Fuel assembly and method for manufacturing fuel assembly

Abstract

To provide a fuel assembly including a coating layer achieving both oxidation resistance under high temperature in an accident of a nuclear reactor and corrosion resistance in an aqueous environment in contact with cooling water during normal operation of the nuclear reactor, and to provide a method of manufacturing the fuel assembly.SOLUTION: The fuel assembly includes a base material 1 formed of a zirconium alloy and a coating layer 10, and the coating layer 10 has a chromium layer 2 formed of chromium or a chromium alloy and a corrosion resistant layer 3 formed of a zirconium alloy or a titanium alloy. A method for manufacturing a fuel assembly includes a step of preparing a base material 1, a step of forming a chromium layer 2 on a surface of the base material 1 in contact with cooling water, a step of forming a corrosion-resistant layer 3 on a surface of the chromium layer 2, and a step of assembling the fuel assembly using the base material 1, in which the chromium layer 2 and the corrosion-resistant layer 3 are formed by a thin plate cladding method, a physical vapor deposition method, a thermal spraying method, a cold spraying method, or a plating method before assembly using the base material 1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a fuel assembly for a water-cooled nuclear reactor that uses light water or the like as a coolant, and to a method for manufacturing a fuel assembly for a water-cooled nuclear reactor. [Background technology]

[0002] Water-cooled reactors use water, such as light water, as a coolant. Water-cooled reactors include boiling water reactors (BWRs) and pressurized water reactors (PWRs). In water-cooled reactors, zirconium alloys containing niobium, tin, iron, chromium, nickel, etc. have been used as the core structure material because of their small thermal neutron absorption cross section and excellent corrosion resistance.

[0003] In the event of a severe accident in a water-cooled nuclear reactor, the materials of the reactor core structure are exposed to a high-temperature oxidizing environment in the presence of high-temperature steam. Therefore, in recent years, chromium coating has been considered for the zirconium alloy components that make up the fuel assemblies of the reactor core in order to improve their resistance to high-temperature oxidizing environments. This technology involves coating the surface of a substrate made of a zirconium alloy with chromium or chromium compounds such as oxides, nitrides, and carbides to suppress oxidation of the substrate and hydrogen generation at high temperatures.

[0004] Fuel assemblies are loaded into the core of light water reactors such as BWRs and PWRs. A fuel assembly is a structure in which multiple fuel rods filled with fuel pellets are aligned and supported by upper and lower tie plates. In a BWR fuel assembly, the aligned fuel rods are surrounded from the outside by a channel box. The channel box serves to rectify the flow of cooling water and to prevent the control rods, which move around on the outside, from coming into contact with the fuel rods.

[0005] A fuel rod is a component in which multiple fuel pellets are stacked and loaded into a fuel cladding tube approximately 4 m long. Both ends of the fuel rod are sealed with end plugs. Zirconium alloys, which have a small thermal neutron absorption cross section and excellent corrosion resistance, have traditionally been used as the material for fuel cladding tubes and end plugs. Therefore, the fuel cladding tubes and end plugs that form the fuel rods are excellent in neutron economy and have been used safely in the reactor environment during normal reactor operation.

[0006] The channel box is a cylindrical part that is rectangular in top view and is provided in a structure that surrounds the fuel rods arranged in a matrix. The channel box is in contact with the outer surfaces of the upper tie plate and the lower tie plate and is supported by the upper tie plate. The channel box is made of a zirconium alloy, the same material as the fuel cladding tubes and end plugs.

[0007] In a water-cooled reactor, if a loss-of-coolant accident occurs, the temperature inside the reactor rises due to the heat generated by the nuclear fuel, generating high-temperature steam inside the reactor. Furthermore, if the cooling water decreases and the fuel rods become exposed, the temperature of the fuel rods rises to over 1000°C, causing a reaction between the zirconium alloy that forms the fuel rods and the steam. The hydrogen generated by the oxidation-reduction reaction between zirconium and water could cause an explosion. The generation of large amounts of hydrogen due to the oxidation of zirconium is an event that should be strictly avoided.

[0008] In recent nuclear reactors, multiple emergency equipment such as emergency power supplies and emergency core cooling systems have been installed to prevent loss of coolant and explosion accidents. System designs have been implemented to enhance safety, and further improvements and modifications to emergency equipment have been made.

[0009] For example, emergency core cooling systems are required to have the ability to contain an accident while maintaining a shape that allows the fuel assemblies to be cooled in the event of a loss of coolant accident. Performance evaluation guidelines include maintaining the maximum temperature of the fuel cladding below 1200°C, maintaining the amount of oxidation of the fuel cladding below 15% of the level before oxidation reactions become significant, keeping the amount of hydrogen generated by the reactor internals low enough to ensure the integrity of the containment vessel, and continuing to remove decay heat for a long period of time even when changes in the shape of the nuclear fuel are taken into account.

[0010] In the fields of design and evaluation of nuclear facilities, an accident that progresses under predetermined criteria assumed for setting and evaluating design conditions is called a DBA (Design Basis Accident). Nuclear facilities are designed to meet the specified criteria for DBAs. On the other hand, an accident that is more serious than a DBA is called a BDBA (Beyond Design Basis Accident). In recent years, efforts have been made to strengthen safety in both software and hardware in preparation for the occurrence of a BDBA.

[0011] Strengthening safety is not limited to system design; materials for the core structure are also being considered. For DBA, materials that reduce the amount of hydrogen generated by oxidation-reduction reactions with water and materials that suppress the leakage of fission products are being considered. For BDBA, where the temperature of the core structure exceeds 1200°C, materials that reduce the amount of hydrogen generated by improving oxidation resistance at high temperatures and materials that slow the rate of temperature rise are being considered.

[0012] For example, instead of zirconium alloys, aluminum-containing stainless steel and ceramics such as silicon carbide, which have excellent oxidation resistance, are being considered for use as materials for fuel cladding tubes and end plugs. Furthermore, the use of ceramics such as silicon carbide is also being considered as materials for channel boxes. Furthermore, methods for coating the surface of zirconium alloys are being investigated. There is technology for forming a coating film on zirconium alloys, which have a proven track record in actual reactors and for which regulatory data is available, to improve their oxidation resistance at high temperatures.

[0013] One technology for forming a coating film is chromium coating, which involves covering the material with chromium. Coating the zirconium alloy that forms the fuel cladding with chromium improves oxidation resistance at high temperatures, thereby reducing the generation of hydrogen due to oxidation-reduction reactions with the coolant. Chromium also has higher high-temperature strength than zirconium alloys, so deformation of chromium-coated zirconium alloy components at high temperatures is suppressed. Since the opening in the event of ballooning or rupture of the fuel cladding is smaller, it is expected that blockage of the coolant flow path, oxidation inside the fuel cladding, and exposure of the fuel pellets will be suppressed.

[0014] Patent Document 1 describes a multilayer material used in the nuclear power field, which includes a zirconium-based substrate covered with a multilayer coating. The multilayer coating is said to be made of metal layers selected from chromium, chromium alloys, or Nb-Cr-Ti ternary alloys. The layer made of the Nb-Cr-Ti ternary alloy is provided between the substrate and a layer made of chromium or chromium alloy. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Special Publication No. 2015-523231 Summary of the Invention [Problem to be solved by the invention]

[0016] While the chromium layer formed by chromium coating has excellent oxidation resistance at high temperatures, it is susceptible to corrosion in the aqueous environment during normal operation of a nuclear reactor, where the corrosion potential is high, and therefore corrosion resistance must be carefully considered. Chromium forms oxides and oxyhydroxides in aqueous environments where water is present. Chromium oxides and oxyhydroxides are easily dissolved in aqueous environments with high dissolved oxygen concentrations and hydrogen peroxide concentrations, resulting in a high corrosion potential, which can weaken their function as a protective film.

[0017] Furthermore, the coated chromium dissolves into the cooling water as chromate ions. Chromate ions affect the electrical conductivity and impurity concentration of the cooling water, which may affect the water quality management and reactor control. An increase in the chromate concentration in the cooling water increases the load on the reactor's purification system.

[0018] Furthermore, chromium has a thermal neutron absorption cross section approximately 15 times that of zirconium, making it a material with high thermal neutron absorption. When coating core structures with chromium, from the perspective of neutron economy, the chromium layer should be as thin as possible. On the other hand, if the chromium layer is too thin, it is likely to disappear if chromium dissolves in the cooling water. If the chromium layer disappears, the effects of increasing oxidation resistance at high temperatures and suppressing deformation of materials at high temperatures will be lost.

[0019] The chromium layer formed by chromium coating is provided adjacent to another layer, such as a substrate made of a zirconium alloy. When the chromium layer is exposed to high temperatures during a nuclear reactor accident, interdiffusion of atoms between the chromium layer and the adjacent layer progresses. Furthermore, oxidation progresses from the surface side that comes into contact with the cooling water. If diffusion of heteroatoms progresses in the chromium layer, there is a risk that the melting point of the chromium layer will decrease. Under high temperatures during a nuclear reactor accident, melting will progress from the region with a low melting point, and the chromium layer may eventually disappear.

[0020] Under these circumstances, a technology is needed that can provide both oxidation resistance under high temperatures during a reactor accident and corrosion resistance in the aqueous environment where the reactor is exposed to cooling water during normal operation. It is desirable for the chromium layer formed by chromium coating to minimize chromium elution to ensure appropriate neutron economy and protective film function. It is also desirable to suppress atomic interdiffusion under high temperatures during a reactor accident, thereby preventing the chromium layer from melting or disappearing.

[0021] In Patent Document 1, in addition to a chromium layer made of chromium or a chromium alloy, a layer made of an Nb-Cr-Ti ternary alloy is formed. However, the Nb-Cr-Ti ternary alloy contains 50% to 75% niobium in atomic percentage. The Nb-Cr-Ti ternary alloy, which is mainly composed of niobium, may generate niobium oxide in an aqueous environment where it comes into contact with cooling water. Niobium oxide is soluble, making it difficult to maintain its function as a protective film for a long period of time.

[0022] Furthermore, in Patent Document 1, a multilayer coating is formed in which many thin metal layers are stacked. This structure presents a problem in that the distance between the layer made of chromium or a chromium alloy and the layer made of an Nb-Cr-Ti ternary alloy is small, which makes interdiffusion of atoms more likely to occur at high temperatures. This interdiffusion of atoms between adjacent layers can lead to melting or disappearance of the chromium layer.

[0023] Therefore, an object of the present invention is to provide a fuel assembly having a coating layer that has both oxidation resistance at high temperatures in the event of a reactor accident and corrosion resistance in an aqueous environment where the fuel assembly comes into contact with cooling water during normal reactor operation, and a method for manufacturing the fuel assembly. [Means for solving the problem]

[0024] In order to solve the above problems, a fuel assembly according to the present invention is a fuel assembly for a water-cooled reactor, and includes a substrate made of a zirconium alloy and a coating layer formed on the substrate, wherein the coating layer has a chromium layer made of chromium or a chromium alloy on a surface of the substrate that comes into contact with cooling water, and a corrosion-resistant layer made of a zirconium alloy or a titanium alloy on the surface of the chromium layer.

[0025] Furthermore, a method for manufacturing a fuel assembly according to the present invention is a method for manufacturing a fuel assembly for a water-cooled reactor, and includes the steps of: preparing a base material made of a zirconium alloy; forming a chromium layer made of chromium or a chromium alloy on a surface of the base material that comes into contact with cooling water; forming a corrosion-resistant layer made of a zirconium alloy or a titanium alloy on the surface of the chromium layer; and assembling a fuel assembly using the base material, wherein the chromium layer and the corrosion-resistant layer are formed by a thin-plate cladding method in which thin plates are stacked and diffusion-bonded, a physical vapor deposition method, a thermal spraying method, a cold spraying method, or a plating method before assembly using the base material. [Effects of the Invention]

[0026] According to the present invention, it is possible to provide a fuel assembly and a method for manufacturing the fuel assembly that are provided with a coating that has both oxidation resistance at high temperatures in the event of a reactor accident and corrosion resistance in an aqueous environment in contact with cooling water during normal reactor operation. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 is a diagram schematically illustrating an example of a coating structure formed on the surface of a fuel assembly according to an embodiment of the present invention. [Figure 2] FIG. 1 is a diagram schematically illustrating an example of a coating structure formed on the surface of a fuel assembly according to an embodiment of the present invention. [Figure 3] FIG. 1 is a vertical cross-sectional view showing an example of a fuel assembly for a BWR. [Figure 4] 1 is a cross-sectional view showing an example of a fuel assembly for a BWR. [Figure 5] FIG. 2 is a longitudinal cross-sectional view showing an example of a fuel rod. [Figure 6] FIG. 2 is a partial cross-sectional view showing an example of a water rod. [Figure 7] FIG. 2 is a perspective view showing an example of a channel box. [Figure 8] 1A to 1C are diagrams illustrating a manufacturing method of a channel box. [Figure 9] FIG. 1 is a perspective view showing an example of a fuel assembly for a PWR. DETAILED DESCRIPTION OF THE INVENTION

[0028] A fuel assembly and a method for manufacturing the same according to an embodiment of the present invention will be described below with reference to the drawings. In the following drawings, common components are designated by the same reference numerals, and duplicated descriptions will be omitted.

[0029] In this specification, "forming a layer on a surface" means forming a layer at a position that covers the substrate or another layer from the outside. Each layer may be laminated directly onto the surface of the substrate or another layer, or may be laminated onto the surface of the substrate or another layer with another layer sandwiched between them. Each layer may be formed by a film formation method such as physical vapor deposition, thermal spraying, cold spraying, or plating, or by lamination using a thin-plate cladding method.

[0030] FIG. 1 is a diagram schematically illustrating an example of a coating structure formed on the surface of a fuel assembly according to an embodiment of the present invention. FIG. 1 shows a structure in which a two-layer coating layer is formed on a substrate that forms the fuel assembly. As shown in FIG. 1, a coating structure 100 including a two-layer coating layer 10 can be formed on the surface of the fuel assembly according to this embodiment. The coating structure 100 includes a substrate 1 made of a zirconium alloy and a two-layer coating layer 10 formed on the substrate 1.

[0031] In FIG. 1, coating layer 10 is composed of a chromium layer 2, a corrosion-resistant layer 3, and an oxide film 3a. Chromium layer 2 is formed on the surface of substrate 1 made of zirconium alloy. Corrosion-resistant layer 3 is formed on the surface of chromium layer 2. Oxide film 3a is formed on the surface of corrosion-resistant layer 3. The surface of coating layer 10 opposite to the interface in contact with substrate 1 is in contact with reactor cooling water 5.

[0032] The coating structure 100 is formed at least on the surface of a fuel assembly for a water-cooled nuclear reactor in an area that comes into contact with the reactor's cooling water 5. The coating structure 100 is preferably formed on an area that comes into contact with high-temperature steam in the event of a nuclear reactor accident. Examples of water-cooled nuclear reactors include boiling water reactors (BWRs), pressurized water reactors (PWRs), advanced boiling water reactors (ABWRs), and pressurized heavy water reactors (PHWRs).

[0033] The coating layer 10 comes into contact with the reactor cooling water 5 and is exposed to an operating environment during normal reactor operation, and to an accident environment during a reactor accident. For example, the operating environment in a BWR is an environment in which the coating layer comes into contact with a single-phase flow of pure water at around 290°C or a two-phase flow of high-temperature pure water and steam. The operating environment in a PWR is an environment in which the coating layer comes into contact with a single-phase flow of water containing boron and lithium at around 325°C. The accident environment is an environment in which the coating layer comes into contact with a single-phase flow of steam at approximately 700 to 1200°C or higher.

[0034] The substrate 1 is a structural material that forms a fuel assembly for a water-cooled nuclear reactor. The substrate 1 is made of a zirconium alloy containing zirconium as the main component. The zirconium alloy that forms the substrate 1 is an alloy whose material structure has been adjusted by adding alloy components and heat treatment for use in a nuclear reactor.

[0035] Zirconium alloys include alloys containing one or more alloying elements selected from niobium, tin, iron, chromium, and nickel, each at a concentration of 3 mass% or less, with the remainder consisting of zirconium and unavoidable impurities. Specific examples of zirconium alloys include Zircaloy-2, Zircaloy-4, zirconium-niobium-tin alloy, and zirconium-niobium alloy.

[0036] Zircaloy-2 contains, by mass%, 1.20 to 1.70% tin, 0.07 to 0.20% iron, 0.05 to 1.15% chromium, and 0.03 to 0.08% nickel, with the remainder consisting of zirconium and unavoidable impurities. Zircaloy-2 may also contain 900 to 1500 ppm of oxygen.

[0037] Zircaloy-4 contains, by mass%, 1.20 to 1.70% tin, 0.18 to 0.24% iron, 0.07 to 1.13% chromium, less than 0.007% nickel, and the balance consisting of zirconium and unavoidable impurities. Zircaloy-4 may also contain 900 to 1500 ppm oxygen.

[0038] The zirconium-niobium-tin alloy contains 0.5 to 2.0% niobium, 0.7 to 1.5% tin, 0.07 to 0.42% of one or more elements selected from the group consisting of iron, nickel, and chromium, and the balance consisting of zirconium and unavoidable impurities. The zirconium-niobium-tin alloy may have an oxygen content of 200 ppm or less.

[0039] The zirconium-niobium alloy contains 0.8 to 1.2% niobium, with the remainder consisting of zirconium and unavoidable impurities. The zirconium-niobium alloy may contain 900 to 1490 ppm of oxygen.

[0040] The chromium layer 2 is a layer whose main component is chromium, and is formed of chromium or a chromium alloy. The chromium layer 2 is formed on the surface of the substrate 1 that comes into contact with the cooling water 5. The chromium layer 2 enhances oxidation resistance at high temperatures. Therefore, even if the substrate 1 is exposed to an accident environment, which is a high-temperature oxidizing environment containing high-temperature steam, the elution of zirconium that forms the substrate 1 and the generation of hydrogen can be suppressed, thereby protecting the substrate 1. The layer also enhances wear resistance. Unlike chromium compounds, chromium and chromium alloys do not generate gas due to corrosion or activation, and therefore can form a stable protective film.

[0041] The chromium layer 2 may contain components diffused from the base material 1 or the corrosion-resistant layer 3 adjacent to the chromium layer 2. The chromium layer 2 may contain one or more of niobium, tin, iron, chromium, nickel, zirconium, titanium, and inevitable impurities diffused from a layer adjacent to the chromium layer 2. Examples of inevitable impurities include hafnium, nitrogen, and carbon, which are inevitably mixed in during the manufacture of the base material 1 or the formation of the corrosion-resistant layer 3.

[0042] The thickness of the chromium layer 2 is preferably 5 μm or more and 1 / 31 or less of the thickness of the base material 1, and is preferably 5 μm or more and 50 μm or less. A thickness of 5 μm or more ensures the function of the chromium layer 2 even if atoms diffuse from adjacent layers in an accident environment, etc. Furthermore, if the thickness is 1 / 31 or less of the thickness of the base material 1, the thermal neutron absorption cross-section of the chromium layer 2 is less than twice the thermal neutron absorption cross-section of the base material 1. Chromium has a larger thermal neutron absorption cross-section than zirconium, making it a disadvantageous material in terms of neutron economy. However, limiting the thickness of the chromium layer 2 can improve oxidation resistance at high temperatures while reducing the impact of neutron absorption on the core design.

[0043] The corrosion-resistant layer 3 is a layer containing zirconium or titanium as a main component, and is formed of a zirconium alloy or a titanium alloy. The corrosion-resistant layer 3 is formed on the surface of the chromium layer 2. An oxide film 3a is formed on the surface of the corrosion-resistant layer 3 that comes into contact with the cooling water 5, from zirconium oxide or titanium oxide produced by oxidation of the corrosion-resistant layer 3. The corrosion-resistant layer 3 enhances corrosion resistance, so that even if the substrate 1 and the chromium layer 2 are exposed to an operating environment with high dissolved oxygen concentrations and hydrogen peroxide concentrations and a high corrosion potential, the elution of chromium that forms the chromium layer 2 and the loss of the chromium layer 2 can be suppressed, thereby protecting the substrate 1 and the chromium layer 2.

[0044] The corrosion-resistant layer 3 can be formed from a zirconium alloy containing one or more alloying elements selected from niobium, tin, iron, chromium, and nickel, each at a concentration of 3% by mass or less, with the balance being zirconium and unavoidable impurities; a titanium alloy containing one or more alloying elements selected from niobium, tin, iron, chromium, and nickel, each at a concentration of 3% by mass or less, with the balance being titanium and unavoidable impurities; or a zirconium-titanium alloy containing these alloying elements. If the concentration of the alloying elements is too high, the corrosion resistance of the corrosion-resistant layer 3 itself will be impaired. If the alloying elements are 3% by mass or less, high corrosion resistance can be ensured.

[0045] The corrosion-resistant layer 3 can have any ratio of zirconium concentration to titanium concentration. The corrosion-resistant layer 3 may have a concentration gradient in which the concentrations of the components vary from region to region within the corrosion-resistant layer 3. For example, a concentration gradient can be formed in which the chromium concentration decreases and the zirconium and titanium concentrations increase from the interface in contact with the chromium layer 2 to the opposite interface. Forming such a concentration gradient can improve adhesion between layers and corrosion resistance.

[0046] The corrosion-resistant layer 3 may contain components diffused from the chromium layer 2 adjacent to the corrosion-resistant layer 3. The corrosion-resistant layer 3 may contain one or more of niobium, tin, iron, chromium, nickel, zirconium, titanium, and inevitable impurities diffused from a layer adjacent to the corrosion-resistant layer 3. Examples of inevitable impurities include hafnium, aluminum, zinc, and the like, which are inevitably mixed in during the production of the base material 1 or the formation of the chromium layer 2.

[0047] The corrosion-resistant layer 3 may have a region containing chromium diffused from the chromium layer 2 at a concentration of 3% by mass or more. Under high temperatures during a nuclear reactor accident, interdiffusion of atoms may occur between the chromium layer 2 and the corrosion-resistant layer 3. Even in such a case, the formation of the oxide film 3a can improve corrosion resistance in an aqueous environment where the layer comes into contact with cooling water during normal operation of the nuclear reactor.

[0048] The thickness of the corrosion-resistant layer 3 is preferably 5 μm or more, and is a thickness that makes the atomic concentration ratio of chromium to zirconium or titanium equal to or less than a predetermined value in the entire combined chromium layer 2 and corrosion-resistant layer 3. If the thickness is 5 μm or more, the function of the corrosion-resistant layer 3 can be ensured even if atoms diffuse from the chromium layer 2 or the like adjacent to the corrosion-resistant layer 3.

[0049] In the coating layer 10, when the corrosion-resistant layer 3 is formed of a zirconium alloy, the ratio of the atomic concentration of zirconium to the atomic concentration of chromium in the entire combination of the chromium layer 2 and the corrosion-resistant layer 3 is preferably 3 / 2 or less. Furthermore, when the corrosion-resistant layer 3 is formed of a titanium alloy, the ratio of the atomic concentration of titanium to the atomic concentration of chromium in the entire combination of the chromium layer 2 and the corrosion-resistant layer 3 is preferably 1 or less. With such an atomic concentration ratio, the melting point of the chromium layer 2 can be ensured to be high.

[0050] In the event of a nuclear reactor accident, not only the reactor cooling water 5 but also the substrate 1 and coating layer 10 become hot. Interdiffusion of atoms between the corrosion-resistant layer 3 and the chromium layer 2 and other components progresses, and oxidation progresses from the surface side that comes into contact with the cooling water 5. The melting point of chromium alone is 1855°C. When chromium is alloyed with zirconium or titanium, the melting point decreases to 1322°C and 1410°C, respectively. Furthermore, the presence of oxygen further decreases the melting point. In contrast, if the atomic concentration ratio in the coating layer 10 is limited, the melting point of the chromium layer 2 is ensured to be high. Even if the chromium layer 2 locally melts in the event of a nuclear reactor accident, there is a sufficient margin before the entire chromium layer 2 melts and disappears, ensuring the functionality of the chromium layer 2.

[0051] With this coating structure 100, the chromium layer 2 provides high oxidation resistance in an accident environment, which is a high-temperature oxidizing environment with high-temperature steam present during a reactor accident. Furthermore, the corrosion-resistant layer 3 provides high corrosion resistance in an operating environment during normal reactor operation, where dissolved oxygen and hydrogen peroxide concentrations are high and corrosion potential is high. This means that the coating structure 100 can achieve both oxidation resistance under high temperatures during a reactor accident and corrosion resistance in an aqueous environment exposed to cooling water during normal reactor operation. The improved oxidation and corrosion resistance suppress chromium elution and loss of the chromium layer, protecting the substrate 1 and other components, thereby enhancing safety during an accident. While ensuring the integrity of the fuel assembly during a reactor accident, the coating structure can reduce degradation of the coating structure and the load on the purification system during normal operation.

[0052] Fig. 2 is a diagram schematically illustrating an example of a coating structure formed on the surface of a fuel assembly according to an embodiment of the present invention. Fig. 2 shows a structure in which a three-layer coating layer is formed on a substrate forming the fuel assembly. As shown in Fig. 2, a coating structure 200 including a three-layer coating layer 20 can also be formed on the surface of the fuel assembly according to this embodiment. The coating structure 200 includes a substrate 1 made of a zirconium alloy and a three-layer coating layer 20 formed on the substrate 1.

[0053] In FIG. 2, the coating layer 20 is composed of an isolation layer 4, a chromium layer 2, a corrosion-resistant layer 3, and an oxide film 3a. The isolation layer 4 is formed on the surface of a substrate 1 made of a zirconium alloy. The chromium layer 2 is formed on the surface of the isolation layer 4. The corrosion-resistant layer 3 is formed on the surface of the chromium layer 2. An oxide film 3a is formed on the surface of the corrosion-resistant layer 3. The surface of the coating layer 20 opposite to the interface in contact with the substrate 1 is in contact with the cooling water 5 of the nuclear reactor.

[0054] Similar to the coating structure 100, the coating structure 200 is formed at least on the surface of a fuel assembly for a water-cooled reactor in an area that comes into contact with the reactor's cooling water 5. The coating structure 200 is preferably formed on an area that comes into contact with high-temperature steam in the event of a reactor accident. The coating layer 20 comes into contact with the reactor's cooling water 5, is exposed to an operating environment during normal reactor operation, and is exposed to an accident environment in the event of a reactor accident.

[0055] The isolation layer 4 is a layer whose main component is niobium or titanium, and is formed from niobium or titanium. The isolation layer 4 is formed between the substrate 1 and the chromium layer 2, on the surface of the substrate 1. The isolation layer 4 suppresses atomic diffusion between the substrate 1 and the chromium layer 2, thereby preventing a decrease in the melting point of the chromium layer 2. In the event of a nuclear reactor accident, melting or loss of the chromium layer 2 is reduced, thereby ensuring the function of the chromium layer 2.

[0056] The isolation layer 4 may contain components diffused from the substrate 1, the chromium layer 2, or the like adjacent to the isolation layer 4. The isolation layer 4 may contain one or more of niobium, tin, iron, chromium, nickel, zirconium, titanium, and inevitable impurities diffused from layers adjacent to the isolation layer 4. Examples of inevitable impurities include hafnium, aluminum, zinc, and the like that are inevitably mixed in during the production of the substrate 1 or the formation of the chromium layer 2.

[0057] The thickness of the isolation layer 4 is preferably 1 μm or more and 20 μm or less. A thickness of 1 μm or more can sufficiently suppress atomic diffusion between the substrate 1 and the chromium layer 2. Furthermore, a thickness of 20 μm or less reduces the absorption of thermal neutrons by the isolation layer 4. Niobium and titanium have a larger absorption cross section for thermal neutrons than zirconium, making them disadvantageous materials in terms of neutron economy. However, limiting the thickness of the isolation layer 4 can suppress the impact on the core design. Furthermore, it becomes possible to make the chromium layer 2, the corrosion-resistant layer 3, etc., thicker accordingly.

[0058] Like the coating structure 100, the coating structure 200 can provide both oxidation resistance at high temperatures during a reactor accident and corrosion resistance in an aqueous environment where the reactor is in contact with cooling water during normal reactor operation. The isolation layer 4 also suppresses atomic diffusion between the substrate 1 and the chromium layer 2, thereby preventing a decrease in the melting point of the chromium layer 2. When the temperatures of the substrate 1 and the chromium layer 2 rise during a reactor accident, atomic diffusion between the substrate 1 and the chromium layer 2 progresses. The chromium layer 2 begins to melt at 1322°C. By providing the isolation layer 4, the melting temperature of the chromium layer 2 can be increased to 1410-1620°C. In the event of a reactor accident, melting and loss of the chromium layer 2 are suppressed, protecting the substrate 1 and other components, thereby enhancing safety during an accident. While ensuring the integrity of the fuel assembly during a reactor accident, deterioration of the coating structure and the load on the purification system during normal operation can be reduced.

[0059] Next, a method for manufacturing the fuel assembly according to this embodiment for forming the coating structures 100, 200 will be described.

[0060] The fuel assembly according to this embodiment can be manufactured by forming coating layers 10, 20 on the surface of substrate 1 before assembling the fuel assembly. The method for manufacturing the fuel assembly according to this embodiment includes the steps of preparing a substrate made of a zirconium alloy, forming a coating layer on the surface of the substrate that comes into contact with the coolant, and assembling a fuel assembly using the substrate on which the coating layer has been formed.

[0061] The step of forming the coating layer includes the steps of forming a chromium layer of chromium or a chromium alloy on the surface of the substrate that comes into contact with the cooling water, forming a corrosion-resistant layer of zirconium alloy or titanium alloy on the surface of the chromium layer, and forming an oxide film on the surface of the corrosion-resistant layer that comes into contact with the cooling water. When an isolation layer is formed between the substrate and the chromium layer, a step of forming an isolation layer of niobium or titanium on the surface of the substrate that comes into contact with the cooling water is performed before the step of forming the chromium layer.

[0062] In the substrate preparation step, a substrate made of a zirconium alloy is prepared as a structural material for forming a fuel assembly. As described below, the substrate may be a pipe material for forming a fuel cladding tube or a thimble, a plate material for forming a channel box, a water rod with holes, or a formed end plug or support grid. The substrate may be a primary processed material such as a pipe material or a plate material, or a secondary processed material obtained by bending or sleeve processing a primary processed material. The surface of the substrate that comes into contact with the cooling water is preferably subjected to a degreasing treatment, pickling treatment, polishing treatment, or the like before forming a coating layer.

[0063] The chromium layer, corrosion-resistant layer, and isolation layer that make up the coating layer can be formed by a thin-plate cladding method in which thin plates are stacked and diffusion-bonded; a physical vapor deposition (PVD) method; a thermal spraying method in which a metal or the like is melted and sprayed; or a cold spraying method in which a metal or the like is accelerated to a high speed at a temperature lower than its melting point and sprayed.

[0064] The chromium layer can be formed by plating methods such as electrolytic plating and electroless plating. On the other hand, the corrosion-resistant layer and isolation layer are formed of zirconium, titanium, or niobium, and therefore cannot be coated by electrolytic plating using a plating solution with water as the solvent. Therefore, the corrosion-resistant layer and isolation layer are coated by thin-plate cladding, physical vapor deposition, thermal spraying, or cold spraying.

[0065] After the formation of each layer constituting the coating layer, the formed layer can be subjected to heat treatment for the purpose of improving adhesion between layers, removing distortion, etc. When the method of forming each layer involves heating, or when the formed layer is subjected to heat treatment, atomic diffusion progresses due to the increase in temperature, and each layer will contain components diffused from adjacent layers. Each layer may contain components different from the charged components, or the interface with other layers may be unclear.

[0066] The coating layer forming step is performed after preparing the substrate and before assembling a fuel assembly using the substrate. For example, for fuel cladding tubes, the step may be performed before welding end plugs or after welding end plugs to one end. For water rods, the step may be performed before welding end plugs or after welding end plugs to one or both ends. For thimbles, the step may be performed before welding to the nozzle or after welding to the nozzle. For channel boxes, the step is preferably performed before joining the bent members together. For end plugs, when welding to one end of a fuel cladding tube or one or both ends of a water rod, the step may be performed all at once after welding. Furthermore, when welding to the other end of a fuel cladding tube with an end plug welded to one end, the step is preferably performed separately before welding to the other end. For support grids, the step is preferably performed after forming the grid.

[0067] However, the coating layer can also be formed additionally on a localized region after assembling a fuel assembly using substrates. For example, the coating layer can be formed by thermal spraying, cold spraying, or the like on the surface of a joint where substrates are joined by welding or the like. Specific examples of such joints include joints between a fuel cladding tube or a water rod and an end plug, joints between a thimble and a nozzle or a support grid, and joints between members that form a channel box.

[0068] The oxide film on the surface of the corrosion-resistant layer can be formed by exposing the corrosion-resistant layer to high-temperature water or high-temperature steam in the absence of radiation after assembling a fuel assembly using the substrate and before putting the fuel assembly into service. The exposure to high-temperature water or high-temperature steam is preferably carried out by pressurizing the atmosphere to which the substrate is exposed to a pressure equal to or higher than atmospheric pressure. The exposure to high-temperature water or high-temperature steam can be carried out using an autoclave.

[0069] The temperature of the high-temperature water or high-temperature steam is preferably 100°C or higher, more preferably 150°C or higher, and even more preferably 200°C or higher, from the viewpoint of shortening the time required to form an oxide film. Furthermore, from the viewpoint of forming a dense oxide film that is not porous, the temperature is preferably 400°C or lower, more preferably 340°C or lower. The atmospheric pressure is preferably 150 atmospheres or lower, and from the viewpoint of forming a dense oxide film, it is preferably 10 atmospheres or lower at 400°C. The exposure time to the high-temperature water or high-temperature steam can be set as desired depending on the temperature conditions, pressure conditions, the target thickness of the oxide film, and the like. Depending on the exposure time and other factors, the high-temperature water or high-temperature steam that can be used may be water that is higher than room temperature (5 to 35°C) or steam that is higher than 100°C.

[0070] According to this manufacturing method, the step of forming the coating layer is performed before assembling the fuel assembly using the substrate, so that the coating layer can be easily formed on the substrate that forms the fuel assembly. Even if the area that comes into contact with the coolant is inside the structural material that forms the fuel assembly or in an area that is covered by other parts, a layer with high uniformity in composition and thickness can be formed. Therefore, it is possible to stably manufacture fuel assemblies that have a coating that combines oxidation resistance at high temperatures in the event of a reactor accident and corrosion resistance in an aqueous environment that comes into contact with the coolant during normal reactor operation.

[0071] Next, a specific example of the structure of the fuel assembly according to this embodiment to which the coating structures 100, 200 are applied will be described.

[0072] Fig. 3 is a longitudinal sectional view showing an example of a fuel assembly for a BWR. Fig. 4 is a transverse sectional view showing an example of a fuel assembly for a BWR. Fig. 4 corresponds to a cross-sectional view taken along line II in Fig. 3. As shown in Figs. 3 and 4, a fuel assembly 300 for a BWR includes a plurality of fuel rods 31, water rods 32, a channel box 33, an upper tie plate 34, a lower tie plate 35, a plurality of spacers 36, and a handle 37.

[0073] The fuel assembly 300 is a structure in which a plurality of fuel rods 31 are bundled together, and is loaded into the core of a boiling water reactor (BWR). In a BWR, four fuel assemblies 300 are loaded in a 2x2 lattice arrangement. Control rods, which are cross-shaped when viewed from above, are inserted between the fuel assemblies 300 so as to be movable up and down.

[0074] Fuel rods 31 are formed by loading fuel pellets, which are formed from nuclear fuel, into fuel cladding tubes and sealing the ends of the fuel cladding tubes with end plugs. The fuel rods 31 are arranged in a regular matrix with spaces between them inside the fuel assembly 300. The upper and lower openings of the fuel rods 31 are sealed with end plugs with fuel pellets and plenum springs loaded inside the fuel cladding tubes.

[0075] 3 and 4, the fuel rods 31 include standard fuel rods 31a having a length that extends substantially the entire length of the fuel assembly 300, and part-length fuel rods 31b that are shorter than the standard fuel rods 31a. The part-length fuel rods 31b have a shorter internal effective fuel length than the standard fuel rods 31a, and their height does not reach the upper tie plate 34. The upper portions of the standard fuel rods 31a are supported by the upper tie plate 34 via springs. The lower portions of the standard fuel rods 31a and the part-length fuel rods 31b are inserted into the lower tie plate 35. The vertical intermediate portions of the standard fuel rods 31a and the part-length fuel rods 31b are supported by a plurality of spacers 36.

[0076] The water rods 32 are hollow tubes that support the spacers 36 and are components that adjust the output, void fraction, etc. of the fuel assembly 300. The water rods 32 are disposed near the center of the channel box 33 in a top view, at the center of the plurality of fuel rods 31 arranged in a matrix. The upper portions of the water rods 32 are supported by an upper tie plate 34. The lower portions of the water rods 32 are supported by a lower tie plate 35. A plurality of spacers 36 are fixed to the middle portions of the water rods 32 in the vertical direction, spaced apart from one another.

[0077] The channel box 33 is provided so as to surround the periphery of the plurality of fuel rods 31 arranged in a matrix. The channel box 33 is provided in a cylindrical shape that is rectangular in top view. The plurality of fuel rods 31 are inserted into the channel box 33 so that they are bundled parallel to each other and arranged in a matrix in top view. The channel box 33 is supported by springs that support the upper parts of the fuel rods 31.

[0078] The upper tie plate 34 is a component that supports the upper portions of the fuel rods 31 and the water rods 32. The upper portions of the standard fuel rods 31a are supported by the upper tie plate 34 via springs in a spaced-apart relationship. The upper portions of the water rods 32 are also supported by the upper tie plate 34.

[0079] The lower tie plate 35 is a component that supports the lower portions of the fuel rods 31 and the water rods 32. The lower portions of the standard fuel rods 31a and the part-length fuel rods 31b are supported with end plugs inserted into the lower tie plate 35 and spaced apart from each other. The lower portions of the water rods 32 are also supported by the lower tie plate 35.

[0080] The spacers 36 are components that support the vertical middle portions of the fuel rods 31, etc. The spacers 36 are arranged at intervals in the vertical direction. The spacers 36 are supported by the water rods 32. The vertical middle portions of the standard fuel rods 31a and the vertical middle portions of the part-length fuel rods 31b are bundled by the spacers 36 and supported at intervals.

[0081] The handle 37 is a component for suspending the fuel assembly 300 by a crane or the like. The handle 37 is installed above the upper tie plate 34. Both ends of the handle 37 are fixed to the upper tie plate 34.

[0082] In a fuel assembly 300 for a BWR, the coating layer 10 composed of the chromium layer 2 and the corrosion-resistant layer 3, and the coating layer 20 composed of the isolation layer 4, the chromium layer 2, and the corrosion-resistant layer 3, are preferably formed on the outer surface of the fuel cladding tube of the fuel rod 31, the outer surface of the end plug of the fuel rod 31, the outer surface of the water rod 32, and the inner surface or the inner and outer surfaces of the channel box 33. The substrate 1 on whose surface the coating layers 10, 20 are formed is preferably a portion that forms one or more of the fuel cladding tube of the fuel rod 31, the end plug of the fuel rod 31, the water rod 32, and the channel box 33.

[0083] The outer surfaces of the fuel cladding tubes of the fuel rods 31, the outer surfaces of the end plugs of the fuel rods 31, the outer surfaces of the water rods 32, and the inner and outer surfaces of the channel box 33 are made of zirconium alloy and are the parts that come into contact with the reactor cooling water 5. When the coating layers 10, 20 are formed on these parts, the chromium layer 2 provides high oxidation resistance in an accident environment. Furthermore, the corrosion-resistant layer 3 provides high corrosion resistance in an operating environment. In other words, it is possible to achieve both oxidation resistance at high temperatures during a reactor accident and corrosion resistance in an aqueous environment where the reactor comes into contact with cooling water during normal operation.

[0084] Fig. 5 is a longitudinal cross-sectional view showing an example of a fuel rod. Fig. 5 shows the structure of a fuel rod for a BWR. As shown in Fig. 5, a fuel rod 31 includes fuel pellets 311, a fuel cladding tube 312, an end plug 313, and a plenum spring 314. The end plug 313 includes an upper end plug 313a that closes the upper opening of the fuel cladding tube 312, and a lower end plug 313b that closes the lower opening of the fuel cladding tube 312.

[0085] The upper side of the fuel rod 31 for a BWR is elastically supported by an upper tie plate 34, and the lower side is inserted into and supported by a lower tie plate 35. The fuel pellets 311 are formed by molding sintered nuclear fuel such as uranium oxide into pellets. The upper end plugs 313a and lower end plugs 313b are formed into plug shapes using a zirconium alloy. The plenum spring 314 is a spring installed in the upper plenum inside the fuel cladding tube 312.

[0086] The fuel pellets 311 are loaded inside the fuel cladding tube 312 in a stacked state. A plenum spring 314 is interposed between the stacked fuel pellets 311 and the upper end plug 313a, and presses the fuel pellets 311 from above to secure them in place. An inert gas such as helium is sealed inside the fuel cladding tube 312. The upper opening of the fuel cladding tube 312 is sealed by welding the upper end plug 313a. The lower opening of the fuel cladding tube 312 is sealed by welding the lower end plug 313b.

[0087] In the fuel rod 31 for a BWR, the coating layer 10 composed of the chromium layer 2 and the corrosion-resistant layer 3, and the coating layer 20 composed of the isolation layer 4, the chromium layer 2, and the corrosion-resistant layer 3 are preferably formed at least on the outer surface of the fuel cladding tube 312, and more preferably on the outer surfaces of the fuel cladding tube 312 and the end plug 313. The coating layers 10 and 20 are preferably formed on both the outer surface of the upper end plug 313a and the outer surface of the lower end plug 313b.

[0088] The outer surface of the fuel rod 31 is exposed to a single-phase flow of high-temperature water (cooling water) or a two-phase flow of high-temperature hot water or high-temperature water and steam during normal reactor operation, and is therefore susceptible to exposure to high-temperature steam in the event of a reactor accident. Forming the coating layers 10, 20 on these areas provides both oxidation resistance at high temperatures during a reactor accident and corrosion resistance in the water environment during normal reactor operation. In the event of an accident that unexpectedly occurs during normal operation, a healthy chromium layer can be maintained on the surface of the fuel rod, delaying the dissolution of zirconium, which forms the fuel rod, and the generation of hydrogen.

[0089] Fig. 6 is a partial cross-sectional view showing an example of a water rod. Fig. 6 shows the partial cross-section with a portion of the water rod cut away. As shown in Fig. 6, water rod 32 includes a body portion 321, an end plug 322, a handle portion 323, and a hole portion 324. End plug 322 includes an upper end plug 322a that closes the upper opening of body portion 321, and a lower end plug 322b that closes the lower opening of body portion 321.

[0090] Body 321 is formed in a tubular shape from zirconium alloy. Upper end plug 322a and lower end plug 322b are also formed in a plug-like shape from zirconium alloy. Body 321 has a plurality of holes 324 that penetrate from the inside to the outside. Body 321 is designed to allow cooling water to flow into the interior through holes 324. The upper opening of body 321 is sealed by welding upper end plug 322a. The lower opening of body 321 is sealed by welding lower end plug 322b to handle 323 joined to the lower part of body 321.

[0091] In water rod 32, coating layer 10 composed of chrome layer 2 and corrosion-resistant layer 3, and coating layer 20 composed of isolation layer 4, chrome layer 2, and corrosion-resistant layer 3 are preferably formed at least on the outer surface of body 321, more preferably on the outer surfaces of body 321 and end plug 322, and even more preferably on the outer surfaces of body 321, end plug 322, handle 323, and the inner surface of hole 324. Coating layers 10 and 20 are preferably formed on the outer surfaces of both upper end plug 322a and lower end plug 322b.

[0092] Fig. 7 is a perspective view showing an example of a channel box. Fig. 7 shows the structure of a channel box for a BWR. As shown in Fig. 7, the channel box 33 is formed in a square tubular shape that is square in top view. The channel box 33 is formed by joining a pair of U-shaped members 331 that are U-shaped in cross section.

[0093] The U-shaped member 331 is made of a zirconium alloy. The channel box 33 has a structure in which two U-shaped members 331 are joined to each other via a joint 332 that passes through the center of both sides of the channel box 33 and extends along the longitudinal direction. A flat clip 333 that is used to fix the channel box 33 to a fuel assembly is provided at the corner of the upper end of the channel box 33 so as to protrude inward.

[0094] In the channel box 33, the coating layer 10 composed of the chromium layer 2 and the corrosion-resistant layer 3, and the coating layer 20 composed of the isolation layer 4, the chromium layer 2, and the corrosion-resistant layer 3 are preferably formed at least on the inner surface of the U-shaped member 331, and more preferably on both the inner and outer surfaces of the U-shaped member 331.

[0095] The inner surface of the U-shaped member 331 is likely to be exposed to high-temperature steam due to heat generated by the fuel rods in the event of a reactor accident. This material offers both oxidation resistance under high temperatures during a reactor accident and corrosion resistance in the water environment during normal reactor operation. In the event of an accident that unexpectedly occurs during normal operation, a healthy chrome layer can be maintained on the surface of the channel box, delaying the elution of zirconium that forms the channel box and the generation of hydrogen.

[0096] Fig. 8 is a diagram showing a manufacturing method of a channel box. Fig. 8 shows a process for forming a channel box for a BWR by joining U-shaped members together. The upper part of Fig. 8 shows a plate material 330 which is a primary processed material used as a raw material. The middle part of Fig. 8 shows a U-shaped member 331 which is a secondary processed material obtained by bending the plate material 330. The lower part of Fig. 8 shows a channel box 33 to which the U-shaped members 331 are joined.

[0097] 8, the channel box 33 can be manufactured by forming a plate material 330 to form U-shaped members 331, and then butting and joining the pair of U-shaped members 331 to each other. As the plate material 330, a long flat plate made of a zirconium alloy is prepared.

[0098] The U-shaped member 331 can be formed by bending both ends of the plate material 330 in the short direction at right angles to form a U shape. The U-shaped member 331 is preferably formed by roll forming. This is because bending the plate material 330 on which the coating layers 10 and 20 are formed may cause cracks in the coating layers 10 and 20.

[0099] The pair of U-shaped members 331 can be joined by butting both end faces extending along the longitudinal direction against each other and welding them together. Examples of welding methods that can be used include arc welding such as TIG welding and plasma welding, laser welding, and electron beam welding.

[0100] In the process of forming the channel box 33, the coating layer 10 composed of the chromium layer 2 and the corrosion-resistant layer 3, and the coating layer 20 composed of the isolation layer 4, the chromium layer 2, and the corrosion-resistant layer 3 are preferably formed after the base plate material 330 is prepared and before the U-shaped members 331 are joined together. The coating layers 10 and 20 can be formed on one or both sides of the plate material 330 in a flat plate state, or on one or both sides of the U-shaped member 331 in a U-shaped state.

[0101] The channel box 33 is long, exceeding 4 m, and has a structure that makes it difficult to apply a coating to the inner surface after assembly. In contrast, if the coating layers 10, 20 are formed on the plate material 330 or U-shaped member 331 before assembly, the coating layers 10, 20 can be formed on either one or both surfaces with high uniformity in composition and thickness.

[0102] After assembling the U-shaped members 331 together and before using the channel box 33, local coating layers 10, 20 can be additionally formed on the surfaces of the joints 332 where the U-shaped members 331 are joined. The surface of the weld metal, which is susceptible to corrosion, can be coated with the coating layers 10, 20 by using a thermal spraying method, a cold spraying method, or the like.

[0103] Fig. 9 is a perspective view showing an example of a fuel assembly for a PWR. Fig. 9 shows a schematic view of the structure including the central part of the fuel assembly for a PWR, with a portion of the fuel assembly seen through. As shown in Fig. 9, a fuel assembly 400 for a PWR includes a plurality of fuel rods 41, a control rod guide thimble 42, an in-core instrumentation guide thimble 43, an upper nozzle 44, a lower nozzle 45, and a support grid 46.

[0104] The fuel assembly 400 is a structure in which a plurality of fuel rods 41 are bundled together, and is loaded into the core of a pressurized water reactor (PWR). In a PWR, a control rod cluster made up of a plurality of control rods is inserted into the large fuel assembly 400 so that it can move freely in the vertical direction. The fuel rods and control rods are arranged in a matrix in a lattice-like space inside the fuel assembly 400.

[0105] The fuel rods 41 are formed by filling fuel pellets, which are formed from nuclear fuel, into fuel cladding tubes. The fuel rods 41 are arranged in a regular matrix with spaces between them inside the fuel assembly 400. The upper and lower openings of the fuel rods 41 are sealed with end plugs when filled with fuel pellets and plenum springs.

[0106] The control rod guide thimbles 42 are hollow tubes that guide the insertion of the control rods 41. The control rod guide thimbles 42 are arranged intermittently in a lattice-like space inside the fuel assembly 400, and are regularly aligned together with the multiple fuel rods. The control rods are inserted into the control rod guide thimbles 42 so that they can move freely in the vertical direction.

[0107] The in-core instrumentation guide thimble 43 is a hollow tube that guides the insertion of in-core instrumentation devices such as neutron detectors. The in-core instrumentation guide thimble 43 is arranged in the center of the lattice-like space inside the fuel assembly 400 and is aligned with a plurality of fuel rods. A thimble containing an in-core instrumentation device is inserted into the in-core instrumentation guide thimble 43 from outside the reactor.

[0108] The top nozzle 44 is disposed above the fuel rods 41 to form a framework for supporting the fuel rods 41, etc., and also serves to ensure a flow path for the coolant, and is used for positioning and transporting the fuel assembly 400. The top nozzle 44 supports the tops of the control rod guide thimbles 42 and the in-core instrumentation guide thimbles 43.

[0109] The bottom nozzle 45 is disposed below the fuel rods 41 to form a framework for supporting the fuel rods 41, etc., and also serves to secure a flow path for the coolant and is used to control the flow of the coolant. The bottom nozzle 45 supports the lower parts of the control rod guide thimbles 42 and the lower parts of the in-core instrumentation guide thimbles 43.

[0110] The support grid 46 is a component that forms a grid-like space into which the fuel rods 41, etc. are inserted, and that bundles and supports the fuel rods 41, etc., and is disposed in the vertical middle of the plurality of fuel rods 41 to form a framework that supports the fuel rods 41, etc. The plurality of support grids 46 are arranged at intervals from one another along the vertical direction of the fuel assembly 400. The support grid 46 is supported by the control rod guide thimbles 42, and bundles and maintains the plurality of fuel rods 41, etc., in a state where they are spaced apart from one another.

[0111] The fuel rods 41 are individually inserted into lattice-like spaces formed by the support grids 46. The top, bottom, and vertically intermediate portions of the fuel rods 41 are supported by protrusions, leaf springs, and the like formed on the support grids 46. The tops of the control rod guide thimbles 42 are supported by the top nozzle 44. The bottoms of the control rod guide thimbles 42 are supported by the bottom nozzle 45. A plurality of support grids 46 are supported at the vertically intermediate portions of the control rod guide thimbles 42.

[0112] In a PWR fuel assembly 400, the coating layer 10 composed of the chromium layer 2 and the corrosion-resistant layer 3, and the coating layer 20 composed of the isolation layer 4, the chromium layer 2, and the corrosion-resistant layer 3, are preferably formed on the outer surfaces of the fuel cladding tubes of the fuel rods 41, the outer surfaces of the end plugs of the fuel rods 41, the outer surfaces of the control rod guide thimbles 42, the outer surfaces of the in-core instrumentation guide thimbles 43, and the outer surfaces of the support grids 46 made of zirconium. The substrate 1 on whose surface the coating layers 10, 20 are formed is preferably a portion that forms one or more of the fuel cladding tubes of the fuel rods 31, the end plugs of the fuel rods 31, the control rod guide thimbles 42, the in-core instrumentation guide thimbles 43, and the support grids 46.

[0113] The outer surfaces of the fuel cladding tubes of the fuel rods 41, the outer surfaces of the end plugs of the fuel rods 41, the outer surfaces of the control rod guide thimbles 42, the outer surfaces of the in-core instrumentation guide thimbles 43, and the outer surfaces of the support grids 46 are made of zirconium alloy and are in contact with the reactor coolant 5. When the coating layers 10, 20 are formed on these areas, the chromium layer 2 provides high oxidation resistance in an accident environment. Furthermore, the corrosion-resistant layer 3 provides high corrosion resistance in an operating environment. In other words, it is possible to achieve both oxidation resistance at high temperatures during a reactor accident and corrosion resistance in an aqueous environment where the reactor comes into contact with coolant during normal operation.

[0114] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the present invention. For example, the present invention is not necessarily limited to those having all of the configurations of the above-described embodiments. Part of the configuration of an embodiment can be replaced with another configuration, part of the configuration of an embodiment can be added to another form, or part of the configuration of an embodiment can be omitted. [Example]

[0115] The present invention will be specifically described below with reference to examples, but the technical scope of the present invention is not limited to these examples.

[0116] (1) Evaluation of coating structure Test specimens were prepared in which a chromium layer and a corrosion-resistant layer were coated in that order on a substrate made of Zircaloy-2, as well as test specimens in which an isolation layer was additionally coated. Corrosion and oxidation tests were then conducted to evaluate the corrosion resistance and oxidation resistance of the coating structures.

[0117] When the corrosion-resistant layer was made of a zirconium alloy, the chromium layer and the corrosion-resistant layer were formed by a thin-plate cladding method, and when the corrosion-resistant layer was made of a titanium alloy, the chromium layer and the corrosion-resistant layer were formed by a physical vapor deposition method or a thin-plate cladding method.

[0118] The corrosion test was conducted by immersing the coated test specimens in high-temperature, high-pressure pure water for 500 hours. The test conditions were 290°C, approximately 70 atmospheres, and a dissolved oxygen concentration of 8 mg / L. The oxidation test was conducted by heating the coated test specimens in water vapor at atmospheric pressure and exposing them for 1 minute after reaching a specified temperature. The test temperatures were 1200°C or 1350°C.

[0119] Corrosion and oxidation tests were conducted by changing the composition and thickness of each layer that makes up the coating structure. The results of the corrosion test were evaluated based on whether or not the test piece lost weight due to corrosion. The results of the oxidation test were evaluated by microscopically observing the interfaces of each layer to see whether or not the test piece melted due to oxidation.

[0120] [Table 1]

[0121] The evaluation results of the corrosion and oxidation tests are shown in Table 1. Table 1 shows the composition and thickness of each layer that makes up the coating structure of each test piece, as well as the results of the corrosion and oxidation tests for each test piece.

[0122] No. 1 is a test piece in which only a chromium layer is coated on the substrate. The thickness of the chromium layer was set to 10 μm.

[0123] For No. 1, a loss in the coating layer was confirmed in a corrosion test in which the specimen was exposed to a high-temperature, high-pressure water environment. It is believed that chromium was eluted from the chromium layer. Furthermore, in an oxidation test in which the specimen was exposed to a 1200°C steam environment, no melting was confirmed. It is believed that an oxide film made of chromium oxide, which has excellent oxidation resistance at high temperatures, had formed on the surface of the chromium layer. Furthermore, in an oxidation test in which the specimen was exposed to a 1350°C steam environment, an oxide film made of chromium oxide remained, but evidence of melting was confirmed at the interface between the substrate and the chromium layer.

[0124] No. 2 is a test piece in which a chromium layer and a corrosion-resistant layer are coated on a substrate. The thickness of the chromium layer was 10 μm. The corrosion-resistant layer was formed only from a zirconium alloy layer made from Zircaloy-2. The thickness of the corrosion-resistant layer was 10 μm.

[0125] For No. 2, no weight loss of the coating layer was observed in a corrosion test in which the specimen was exposed to a high-temperature, high-pressure water environment. It is believed that the corrosion resistance was improved by the corrosion-resistant layer and that the chromium layer was protected. Furthermore, no melting was observed in an oxidation test in which the specimen was exposed to a 1200°C steam environment. It is believed that an oxide film made of zirconium oxide or chromium oxide, which has excellent oxidation resistance at high temperatures, had formed on the surface of the corrosion-resistant layer and the surface of the chromium layer. However, in an oxidation test in which the specimen was exposed to a 1350°C steam environment, although an oxide film made of zirconium oxide or chromium oxide remained, evidence of melting was observed at the interface between the substrate and the chromium layer and at the interface between the chromium layer and the corrosion-resistant layer.

[0126] No. 3 is a test piece in which a chromium layer and a corrosion-resistant layer are coated on a substrate. The thickness of the chromium layer was 10 μm. The corrosion-resistant layer was formed as a two-layer structure consisting of a compositionally graded layer with a concentration gradient from pure chromium to pure titanium, and a titanium layer made of pure titanium. The thickness of the compositionally graded layer was 2 μm. The thickness of the titanium layer was 3 μm.

[0127] For No. 3, no weight loss of the coating layer was observed in a corrosion test in which the specimen was exposed to a high-temperature, high-pressure water environment. It is believed that the corrosion-resistant layer improved corrosion resistance and protected the chromium layer. Furthermore, no melting was observed in an oxidation test in which the specimen was exposed to a 1200°C steam environment. It is believed that an oxide film made of titanium oxide or chromium oxide, which has excellent oxidation resistance at high temperatures, had formed on the surface of the corrosion-resistant layer and the surface of the chromium layer. However, in an oxidation test in which the specimen was exposed to a 1350°C steam environment, although an oxide film made of titanium oxide or chromium oxide remained, evidence of melting was confirmed at the interface between the base material and the chromium layer. It can be said that the formation of the corrosion-resistant layer suppressed reaction and melting at the interface between the chromium layer and the corrosion-resistant layer.

[0128] No. 4 is a test piece in which an isolation layer, a chromium layer, and a corrosion-resistant layer are coated on a substrate. The isolation layer was formed only from niobium. The isolation layer was 5 μm thick. The chromium layer was 10 μm thick. The corrosion-resistant layer was formed only from a zirconium alloy layer made of Zircaloy-2. The corrosion-resistant layer was 10 μm thick.

[0129] No. 5 is a test piece in which an isolation layer, a chromium layer, and a corrosion-resistant layer are coated on a substrate. The isolation layer was formed only from a titanium layer. The isolation layer was 5 μm thick. The chromium layer was 10 μm thick. The corrosion-resistant layer was formed only from a zirconium alloy layer made of Zircaloy-2. The corrosion-resistant layer was 10 μm thick.

[0130] In the corrosion test for No. 4 and No. 5, in which they were exposed to a high-temperature, high-pressure water environment, no weight loss of the coating layer was observed. It is believed that the corrosion-resistant layer improved corrosion resistance and protected the chromium layer. Furthermore, in the oxidation test, in which they were exposed to a 1200°C steam environment, no melting was observed. It is believed that an oxide film made of zirconium oxide, titanium oxide, or chromium oxide, which has excellent oxidation resistance, was formed on the surface of the corrosion-resistant layer and the chromium layer. However, in the oxidation test, in which they were exposed to a 1350°C pure steam environment, although an oxide film made of zirconium oxide, titanium oxide, or chromium oxide remained, evidence of melting was confirmed at the interface between the chromium layer and the corrosion-resistant layer. It can be said that the formation of an isolation layer suppressed reaction and melting at the interface between the base material and the chromium layer.

[0131] No. 6 is a test piece in which an isolation layer, a chromium layer, and a corrosion-resistant layer are coated on a substrate. The isolation layer was formed as a two-layer structure consisting of a niobium layer made of pure niobium and a compositionally graded layer with a concentration gradient from pure niobium to pure chromium. The niobium layer was 1 μm thick. The compositionally graded layer was 2 μm thick. The chromium layer was 5 μm thick. The corrosion-resistant layer was formed as a two-layer structure consisting of a compositionally graded layer with a concentration gradient from pure chromium to pure titanium, and a titanium layer made of pure titanium. The compositionally graded layer was 3 μm thick. The titanium layer was 3 μm thick.

[0132] No. 7 is a test piece in which an isolation layer, a chromium layer, and a corrosion-resistant layer are coated on a substrate. The isolation layer was formed solely from a niobium layer. The isolation layer thickness was 3 μm. The chromium layer thickness was 10 μm. The corrosion-resistant layer was formed solely from a titanium layer. The corrosion-resistant layer thickness was 5 μm.

[0133] No. 8 is a test piece in which an isolation layer, a chromium layer, and a corrosion-resistant layer are coated on a substrate. The isolation layer was formed as a two-layer structure consisting of a titanium layer made of pure titanium and a compositionally graded layer with a concentration gradient from pure titanium to pure chromium. The titanium layer was 1 μm thick. The compositionally graded layer was 2 μm thick. The chromium layer was 5 μm thick. The corrosion-resistant layer was formed as a two-layer structure consisting of a compositionally graded layer with a concentration gradient from pure chromium to pure titanium and a titanium layer made of pure titanium. The compositionally graded layer was 2 μm thick. The titanium layer was 3 μm thick.

[0134] No. 9 is a test piece in which an isolation layer, a chromium layer, and a corrosion-resistant layer are coated on a substrate. The isolation layer was formed solely from a niobium layer. The isolation layer was 20 μm thick. The chromium layer was 20 μm thick. The corrosion-resistant layer was formed solely from a titanium layer. The corrosion-resistant layer was 10 μm thick.

[0135] No. 10 is a test piece in which an isolation layer, a chromium layer, and a corrosion-resistant layer are coated on a substrate. The isolation layer was formed solely from a titanium layer. The isolation layer was 10 μm thick. The chromium layer was 20 μm thick. The corrosion-resistant layer was formed solely from a titanium layer. The corrosion-resistant layer was 10 μm thick.

[0136] For Nos. 6 to 10, no weight loss of the coating layer was observed in a corrosion test in which they were exposed to a high-temperature, high-pressure water environment. It is believed that the corrosion-resistant layer improved corrosion resistance and protected the chromium layer. Furthermore, no melting was observed in an oxidation test in which they were exposed to a water vapor environment at 1200°C. It is believed that an oxide film made of titanium oxide or chromium oxide, which has excellent oxidation resistance, was formed on the surface of the corrosion-resistant layer and the surface of the chromium layer. Furthermore, no melting was observed in an oxidation test in which they were exposed to a water vapor environment at 1350°C. It can be said that the formation of the corrosion-resistant layer and isolation layer suppressed reaction and melting at the interface between the chromium layer and the corrosion-resistant layer and at the interface between the base material and the chromium layer.

[0137] In Nos. 2 to 10, when the corrosion-resistant layer is formed of a zirconium alloy, the ratio of the atomic concentration of zirconium to the atomic concentration of chromium in the entire chromium layer and corrosion-resistant layer is 3 / 2 or less. Also, when the corrosion-resistant layer is formed of a titanium alloy, the ratio of the atomic concentration of titanium to the atomic concentration of chromium in the entire chromium layer and corrosion-resistant layer is 1 or less. Therefore, even at high temperatures of 1200 to 1350°C, the chromium layer did not melt and disappear.

[0138] During a nuclear reactor accident, the temperature inside the reactor may exceed 1200°C due to factors such as decay heat from the nuclear fuel. In a high-temperature steam environment exceeding 1200°C, the oxidation rate of metals and the atomic diffusion rate are rapid, leading to the overall oxidation of the materials that make up the fuel assembly and the average chemical composition. However, the chromium layer formed on the surface of the substrate functions as a protective film that enhances oxidation resistance at high temperatures. This is expected to suppress the rapid oxidation of the zirconium alloy, the temperature rise of the zirconium alloy due to the heat of the oxidation reaction, and the generation of hydrogen due to the reaction between the zirconium alloy and steam. Furthermore, the corrosion-resistant layer formed on the surface of the chromium layer protects the chromium layer from corrosion during normal operation until a nuclear reactor accident occurs, ensuring a protective film that will function in the event of an accident.

[0139] (2) Fuel rod fabrication This section describes an example of the fabrication of a standard fuel rod in which a coating layer is formed on a substrate made of a zirconium alloy. Water rods and part-length fuel rods can be fabricated in a similar manner to the fabrication method for standard fuel rods. The coating layer is formed on the outer surface of the fuel cladding tube, the outer surface of the upper end plug, and the outer surface of the lower end plug. The coating layer can also be formed on the outer surface of the fuel cladding tube to which the lower end plug is joined, or on the outer surface of the lower end plug joined to the fuel cladding tube.

[0140] First, a coating layer was formed on the outer surface of the fuel cladding tube, the outer surface of the upper end plug, and the outer surface of the lower end plug. The thickness of the substrate on which the fuel cladding tube was formed was 0.8 mm. The overall thickness of the coating layer was 8 to 25 μm, the thickness of the chromium layer was 5 to 15 μm, and the thickness of the corrosion-resistant layer was 5 to 10 μm. When an isolation layer was formed, the thickness of the isolation layer was 1 to 5 μm. The thickness of the chromium layer was 1 / 31 or less of the thickness of the substrate. Furthermore, when the corrosion-resistant layer was formed from a zirconium alloy, the ratio of the atomic concentration of zirconium to the atomic concentration of chromium in the entire chromium layer and corrosion-resistant layer was 3 / 2 or less. Furthermore, when the corrosion-resistant layer was formed from a titanium alloy, the ratio of the atomic concentration of titanium to the atomic concentration of chromium in the entire chromium layer and corrosion-resistant layer was 1 or less.

[0141] The areas near the joints between the fuel cladding tube and the lower end plug, and between the fuel cladding tube and the upper end plug, were masked before joining. This was because heating the coating layer during welding of the joints could lead to atomic diffusion and melting of each layer. When the coating layers were formed by thermal spraying or cold spraying, the surfaces were polished and flattened after coating to ensure that each layer had the specified thickness. When the coating layers were formed by physical vapor deposition, the coating layers were heat-treated after coating to improve adhesion between the layers.

[0142] Next, a lower end plug was bonded to the lower part of the fuel cladding tube. Then, fuel pellets and a plenum spring were loaded into the fuel cladding tube, and an upper end plug was bonded to the upper part of the fuel cladding tube in a helium gas atmosphere adjusted to a predetermined pressure. After bonding the lower and upper end plugs, the joints were subjected to nondestructive ultrasonic testing to confirm the absence of defects. The zirconium alloy was exposed near the joint due to masking. However, because the area of ​​the joint was small relative to the entire surface of the fuel rod, the fuel rod could still be used even with the exposed zirconium alloy. However, a localized coating layer could also be formed near the joint. Alternatively, the coating layer could be applied to the fuel cladding tube and lower end plug after bonding the lower end plug to the lower part of the fuel cladding tube. In this case, pre-masking was only required near the joint between the fuel cladding tube and the upper end plug.

[0143] (3) Fabrication of the channel box This shows an example of fabricating a channel box in which a coating layer is formed on a substrate made of a zirconium alloy. The coating layer is formed on the inner surface and outer surface of the U-shaped member. The plate material and U-shaped member used to fabricate the channel box are approximately 3 mm thick, which is thicker than a fuel cladding tube. Therefore, a thicker coating layer can be formed compared to the case of a fuel cladding tube. Also, unlike the case of a fuel cladding tube, it can be formed on a flat surface, so a layer with high uniformity in thickness can be formed.

[0144] First, a long rectangular plate made of a zirconium alloy was prepared. A metal plate having a predetermined composition and thickness for forming a chromium layer and a metal plate having a predetermined composition and thickness for forming a corrosion-resistant layer were stacked on the prepared plate in this order, and the stack was then hot-rolled to form a coating layer. When forming an isolation layer, a metal plate having a predetermined composition and thickness for forming the isolation layer was stacked below the metal plate for forming the chromium layer. The stack was then hot-rolled to form a U-shape to produce a U-shaped component. U-shaped components were also produced by forming coating layers on other plate materials using physical vapor deposition, thermal spraying, or cold spraying.

[0145] When forming a corrosion-resistant layer using a zirconium alloy, it is necessary to appropriately control the chemical composition of the zirconium alloy in order to form a highly protective oxide film on the surface of the corrosion-resistant layer. When forming a corrosion-resistant layer using physical vapor deposition, thermal spraying, or cold spraying, differences in chemical composition between the material used for coating and the formed corrosion-resistant layer are likely to occur, making it difficult to control the chemical composition of the zirconium alloy. In contrast, thin-plate cladding allows the use of thin plates with a pre-adjusted chemical composition, making it easier to control the chemical composition. Furthermore, it is possible to form a denser layer than thermal spraying or cold spraying, which are prone to forming pores within the layer.

[0146] Next, the U-shaped components were butted together at both ends and joined together by plasma welding. After joining the U-shaped components, the joint was subjected to non-destructive ultrasonic testing to confirm the absence of defects. The zirconium alloy weld metal was exposed near the joint. However, because the area of ​​the joint was small relative to the entire surface of the channel box, the channel box could still be used even with the zirconium alloy exposed. However, a localized coating layer could also be applied near the joint.

[0147] After joining, the channel box was hardened and annealed using high-frequency induction heating in order to control the material structure. It was then shaped to fit the dimensions and polished to remove the oxide film. The corrosion-resistant layer was coated thickly in advance to prevent thickness reduction due to polishing. Next, clips were welded to the end of the channel box. The outer surface of the channel box was then cleaned, and a dense oxide film was formed on the outer surface in high-temperature steam under high pressure. After that, parts such as channel spacers were attached to the outer surface of the channel box to complete the channel box.

[0148] (4) Evaluation and fabrication results As described above, when a coating layer consisting of a chromium layer and a corrosion-resistant layer, or a coating layer consisting of an isolation layer, a chromium layer, and a corrosion-resistant layer, is formed on a substrate made of a zirconium alloy that forms a fuel assembly, the chromium layer provides high oxidation resistance at high temperatures, and the corrosion-resistant layer provides high corrosion resistance during normal operation. It was also confirmed that the desired coating layer can be formed by coating before assembly during the fabrication of fuel rods or channel boxes. It was shown that the integrity of the fuel assembly in the event of a reactor accident can be ensured, while the deterioration of the coating structure and the load on the purification system during normal operation can be reduced. [Explanation of symbols]

[0149] 1 Base material 2 chrome layers 3. Corrosion-resistant layer 3a Oxide film 4 isolation layer 5 Cooling water 10 Coating Layer 20 coating layers 100 Coating Structure 200 Coating Structure

Claims

1. 1. A fuel assembly for a water-cooled nuclear reactor, comprising: a substrate formed of a zirconium alloy; a coating layer formed on the substrate, The coating layer comprises a chromium layer formed of chromium or a chromium alloy on a surface of the base material that comes into contact with cooling water; a corrosion-resistant layer formed of a zirconium alloy or a titanium alloy on the surface of the chromium layer.

2. 2. The fuel assembly of claim 1, The coating layer includes an isolation layer formed of niobium or titanium between the substrate and the chromium layer.

3. 2. The fuel assembly of claim 1, The coating layer has an oxide film on the surface of the corrosion-resistant layer that comes into contact with the cooling water.

4. 2. The fuel assembly of claim 1, the substrate is formed of a zirconium alloy containing one or more alloying elements selected from niobium, tin, iron, chromium, and nickel, each at a concentration of 3 mass% or less, with the remainder being zirconium and unavoidable impurities; the chromium layer contains one or more of niobium, tin, iron, chromium, nickel, zirconium, titanium, and unavoidable impurities diffused from a layer adjacent to the chromium layer; A fuel assembly, wherein the corrosion-resistant layer contains one or more of niobium, tin, iron, chromium, nickel, zirconium, titanium, and unavoidable impurities diffused from a layer adjacent to the corrosion-resistant layer.

5. 5. The fuel assembly of claim 4, a corrosion-resistant layer having a region containing chromium diffused from the chromium layer at a concentration of 3 mass % or more;

6. 3. The fuel assembly of claim 2, A fuel assembly, wherein the isolation layer contains one or more of niobium, tin, iron, chromium, nickel, zirconium, titanium, and unavoidable impurities diffused from layers adjacent to the isolation layer.

7. 2. The fuel assembly of claim 1, A fuel assembly, wherein the thickness of the chromium layer is 5 μm or more and 1 / 31 or less of the thickness of the base material.

8. 2. The fuel assembly of claim 1, The thickness of the corrosion-resistant layer is 5 μm or more, A fuel assembly wherein, in the coating layer, when the corrosion-resistant layer is formed of a zirconium alloy, the ratio of the atomic concentration of zirconium to the atomic concentration of chromium is 3 / 2 or less, and when the corrosion-resistant layer is formed of a titanium alloy, the ratio of the atomic concentration of titanium to the atomic concentration of chromium is 1 or less.

9. 3. The fuel assembly of claim 2, A fuel assembly, wherein the thickness of the isolation layer is 1 μm or more and 20 μm or less.

10. 2. The fuel assembly of claim 1, a plurality of fuel rods each having fuel pellets loaded into a fuel cladding tube and sealed with an end plug; a water rod disposed centrally of a plurality of the fuel rods; a channel box surrounding the fuel rods; an upper tie plate supporting upper portions of the fuel rods in a spaced-apart relationship; a lower tie plate supporting lower portions of the fuel rods in a spaced relationship; a plurality of spacers that support the intermediate portions of the fuel rods in a spaced-apart relationship; the substrate is a portion that forms one or more of the fuel cladding tube, the end plug, the water rod, and the channel box, A fuel assembly in which the coating layer is formed on the outer surface of the fuel cladding tube, the outer surface of the end plug, the outer surface of the water rod, or the inner surface or the inner and outer surfaces of the channel box.

11. 2. The fuel assembly of claim 1, a plurality of fuel rods each having fuel pellets loaded into a fuel cladding tube and sealed with an end plug; control rod guide thimbles aligned with the fuel rods to guide the insertion of control rods; an in-core instrumentation guide thimble that is aligned with the fuel rod and guides the insertion of an in-core instrumentation device; an upper nozzle disposed above the fuel rods to form a framework for supporting the fuel rods; a bottom nozzle disposed below the fuel rod to form a framework for supporting the fuel rod; a plurality of support grids disposed in intermediate portions of the fuel rods to form a framework for supporting the fuel rods; the base material is a portion that forms one or more of the fuel cladding tube, the end plug, the control rod guide thimble, the in-core instrumentation guide thimble, and the support grid; A fuel assembly in which the coating layer is formed on the outer surface of the fuel cladding tube, the outer surface of the end plug, the outer surface of the control rod guide thimble, the outer surface of the in-core instrumentation guide thimble, or the outer surface of the support grid.

12. A method for manufacturing a fuel assembly for a water-cooled nuclear reactor, comprising: providing a substrate formed of a zirconium alloy; forming a chromium layer of chromium or a chromium alloy on a surface of the substrate that comes into contact with cooling water; forming a corrosion-resistant layer on the surface of the chromium layer using a zirconium alloy or a titanium alloy; and assembling a fuel assembly using the substrate, A method for manufacturing a fuel assembly, in which the chromium layer and the corrosion-resistant layer are formed by a thin-plate cladding method in which thin plates are stacked and diffusion-bonded, a physical vapor deposition method, a thermal spraying method, a cold spraying method, or a plating method before assembly using the base material.

13. 13. The method for manufacturing a fuel assembly according to claim 12, forming an isolation layer of niobium or titanium on a surface of the substrate that comes into contact with cooling water before the step of forming the chromium layer; A method for manufacturing a fuel assembly, wherein the isolation layer is formed by a thin plate cladding method, a physical vapor deposition method, a thermal spraying method, or a cold spraying method in which thin plates are stacked and diffusion bonded before assembly using the base material.

14. 13. The method for manufacturing a fuel assembly according to claim 12, forming an oxide film on a surface of the corrosion-resistant layer that comes into contact with cooling water, a method for manufacturing a fuel assembly, the method comprising: forming the oxide film by exposing the corrosion-resistant layer to high-temperature water or high-temperature steam after assembly using the base material and before putting the fuel assembly into service.

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