Pre-oxidation method for boron-free nuclear reactor primary circuit system
By employing a pre-oxidation method for the primary loop system of a boron-free nuclear reactor, a dense passivation film is formed using hydrazine, an alkaline pH control agent, hydrogen, and hydrogen peroxide solution. This solves the problems of corrosion of structural materials and increased radiation field in boron-free nuclear reactors, thereby improving the stability and safety of structural materials.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- CHINA NUCLEAR POWER TECH RES INST CO LTD
- Filing Date
- 2023-09-13
- Publication Date
- 2026-06-05
AI Technical Summary
In boron-free nuclear reactors, the lack of effective pre-oxidation methods in existing technologies leads to the release and deposition of corrosion products on the surface of structural materials, affecting core safety. Furthermore, the corrosion activation products caused by changes in the hydrochemical environment increase the radiation field.
A boron-free pre-oxidation method is adopted, which forms a dense passivation film by adding hydrazine, an alkaline pH control agent, hydrogen gas and hydrogen peroxide solution to the coolant. This avoids the addition of boric acid, controls oxygen and impurity ions, and forms a dense oxide film to inhibit corrosion and deposition.
In boron-free nuclear reactors, a dense passivation film is formed to inhibit metal corrosion, reduce the release of radioactive impurities, maintain the consistency of the water chemical environment, improve the heat transfer and safety performance of the fuel cladding, and reduce the dose rate of the power plant.
Smart Images
Figure CN117165932B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of nuclear power material corrosion protection technology, and in particular to a pre-oxidation method for the primary loop system of a boron-free nuclear reactor. Background Technology
[0002] The primary loop system of a pressurized water reactor nuclear power plant includes a pressure vessel, steam generator, main pumps, pressurizer, and main piping. The primary coolant transfers heat generated in the reactor core from the pressure vessel to the steam generator, and then further transfers the heat to the secondary loop system through the steam generator's heat exchange tubes. During nuclear power plant operation, the structural materials of the primary loop system are exposed to the reactor coolant, resulting in surface corrosion. Corrosion products are subsequently released and deposited. When these corrosion products are released into the primary coolant, they may deposit on the surface of the fuel cladding in the reactor core, affecting core safety. Simultaneously, after migrating to the core with the coolant, the corrosion products are converted into corrosion activation products by neutron irradiation. These activated corrosion products can migrate with the coolant and deposit in other locations within the primary loop, increasing the radiation field.
[0003] Given the above, a hot functional test is conducted before the initial startup of the primary loop of a nuclear reactor. This involves increasing the temperature and pressure without loading nuclear fuel to simulate the actual operating conditions of a nuclear power plant as closely as possible, including anticipated operating events under typical temperature, pressure, and flow rates. A series of tests are then conducted during the subsequent cooling and depressurization process to verify the safety and practicality of the reactor unit and ensure its normal and stable operation. Pre-oxidation of the primary loop system materials refers to the process during the hot functional test where the temperature, pressure, and water chemistry parameters of the coolant in the primary loop system are adjusted to create a dense oxide film on the metal surfaces of the pressure vessel, main pipes, and in-core components that come into contact with the coolant. This dense oxide film is crucial for the normal operation of the nuclear power unit after nuclear fuel loading, as it inhibits metal corrosion and reduces the release of radioactive impurities. Furthermore, the morphology and properties of the oxide film are closely related to the coolant water chemistry environment.
[0004] Large pressurized water reactor (PWR) nuclear power plants typically employ a boron-containing water chemistry design for the primary coolant, using boric acid dissolved in the coolant for reactivity compensation and control. This, along with control rods and solid combustible poisons, controls the reactivity of the reactor core. Traditional large PWR nuclear power plants, to protect the integrity of the oxide film and inhibit the release and deposition of corrosion products on material surfaces, undergo pre-oxidation of the primary coolant materials under boron-containing conditions before the first start-up. With the development of small modular reactor (SMR) technology, the use of a boron-free design in the primary coolant, constrained by the space layout of SMRs, simplifies boron control systems such as boron recovery and boron water systems, significantly saving space and reducing construction, operation, and maintenance costs. Therefore, boron-free primary coolant design is an important development direction for advanced SMRs. Based on the differences in the primary coolant water chemistry environment of boron-free reactors, a primary coolant pre-oxidation process suitable for boron-free nuclear reactors needs to be proposed. Summary of the Invention
[0005] The technical problem to be solved by the present invention is to provide a pre-oxidation method for the primary loop system of a boron-free nuclear reactor.
[0006] The technical solution adopted by this invention to solve its technical problem is: to provide a pre-oxidation method for the primary loop system of a boron-free nuclear reactor, comprising the following steps:
[0007] S1. Before the first start-up of the boron-free nuclear reactor, when the temperature of the primary coolant is below 80°C, hydrazine is added to control the oxygen content in the primary coolant to below 0.01 mg / kg.
[0008] S2. When the primary coolant temperature rises to 80℃~120℃, add an alkaline pH control agent to adjust the pH of the primary coolant. T The value is 6.9–7.4, where T is 300℃;
[0009] S3. When the primary coolant is heated to 180℃~290℃, hydrogen is injected to make the dissolved hydrogen concentration in the primary coolant 5cc / kg~30cc / kg (STP).
[0010] S4. The primary coolant is passivated at a constant temperature of 290℃ for one cycle, so that a passivation film is formed on the surface of the structural materials of the primary system.
[0011] S5. After passivation is completed, hydrogen peroxide solution is injected into the primary coolant and the primary system is cleaned to remove the loose corrosion products on the surface of the structural material, leaving a dense passivation film on the surface of the structural material.
[0012] Preferably, in step S1, the primary loop system is physically vented before adding hydrazine.
[0013] Preferably, in step S2, an alkaline pH control agent is added to adjust the pH of the primary coolant. 25℃ The value is 9.5 to 10.1.
[0014] Preferably, the pH control agent is lithium hydroxide, potassium hydroxide, or ammonia.
[0015] Preferably, in step S2, when the pH control agent is lithium hydroxide, the concentration of lithium ions in the primary coolant is controlled to be 0.2 ppm to 0.6 ppm.
[0016] Preferably, in step S2, the pH of the primary coolant... T At a concentration of 7.2, the lithium ion concentration is 0.5 mg / kg.
[0017] Preferably, in step S4, one cycle is 10 to 30 days.
[0018] Preferably, step S3 further includes: adding depleted zinc acetate when the primary coolant temperature is 180°C to 290°C, so that the zinc concentration in the primary coolant is 10 ppb to 100 ppb.
[0019] Preferably, in step S5, before injecting the hydrogen peroxide solution, the hydrogen injection into the primary coolant is stopped, the temperature and pressure are reduced, and the hydrogen peroxide solution is injected after the temperature drops to 80°C.
[0020] Preferably, in step S5, before injecting the hydrogen peroxide solution, the hydrogen injection into the primary coolant is stopped, the pH of the primary coolant is adjusted to neutral, the temperature and pressure are reduced, and the hydrogen peroxide solution is injected after the temperature drops to 80°C.
[0021] The boron-free nuclear reactor primary loop system pre-oxidation method of the present invention does not add boric acid during pH control and adjustment, does not perform boration during the cleaning and loosening of corrosion products, avoids the introduction of other impurity ions, and maintains a certain concentration of dissolved hydrogen during passivation to obtain a better passivation effect. The above ensures the integrity of the passivation film on the surface of the primary loop system structural materials during the pre-oxidation stage, maintains the consistency of the water chemical environment of the primary loop system materials during the pre-oxidation stage and the power operation stage, and avoids corrosion release and deposition on the surface of structural materials caused by changes in the water chemical environment. In this way, it effectively improves the heat transfer and safety performance of the fuel cladding and reduces the power plant dose rate. Attached Figure Description
[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments. In the accompanying drawings:
[0023] Figure 1 This is a SEM image of the surface of a primary loop structural material after pre-oxidation using the boron-free nuclear reactor primary loop pre-oxidation method of the present invention.
[0024] Figure 2 This is a SEM image of the surface of another primary loop structural material after pre-oxidation using the boron-free nuclear reactor primary loop pre-oxidation method of the present invention. Detailed Implementation
[0025] The pre-oxidation method for the primary loop system of the boron-free nuclear reactor of the present invention serves as a pre-oxidation stage before the first start-up of the boron-free nuclear reactor, and pre-oxidizes the primary loop system to form a complete passivation film on the surface of the structural materials in the primary loop system.
[0026] The pre-oxidation method for the primary loop system of a boron-free nuclear reactor of the present invention may include the following steps:
[0027] S1. Before the first start-up of the boron-free nuclear reactor, when the temperature of the primary coolant in the primary loop system is below 80°C, hydrazine is added to control the oxygen content in the primary coolant to below 0.01 mg / kg.
[0028] In addition, in this step, before adding hydrazine, the primary loop system is physically vented to remove air (including oxygen) above the primary loop coolant. Then, the oxygen in the primary loop coolant is chemically removed by adding hydrazine.
[0029] S2. When the primary coolant temperature reaches 80℃~120℃, add an alkaline pH control agent to adjust the pH of the primary coolant. T The value is 6.9 to 7.4, where T is 300℃.
[0030] After adjusting the oxygen content in the primary coolant through step S1, the primary coolant is heated by means of a main pump and an electric heater. When the temperature of the primary coolant rises to 80℃~120℃, a pH control agent is added to adjust the pH value of the primary coolant.
[0031] The pH control agent is consistent with that used during actual operation of the boron-free reactor. In this invention, the pH control agent can be lithium hydroxide, potassium hydroxide, or ammonia, and is added in solution form. The pH of the primary coolant is adjusted by adding an alkaline pH control agent. 300℃ The pH ranges from 6.9 to 7.4; the primary coolant at 25°C corresponds to a pH of 9.5 to 10.1, i.e., pH... 25℃ The value is 9.5 to 10.1.
[0032] No boric acid is added during pH control, which effectively avoids the introduction of other impurities during the hot functional test stage. Combined with the water chemistry control conditions during the power operation stage, a more stable and dense passivation film (also known as an oxide film) can be obtained.
[0033] Furthermore, when lithium hydroxide is used as the pH control agent, the concentration of lithium ions in the primary coolant is controlled to be between 0.2 ppm and 0.6 ppm. Preferably, the pH of the primary coolant is... T The value is 7.2 (T is 300℃), corresponding to a lithium ion concentration of 0.5 mg / kg.
[0034] S3. The primary coolant is heated by the main pump and electric heater. When the primary coolant reaches 180℃~290℃, hydrogen is injected into the primary coolant to make the dissolved hydrogen concentration in the primary coolant 5cc / kg~30cc / kg (STP).
[0035] In dissolved hydrogen concentration, STP represents the standard temperature and pressure conditions (0°C, 1 atmosphere).
[0036] In step S3, hydrogen is preferably injected when the primary coolant temperature reaches 180°C, so that when the primary coolant temperature reaches 290°C or before 290°C, the dissolved hydrogen concentration in the primary coolant reaches 5cc / kg to 30cc / kg (STP).
[0037] Understandably, hydrogen can be injected after the primary coolant is heated to 180°C and before the temperature reaches 290°C. Preferably, the dissolved hydrogen concentration in the primary coolant reaches 5 cc / kg to 30 cc / kg (STP) when the temperature reaches 290°C.
[0038] The above-mentioned addition of a certain concentration of hydrogen during the passivation process is beneficial to the formation of a good double-layer passivation film on the surface of the structural material. The higher the concentration of dissolved hydrogen, the smaller and denser the inner crystals. However, the excessive addition of hydrogen can also bring certain negative effects, such as hydrogen absorption effect and influence on electrochemical potential. Therefore, in this invention, the concentration of dissolved hydrogen is controlled to be 5cc / kg to 30cc / kg (STP).
[0039] In addition, the decision on whether to add zinc to the primary coolant will depend on whether a zinc-injected aqueous chemistry environment is used during the subsequent power operation phase of the boron-free nuclear reactor's primary circuit.
[0040] For example, in the primary circuit power operation phase of a boron-free nuclear reactor, where a zinc-injected aqueous chemical environment is used, step S3 may further include: adding depleted zinc acetate when the primary circuit coolant temperature is 180°C to 290°C, so that the zinc concentration in the primary circuit coolant is 10 ppb to 100 ppb.
[0041] Preferably, when the primary coolant temperature reaches 180°C, depleted zinc acetate is added so that the zinc concentration in the primary coolant reaches 10 ppb to 100 ppb before the primary coolant temperature reaches 290°C.
[0042] The addition of zinc exposes the structural materials in the primary loop system to a sufficient zinc concentration. Taking advantage of the fact that zinc has a higher lattice energy than Fe, Ni, Co, and other atoms, zinc preferentially enters the lattice vacancies in the oxide film, forming a more stable oxide film. This reduces corrosion of the structural materials and also reduces the deposition of corrosion activation products during power operation.
[0043] If a zinc-free aqueous chemistry environment is used during the primary loop power operation phase of a boron-free nuclear reactor, then zinc does not need to be added in step S3.
[0044] S4. The primary coolant is passivated at a constant temperature of 290℃ for one cycle, resulting in the formation of a passivation film (oxide film) on the surface of the structural materials of the primary system.
[0045] Specifically, after step S3, the temperature of the primary coolant is maintained at 290°C for one cycle (i.e., passivation cycle) to form a passivation film on the surface of the structural material of the primary system (i.e., primary equipment).
[0046] One cycle (passivation cycle) is 10 to 30 days, and the specific time can be flexibly determined according to the actual situation, such as 15 days, which can be increased or decreased according to the actual situation.
[0047] S5. After passivation is completed, hydrogen peroxide solution is injected into the primary coolant and the primary system is cleaned to remove loose corrosion products on the surface of structural materials, leaving a dense passivation film on the surface of structural materials.
[0048] After the aforementioned passivation cycle, the passivation film formed on the surface of the structural materials of the primary loop system will have loose corrosion products. These corrosion products need to be cleaned and removed to retain the dense inner passivation film. This is to further reduce the release of corrosion products during the normal operation of the boron-free nuclear reactor.
[0049] Furthermore, the cleaning in step S5 can be performed in two ways: alkaline oxidation and neutral oxidation.
[0050] For alkaline oxidation methods: Before injecting hydrogen peroxide solution, hydrogen injection into the primary coolant is stopped, and the temperature and pressure are reduced. After the temperature drops to 80°C, hydrogen peroxide solution is injected. The primary coolant flows through treatment equipment such as a desalination bed or resin bed to remove loose corrosion products from the primary coolant.
[0051] For the neutral oxidation method, the alkaline substances (such as lithium, potassium, or ammonia) introduced in the previous steps by adding a pH control agent are first removed, and the pH of the primary coolant is adjusted to neutral. Before injecting hydrogen peroxide solution, hydrogen injection into the primary coolant is stopped, and the pH of the primary coolant is adjusted to neutral. The temperature and pressure are then reduced, and the hydrogen peroxide solution is injected after the temperature drops to 80°C. For example, when lithium hydroxide is added as the pH control agent, lithium is removed from the primary coolant in this step; the same applies to potassium hydroxide and ammonia.
[0052] Lithium removal from the primary coolant can be achieved by removing Li from the primary coolant drain stream through the desalination bed of the primary auxiliary system (chemical and volume control system) to reduce the primary coolant pH value.
[0053] In the pre-oxidation method of the primary loop system of the boron-free nuclear reactor of the present invention, when the primary loop coolant temperature is below 80°C, the pressure in the primary loop system is generally 2.5 MPa. As the primary loop coolant temperature rises, the pressure in the primary loop system also rises; when the primary loop coolant temperature rises to 290°C, the pressure in the primary loop system also rises to 15.5 MPa. Therefore, in the cleaning process of step S5, after the temperature drops to 80°C, the pressure in the primary loop system correspondingly drops to 2.5 MPa.
[0054] Figure 1 and Figure 2 The electron microstructure of the structural material surface of the primary loop system after pre-oxidation according to the present invention is shown. As can be seen from the figure, spinel-like oxides are generated on the metal surface of the primary loop device, with the largest particle size being less than 2 μm. The small-sized spinel oxides are distributed relatively uniformly, while the large-sized spinel oxides are distributed in an irregular dotted pattern, which has a good passivation protection effect.
[0055] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.
Claims
1. A pre-oxidation method for the primary loop system of a boron-free nuclear reactor, characterized in that, Includes the following steps: S1. Before the first start-up of the boron-free nuclear reactor, when the temperature of the primary coolant is below 80°C, hydrazine is added to control the oxygen content in the primary coolant to below 0.01 mg / kg. Before adding hydrazine, the primary loop system is physically vented to remove the air above the primary loop coolant, and then the oxygen in the primary loop coolant is removed by chemical means through the added hydrazine. S2. When the primary coolant temperature reaches 80℃~120℃, add an alkaline pH control agent to adjust the pH of the primary coolant. T The value is 6.9~7.4, where T is 300℃; No more boric acid is added during pH control, which effectively avoids the introduction of other impurities during the hot functional test stage. Combined with the water chemistry control conditions during the power operation stage, a more stable and dense passivation film can be obtained. S3. When the primary coolant is heated to 180℃~290℃, hydrogen is injected to make the dissolved hydrogen concentration in the primary coolant 5cc / kg~30cc / kg. S4. The primary coolant is passivated at a constant temperature of 290℃ for one cycle, so that a passivation film is formed on the surface of the structural materials of the primary system. S5. After passivation is completed, hydrogen peroxide solution is injected into the primary coolant and the primary system is cleaned to remove the loose corrosion products on the surface of the structural material, leaving a dense passivation film on the surface of the structural material. The cleaning process in step S5 can be performed using two methods: alkaline oxidation and neutral oxidation. For alkaline oxidation methods, before injecting hydrogen peroxide solution, hydrogen injection into the primary coolant is stopped, and the temperature and pressure are reduced. After the temperature drops to 80°C, hydrogen peroxide solution is injected. The primary coolant flows through a desalination bed or resin bed to remove loose corrosion products from the primary coolant. For the neutral oxidation method, before injecting hydrogen peroxide solution, hydrogen injection into the primary coolant is stopped, and the pH of the primary coolant is adjusted to neutral. The temperature and pressure are then reduced. After the temperature drops to 80°C, hydrogen peroxide solution is injected.
2. The pre-oxidation method for the primary loop system of a boron-free nuclear reactor according to claim 1, characterized in that, In step S2, an alkaline pH control agent is added to adjust the pH of the primary coolant. 25℃ The value is 9.5~10.
1.
3. The pre-oxidation method for the primary loop system of a boron-free nuclear reactor according to claim 1, characterized in that, The pH control agent is lithium hydroxide, potassium hydroxide, or ammonia.
4. The pre-oxidation method for the primary loop system of a boron-free nuclear reactor according to claim 3, characterized in that, In step S2, when the pH control agent is lithium hydroxide, the concentration of lithium ions in the primary coolant is controlled to be 0.2 ppm to 0.6 ppm.
5. The pre-oxidation method for the primary loop system of a boron-free nuclear reactor according to claim 4, characterized in that, In step S2, the pH of the primary coolant... T At a concentration of 7.2, the lithium ion concentration is 0.5 mg / kg.
6. The pre-oxidation method for the primary loop system of a boron-free nuclear reactor according to claim 1, characterized in that, In step S4, one cycle is 10 to 30 days.
7. The pre-oxidation method for the primary loop system of a boron-free nuclear reactor according to any one of claims 1-6, characterized in that, Step S3 further includes: adding depleted zinc acetate when the primary coolant temperature is 180℃~290℃, so that the zinc concentration in the primary coolant is 10ppb~100ppb.