Method for reducing hydrogen and deuterium concentration at key position of heavy water reactor pressure pipe

Abstract

The invention particularly relates to a method for reducing hydrogen and deuterium concentration at a key position of a heavy water reactor pressure pipe, and belongs to the technical field of aging protection of heavy water reactor pressure pipe materials, the pressure pipe is made of zirconium alloy, two ends of the pressure pipe are in expanded connection with end parts, and an expanded connection pressure stress area, an expanded connection tensile stress area and a pipe body area are defined; the method comprises the steps that at least one hydrogen absorption material block is fixedly installed in the area, corresponding to an expanded connection tensile stress area, of the outer surface of the pressure pipe; the hydrogen absorption material block is made of yttrium or yttrium alloy; and the hydrogen absorption material block is used for absorbing hydrogen and deuterium diffused to the expanded connection tensile stress area from the expanded connection pressure stress area of the pressure pipe. According to the method, the hydrogen equivalent concentration of the expanded connection tensile stress area, especially the polishing mark position is obviously reduced, the hydrogen equivalent concentration is prevented from exceeding the safety limit value, and the service life of the pressure pipe is prolonged.

Description

Technical Field

[0001] This invention relates to the field of aging protection technology for pressure tube materials in heavy water reactors, and in particular to a method for reducing the hydrogen-deuterium concentration at critical locations in the pressure tubes of heavy water reactors. Background Technology

[0002] The pressure tube is a core component of the primary loop pressure boundary in a heavy water reactor unit, and is typically made of Zr-2.5Nb alloy. During long-term service, the pressure tube absorbs hydrogen and deuterium, primarily deuterium. When the total amount of absorbed hydrogen and deuterium exceeds the material's solid solubility, hydrogen / deuterides will precipitate, leading to decreased material toughness and increased brittleness. This may trigger hydrogen-induced delayed cracking, threatening nuclear safety.

[0003] Especially in the expansion joint areas where pressure pipes connect to end components, the deuterium concentration is much higher than in the middle of the pipe body due to multiple deuterium adsorption pathways such as galvanic corrosion and crevice corrosion, making it the most critical area prone to hydrogen-induced failure. Currently, the industry mainly relies on periodic testing of hydrogen and deuterium concentrations to assess risk, and there is a lack of effective engineering methods to reduce the hydrogen and deuterium concentration in this area.

[0004] Therefore, there is an urgent need for a hydrogen-deuterium concentration mitigation technology that is easy to implement and does not affect the structural integrity of the pressure tube or the system function. Summary of the Invention

[0005] The purpose of this invention is to provide a method for reducing the hydrogen-deuterium concentration at critical locations in the pressure tube of a heavy water reactor. By installing a highly efficient hydrogen-absorbing material at a specific location on the outside of the pressure tube, the hydrogen-deuterium that diffuses from the expansion joint compressive stress zone to the expansion joint tensile stress zone is actively absorbed and fixed, thereby significantly reducing the equivalent hydrogen concentration in the expansion joint tensile stress zone, especially at the polished marking location, preventing it from exceeding the safety limit, extending the service life of the pressure tube, and improving the operational safety of the heavy water reactor.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for reducing hydrogen and deuterium concentrations at critical locations in a heavy water reactor pressure tube, wherein the pressure tube is made of zirconium alloy, and its two ends are connected to end components by expansion joints, and expansion joint compressive stress zones, expansion joint tensile stress zones, and tube body zones are defined; the method includes: At least one hydrogen-absorbing material block is fixedly installed on the outer surface of the pressure tube in the region corresponding to the tensile stress zone of the expansion joint. The hydrogen-absorbing material block is made of yttrium or a yttrium alloy; The hydrogen-absorbing material block is used to absorb hydrogen and deuterium that diffuse from the pressure stress zone of the expansion joint to the tensile stress zone of the expansion joint.

[0007] As one possible approach, the hydrogen-absorbing material block is fixed to the outer surface of the pressure tube by brazing.

[0008] As one possible approach, before installing the hydrogen-absorbing material block, the mounting surfaces of the pressure tube and the hydrogen-absorbing material block are polished to remove the surface oxide film.

[0009] As one possible approach, after installing the hydrogen-absorbing material block, the outer surface of the hydrogen-absorbing material block and the adjacent pressure pipe is oxidized to form a surface oxide film.

[0010] As one possible approach, the hydrogen-absorbing material blocks are arranged discontinuously around the pressure tube.

[0011] As one possible implementation, the number of hydrogen-absorbing material blocks is 1 to 8, and they are distributed circumferentially.

[0012] As one possible approach, the hydrogen-absorbing material block is installed near the BM position of the end component in the expansion tensile stress zone.

[0013] As one possible approach, the proximal end of the hydrogen-absorbing material block is less than 40 mm from the BM location.

[0014] As one possible implementation, the hydrogen-absorbing material block has a thickness of 5-15 mm and a width of 10-40 mm.

[0015] As one possible approach, the total hydrogen absorption capacity of the hydrogen-absorbing material block is designed based on the remaining service life of the pressure tube and the hydrogen-deuterium flux diffusing from the expansion joint compressive stress zone to the expansion joint tensile stress zone.

[0016] Compared with the prior art, the present invention has the following beneficial technical effects: Significantly reduce hydrogen and deuterium concentration in critical areas of the pressure tube: By setting up an efficient "hydrogen trap" along the hydrogen and deuterium diffusion path, hydrogen and deuterium diffused into the expansion joint tensile stress zone can be actively and continuously absorbed, reducing the hydrogen and deuterium concentration at BM by more than 50%, effectively preventing it from exceeding the safety limit.

[0017] Convenient to implement and safe and reliable: This method is achieved through external installation, without involving the processing or modification of the pressure pipe body structure, without affecting the rolling and expansion connection between the pressure pipe and the end components, and without adversely affecting the pressure-bearing function and structural integrity of the pressure pipe. The hydrogen-absorbing material block is small in size and installed in a circumferentially discontinuous manner, without affecting the flow and leakage monitoring function of the annular gas.

[0018] The principle is clear and the effect is long-lasting: It utilizes the physical principle that yttrium or yttrium alloys have a higher hydrogen affinity and solid solubility than zirconium alloys, resulting in a strong driving force. By rationally designing the volume and installation position of the hydrogen-absorbing material block, it can be ensured that it works effectively throughout the entire design life of the pressure tube.

[0019] Good adaptability: This method mainly targets the interception of hydrogen / deuterium diffused into the expansion junction area. It is universally applicable to different operating conditions and sources of hydrogen and deuterium, and can be widely used to mitigate hydrogen-induced aging in pressure tubes of various heavy water reactors. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the connection structure and partitioning between the pressure pipe and the end components; Figure 2 This is a schematic diagram showing the distribution of deuterium concentration, temperature, and irradiation dose along the axial direction of the pressure tube after long-term service. Figure 3 A cross-sectional view showing the installation location of the hydrogen absorption material block on the outer surface of the pressure pipe; Figure 4 Top view of the installation position of the hydrogen absorption material block on the outer surface of the pressure pipe; Figure 5 A comparison of axial deuterium concentration distribution in pressure pipes with and without hydrogen-absorbing material blocks after 250,000 hours of service, obtained from simulation calculations. Figure 6 The deuterium concentration distribution of the expansion joint tensile stress zone from the inlet position after 250,000 hours of service, obtained from simulation calculations, with and without the hydrogen absorption material block installed in the pressure pipe; Figure 7 This is a simulation calculation of the deuterium concentration distribution of the expansion joint tensile stress zone from the outlet position of the pressure pipe with and without hydrogen absorption material blocks after 250,000 hours of service.

[0021] In the image, BM stands for Polishing. Detailed Implementation

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein in the specification of the application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0023] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0024] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments.

[0025] This invention uses the CANDU6 heavy water reactor pressure tube as an example, but it is not limited to this. The CANDU6 heavy water reactor has a horizontal structure and uses heavy water as both the reactor coolant and moderator. Inside a cylindrical tube container filled with cryogenic, low-pressure heavy water moderator, 380 pressure tubes run through the tubes of the container. The fuel rod bundles used in the reactor are installed within the pressure tubes in the tubes. The pressure tubes are approximately 6.3 meters long, and both ends are connected to the end components via a rolling expansion joint process. The outer side is the tube, and the gap between the pressure tubes and the tubes is filled with annular gas. A schematic diagram of the connection and zoning of the pressure tubes and end components is shown below. Figure 1 As shown, the end component is made of 403 stainless steel, and the pressure tube is made of Zr-2.5Nb. According to relevant standards, the axial area from the end face of the pressure tube to the polished mark is called the expansion joint compressive stress zone, where hydrogen-induced delayed cracking will not occur, so there is no limit to the equivalent hydrogen concentration in this section; while the axial area from the BM of the pressure tube to 300mm inside the BM is called the expansion joint tensile stress zone, where the equivalent hydrogen concentration is strictly limited; the other areas of the pressure tube are called the tube body area.

[0026] The hydrogen equivalent concentration is used to uniformly measure the influence of hydrogen and deuterium. It is the total hydrogen mass fraction calculated by taking the molecular weight of deuterium as equivalent to that of hydrogen. The formula is as follows: Heq = [H] + 1 / 2 [D] Where Heq is the equivalent concentration of hydrogen in ppm; [H] is the mass fraction of hydrogen in ppm; and [D] is the mass fraction of deuterium in ppm.

[0027] The initial hydrogen content of the pressure tube is very low, and very little hydrogen is absorbed during service. Therefore, the main consideration is the deuterium absorbed during service. The following case mainly considers the effect of deuterium.

[0028] The axial distribution of deuterium concentration in a pressure tube after long-term service is shown below. Figure 2 As shown, the deuterium concentration in the tensile stress zone at both ends is much higher than that in the tube body zone, and the deuterium effective concentration at BM in the tensile stress zone is the highest, making it the most likely location to exceed the standard. Therefore, the main purpose of this invention is to reduce the hydrogen equivalent concentration near BM in the tensile stress zone.

[0029] Studies have shown that zirconium alloys have a strong affinity for hydrogen and deuterium and can absorb low concentrations of hydrogen in stainless steel. Yttrium and its alloys have an even stronger affinity for hydrogen and deuterium than zirconium alloys and can absorb hydrogen and deuterium from zirconium alloys. Therefore, this invention provides an embodiment of a method for reducing the hydrogen and tritium concentration at critical locations in the pressure tube of a heavy water reactor, in order to reduce the hydrogen and deuterium concentration at the pressure tube BM.

[0030] The method for reducing the hydrogen-deuterium concentration at critical locations in the pressure tube of a heavy water reactor, as described in this embodiment, includes the following steps: At least one hydrogen-absorbing material block is fixedly installed on the outer surface of the pressure pipe, in the area corresponding to the tensile stress zone of the expansion joint; The hydrogen-absorbing material block is made of yttrium or a yttrium alloy; the hydrogen-absorbing material block is used to absorb hydrogen and deuterium that diffuse from the pressure pipe expansion joint compressive stress zone to the expansion joint tensile stress zone.

[0031] In this embodiment, the hydrogen-absorbing material block is fixed to the outer surface of the pressure tube by brazing; Before installing the hydrogen absorption material block, the mounting surfaces of the pressure tube and the hydrogen absorption material block are polished to remove the oxide film at their interface and improve the diffusion efficiency of hydrogen and deuterium at the interface. After the hydrogen-absorbing material block is installed, the outer surface of the hydrogen-absorbing material block and the adjacent pressure pipe is oxidized to form a dense oxide film to prevent the absorption of trace amounts of hydrogen and deuterium in the annular gas.

[0032] Most of the hydrogen and deuterium at the BM (bulk bore) originates from axial diffusion of deuterium absorbed by the galvanic-crevice corrosion near the pressure tube end face. Simultaneously, the hydrogen and deuterium at the BM continue to diffuse into the tube body. The diffusion process is primarily influenced by the concentration gradient, and the final diffusion distance is mainly affected by temperature and time. When the temperature remains constant, the square of the average diffusion distance is proportional to time. Based on experimental results, taking 310℃ as an example, after 30 years of service, the average axial diffusion distance of deuterium is approximately 500 mm. Since the pressure tube wall thickness is only about 4.2 mm, it only takes about 0.7 days for the average diffusion distance to reach the entire wall thickness. Therefore, when... Figure 3 After adding yttrium or yttrium alloy to the indicated location, the hydrogen and deuterium in the pressure tube at that location will be quickly absorbed by the yttrium or yttrium alloy. The equilibrium partial pressure of hydrogen in the two differs by more than an order of magnitude, meaning that when equilibrium is reached, the remaining hydrogen and deuterium in the pressure tube will be less than 1 / 10 of that in the yttrium or yttrium alloy. The distance from the location of the yttrium or yttrium alloy block to BM is less than 1 / 2 of the distance from the end face to BM. Assuming that the end face diffuses continuously towards BM at a saturated concentration of TSSD, and simulating a 30-year service life at 310℃, without the addition of the yttrium or yttrium alloy block, the deuterium concentration at BM will be very close to the TSSD. However, with the addition of the yttrium or yttrium alloy block, assuming that the hydrogen and deuterium concentration in the yttrium or yttrium alloy is not saturated and the interface diffusion is faster, the deuterium concentration at BM will drop to below 1 / 2 TSSD, demonstrating a significant effect.

[0033] Yttrium or yttrium alloys are relatively soft, and adding circumferentially discontinuous yttrium or yttrium alloy blocks to the outside of the pressure tube has virtually no impact on the stress on the pressure tube. The space between the pressure tube and the manifold contains annular gas, mainly used for leak monitoring. The flow rate is relatively slow, and when the height of the yttrium or yttrium alloy block does not exceed the end face height of the end component, it has virtually no effect on the flow of the annular gas.

[0034] like Figure 3As shown, the hydrogen-absorbing material block is installed on the outer surface of the pressure tube near the end component. The dimensions in the figure are for illustrative purposes only. The typical distance from the near end of the hydrogen-absorbing material block to BM is less than 40 mm. The typical thickness of the hydrogen-absorbing material block is 12 mm, the typical width is 40 mm, and the length is designed according to the required hydrogen absorption capacity to ensure that the hydrogen and deuterium absorption capacity is sufficient to support 30 years of service. For ease of installation, the hydrogen-absorbing material block is not a complete ring, but is installed in multiple pieces, such as four pieces. When the circumferential deuterium distribution in the tensile stress zone of the pressure tube expansion joint is uneven, only one piece can be installed in the direction of the highest concentration.

[0035] Figure 4 and Figure 5 Simulation results of deuterium concentration distribution after 250,000 hot hours of service for pressure tubes with and without yttrium blocks show that the deuterium concentration in the expansion joint tensile stress zone of the pressure tube with yttrium blocks is significantly reduced. The highest deuterium concentration in the expansion joint tensile stress zone on both the inlet and outlet sides is more than 50% lower than the highest concentration without yttrium blocks, demonstrating a very significant effect. Furthermore, the highest deuterium concentration in the yttrium alloy block does not exceed 300 ppm, far below the solid solubility of deuterium, and will not cause cracking of the yttrium block due to deuteride formation. The solid solubility of deuterium is >3000 ppm.

[0036] The present invention selects yttrium or yttrium alloys as hydrogen-absorbing materials because yttrium or yttrium alloys absorb hydrogen more easily than zirconium alloys. The hydrogen equilibrium partial pressure of yttrium or yttrium alloys is extremely low. Even at 300°C, when the saturation concentration is reached, the hydrogen equilibrium partial pressure is far below 0.1 Pa, which is about two orders of magnitude lower than that of zirconium alloys. Therefore, theoretically, yttrium or yttrium alloys can steal hydrogen from zirconium alloys.

[0037] The annular gas contains small amounts of hydrogen and deuterium, with an expected value of <0.01% and an allowable value of <0.1%. However, yttrium or yttrium alloys have extremely low hydrogen equilibrium partial pressures, allowing them to absorb even trace amounts of deuterium. When the hydrogen content exceeds the solid solubility, yttrium hydride is formed, which is prone to cracking, pulverizing, and flaking off, becoming foreign matter. Therefore, the parts of yttrium or yttrium alloys that are not in contact with the pressure tube must undergo oxidation treatment to form a dense oxide film. Y₂O₃ is an excellent hydrogen barrier material, almost completely preventing yttrium from absorbing hydrogen. The annular gas contains 0.5%~2.0% oxygen, ensuring the integrity of the surface oxide film of yttrium or yttrium alloys during long-term service.

[0038] When connecting yttrium or yttrium alloys to pressure tubes, the surface oxide film must be cleaned off before brazing, a process known as metallurgical bonding. Pressure tubes undergo surface oxidation treatment before service to prevent hydrogen absorption; without removing the oxide film, hydrogen cannot be released. Even after removing the oxide film, mechanical connections using expansion joints between end components and pressure tubes create micro-gaps at the interface. Hydrogen in the pressure tube requires at least 400°C to desorb into these gaps, which is insufficient at the maximum service temperature of 320°C. Only through metallurgical bonding can hydrogen diffuse more easily.

[0039] The hydrogen solubility in yttrium or yttrium alloys is very high, reaching over 3000 ppm at service temperatures. This allows them to absorb large amounts of hydrogen isotopes in pressure tubes without precipitating hydrides. In contrast, the hydrogen solubility in zirconium alloys is only 100 ppm.

[0040] The yttrium or yttrium alloy must be installed on the outer surface of the pressure tube, close to the end assembly. This is because, firstly, the pressure tube contains the fuel rod bundle, and space does not allow for this; secondly, mounting it externally on the end assembly would be ineffective for the following reasons: From the perspective of the source term, the deuterium in the pressure tube expansion joint area mainly originates from the galvanic gap corrosion between the pressure tube and the end component, rather than from the diffusion of hydrogen / deuterium in the end component into the pressure tube through the interface between the two. From a diffusion perspective, the end component and the pressure tube are mechanically connected, and at the service temperature, hydrogen in the pressure tube cannot be released into the gap and then diffuse into the end component; If the hydrogen is installed on the outer surface of the pressure tube at a location far from the end component, the hydrogen at BM diffuses a greater distance to yttrium, while the hydrogen at the end face diffuses a shorter distance to BM, thus the decrease in deuterium concentration at BM is limited. The outer surface of the pressure tube is filled with annular gas. The yttrium block is close to the end component and its height is lower than that of the end component. It has little impact on the gas flow state and does not affect the original function.

[0041] The applicant experimentally measured parameters such as hydrogen absorption, diffusion, and desorption in end components, pressure tubes, and yttrium blocks. Based on these measurements, modeling and calculations were performed, demonstrating the feasibility of the invention. After 30 years of service, the total hydrogen / deuterium content in the yttrium block does not exceed 300 ppm, far below the solubility, and will not cause the yttrium block to break.

[0042] Installing the components in multiple circumferential sections instead of the entire ring not only facilitates installation by allowing them to be installed after the end components are completed, but also avoids subjecting the pressure pipe to significant additional stress.

[0043] In the later stages of service, the hydrogen concentration distribution in the circumferential direction of the pressure tube becomes uneven, with the hydrogen concentration at the 12 o'clock position at the top of the pressure tube being significantly higher. At least one yttrium block needs to be installed at the top of the pressure tube.

[0044] Some researchers have tried adding yttrium to the pressure tube material to increase its solid hydrogen capacity, but the mechanical properties of the pressure tube material itself decrease after adding yttrium, failing to meet the requirements.

[0045] Therefore, installing a yttrium / yttrium alloy block on the outer surface of the pressure tube near the end component is currently the only engineering solution to reduce the hydrogen concentration at critical locations in the pressure tube.

[0046] The above-described embodiments are merely one implementation of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for reducing hydrogen concentration at critical locations in a heavy water reactor pressure tube, wherein the pressure tube is made of zirconium alloy, and its two ends are connected to end components by expansion joints, and an expansion joint compressive stress zone, an expansion joint tensile stress zone, and a tube body zone are defined; characterized in that, The method includes: At least one hydrogen-absorbing material block is fixedly installed on the outer surface of the pressure tube in the region corresponding to the tensile stress zone of the expansion joint. The hydrogen-absorbing material block is made of yttrium or a yttrium alloy; The hydrogen-absorbing material block is used to absorb hydrogen and deuterium that diffuse from the pressure stress zone of the expansion joint to the tensile stress zone of the expansion joint.

2. The method according to claim 1, characterized in that, The hydrogen-absorbing material block is fixed to the outer surface of the pressure tube by brazing.

3. The method according to claim 1 or 2, characterized in that, Before installing the hydrogen-absorbing material block, the mounting surfaces of the pressure tube and the hydrogen-absorbing material block are polished to remove the surface oxide film.

4. The method according to claim 1 or 2, characterized in that, After the hydrogen-absorbing material block is installed, the outer surface of the hydrogen-absorbing material block and the adjacent pressure pipe is oxidized to form a surface oxide film.

5. The method according to claim 1, characterized in that, The hydrogen-absorbing material blocks are arranged discontinuously around the circumference of the pressure pipe.

6. The method according to claim 5, characterized in that, The number of hydrogen-absorbing material blocks is 1 to 8, and they are distributed circumferentially.

7. The method according to claim 1, characterized in that, The hydrogen-absorbing material block is installed near the BM position of the end component in the expansion tensile stress zone.

8. The method according to claim 7, characterized in that, The proximal end of the hydrogen-absorbing material block is less than 40 mm from the BM position.

9. The method according to claim 1, characterized in that, The thickness of the hydrogen-absorbing material block is 5-15 mm, and the width is 10-40 mm.

10. The method according to claim 1, characterized in that, The total hydrogen absorption capacity of the hydrogen-absorbing material block is designed based on the remaining service life of the pressure tube and the hydrogen-deuterium flux diffusing from the expansion joint compressive stress zone to the expansion joint tensile stress zone.

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