reactor is cooled to a cold state, the hydrogen in the primary circuit is purged; After purging for a period of time, measure the dissolved hydrogen concentration in the primary circuit first. After the dissolved hydrogen concentration is qualified, analyze the hydrogen proportion in the gas overflow area of ​​the control rod drive mechanism sleeve until the purging is completed; Step 3: One-loop natural oxidation; After the nitrogen purge of the reactor is completed, the boric acid solution storage tank and the primary circuit are used to replace water. The strong radioactivity in the primary circuit reacts with the oxygen in the boric acid solution to generate hydrogen peroxide, which causes the activated corrosion products to quickly peel off and turn 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 corrosion products.

2. The control method for natural oxidation of a nuclear power plant primary circuit according to claim 1, characterized in that: In step 1, the operation mode of the ammonia adding pump in one circuit is changed from continuous operation to intermittent operation.

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

4. The control method for natural oxidation of a nuclear power plant primary circuit according to claim 3, characterized in that: In step 1, after the boron injection is completed, the dissolved hydrogen concentration in the primary loop is maintained at 1 mg / L.

5. The control method for natural oxidation of a nuclear power plant primary circuit according to claim 1, characterized in that: In step 2, the hydrogen in the first circuit is purged to the degasser by using the nitrogen supply system, and then the hydrogen is removed by the hydrogen combustion system.

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

7. The control method for natural oxidation of a nuclear power plant primary circuit according to claim 1, characterized in that: In the step 2, after the dissolved hydrogen concentration in the primary circuit 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 proportion of hydrogen in the gas overflow area of ​​the control rod drive mechanism sleeve is less than 0.8%, the purge is completed.

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

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

10. The control method for natural oxidation of a nuclear power plant primary circuit according to claim 9, characterized in that: The preset flow rate is 10 kg / s.

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