Description:FORM 2
THE PATENTS ACT 1970
[39 of 1970]
&
The Patent Rules 2003

COMPLETE SPECIFICATION
[See sections 10 & rule 13]

TITLE: “HIGH-VELOCITY OXYGEN FUEL (HVOF) SPRAYED CHROMIUM COATING FOR NUCLEAR FUEL CLADDING TUBE”

Name and Address of the Applicant:
Metallizing Equipment Company Pvt Ltd
E-602-604, E.P.I.P. BORANADA, JODHPUR, RAJASTHAN, INDIA - 342012

Nationality: INDIAN

The following specification particularly describes the invention and the manner in which it is to be performed.

HIGH-VELOCITY OXYGEN FUEL (HVOF) SPRAYED CHROMIUM COATING FOR NUCLEAR FUEL CLADDING TUBE
FIELD OF INVENTION
[0001] The present invention is generally related to a method for High-Velocity Oxygen Fuel (HVOF) sprayed pure chromium (Cr) coating on Zircaloy-4 and SS316, and more particularly to the process, technology and system for HVOF sprayed chromium coating on Zircaloy-4 and SS316 substrates for enhanced accident tolerance of nuclear fuel claddings.
BACKGROUND OF INVENTION
[0002] The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.
[0003] The quest for accident-tolerant fuel (ATF) claddings in nuclear reactors has gained significant momentum in recent years, driven by the need to enhance reactor safety and operational performance, particularly under accident conditions. Traditional zirconium-based claddings, while effective under normal operating conditions, pose significant challenges during high-temperature transients, especially in loss-of-coolant accident (LOCA) scenarios. The high reactivity of zirconium with steam at elevated temperatures results in rapid oxidation and the generation of hydrogen gas, which presents severe safety risks, as evidenced by incidents like the Fukushima Daiichi nuclear disaster.
[0004] In the aftermath of the Fukushima accident, extensive research efforts have focused on developing accident-tolerant fuel (ATF) claddings. Researchers have primarily pursued two approaches: (1) modifying zirconium cladding alloys to improve oxidation resistance by adding alloying elements such as Mo-Zr, iron-based cladding alloys, SiCf/SiC cladding, Nb, Mo, or W, Ti3SiC2, and Ti2AlC; and (2) modifying the surface of zirconium-based claddings by depositing a coating layer of oxidation-resistant material, such as Fe-based alloys, Al3Ti, CrN, TiAlN, AlCrN, and Chromium (Cr).
[0005] The second approach—applying a coating onto zirconium cladding—has emerged as a practical and efficient method to achieve corrosion resistance without altering the base materials. Various coating techniques have been explored to deposit corrosion-resistant coatings on zirconium cladding, including 3D laser coating, arc ion plating, PVD, plasma spray, vacuum arc, and cold spray.
[0006] Among the materials considered for enhancing the high-temperature oxidation resistance of fuel claddings, chromium (Cr) has shown great promise due to its excellent oxidation resistance. Early studies have demonstrated that Cr-coated claddings significantly reduce oxidation rates and improve mechanical integrity under high-temperature steam conditions. These properties are critical for maintaining the structural integrity of fuel claddings during severe accident scenarios, thereby enhancing overall reactor safety. Chromium's exceptional corrosion resistance, high-temperature oxidation resistance, and favorable mechanical properties make it an attractive choice for protecting zirconium alloys from steam oxidation. Additionally, chromium has a relatively low neutron absorption cross-section compared to other materials like FeCrAl and Al3Ti, and it is widely available at a reasonable cost. Chromium also shares similar mechanical and thermal properties with zirconium-based alloys, ensuring compatibility up to their eutectic temperatures. Moreover, chromium has a history of operational use in reactor cores, as some light water reactors (LWRs) utilize chromium-plated control rods to increase wear resistance, and chromium is also a common alloying element in core structural steels.
[0007] Despite the promising attributes of chromium, the use of the high-velocity oxygen fuel (HVOF) spraying process for depositing Cr coatings on zirconium-based cladding has not been thoroughly explored. The HVOF spraying process is a well-established and widely used technique that offers several advantages for applying Cr coatings, including high deposition rates, strong adhesion, and the ability to produce dense and uniform coatings.
[0008] U.S. Patent US 9721676B2 discloses efforts to apply a double-layered coating. The first layer includes an elemental metal (nickel, copper, palladium, and combinations thereof) wherein the intermediate layer is formed by a physical vapor deposition process and the second layer is an oxidation-resistant layer that includes elemental chromium in some percentage to a zirconium alloy cladding tube using a thermal spray, such as a High-Velocity Oxy Fuel (HVOF) technique wherein, to the high-velocity flame stream, the powder is injected, which partially melts. The stream of hot flame and powder is directed towards the surface to be coated. The resulting dense coating has low porosity and high bond strength, providing many benefits such as wear and corrosion resistance. U.S. Patent US9336909B2 also discloses efforts to apply a protective coating consisting of Zr—Al using the HVOF thermal spray technique.
[0009] The present specification presents the findings from high-temperature steam-oxidation tests conducted on the first-ever HVOF-sprayed Cr coatings on Zircaloy-4 and SS316 substrates. Further, there is a need for a specification to evaluate the microstructural characteristics, oxidation kinetics, and mechanical performance of these innovative Cr coatings when subjected to steam oxidation at 1200°C. By analyzing these factors, the present specification seeks to validate the potential of HVOF-sprayed Cr coatings as a robust protective layer for ATF claddings, contributing to the development of safer and more resilient nuclear power systems
[0010] Thus, in view of the above, there is a long-felt need in the industry to address the aforementioned deficiencies and inadequacies.
[0011] Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art through the comparison of described systems with some aspects of the present disclosure, as set forth in the remainder of the present application and with reference to the drawings.

SUMMARY OF THE INVENTION
[0012] A method and system for HVOF sprayed chromium coating on Zircaloy-4 and SS316 for enhanced accident tolerance of nuclear fuel claddings are provided substantially, as shown in and/or described in connection with at least one of the figures.
[0013] An aspect of the present disclosure relates to a method for enhancing the accident tolerance of Zircaloy-4 and SS316 based nuclear fuel claddings. The method includes a step of preparing a Zircaloy-4 and SS316 substrates in the form of cylindrical tubes or flat samples. The method includes a step of grit blasting the substrate using Aluminium oxide (Al2O3) grit to achieve a surface roughness of 6±2 µm. The method includes a step of depositing a chromium (Cr) coating onto the prepared Zircaloy-4 and SS316 substrates using a High-Velocity Oxygen Fuel (HVOF) spraying process. The Cr powder used has a particle size distribution ranging from 5 to 45 µm. The method includes a step of allowing the coated substrate to cool naturally in air. The method includes a step of conducting a high-temperature steam-oxidation test at 1200°C for 1 hour in a controlled steam atmosphere to assess the oxidation resistance and mechanical integrity of the Cr coating. The method includes a step of analyzing the oxidation kinetics by measuring the mass gain per unit area of the coated substrate during exposure to steam at 1200°C.
[0014] In an aspect, the grit blasting is followed by ultrasonic cleaning in acetone to remove surface contaminants from the Zircaloy-4 and SS316 substrates.
[0015] In an aspect, the chromium coating is characterized by a microstructure evaluated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS).
[0016] In an aspect, the mechanical integrity of the Cr coating is assessed using micro-Vickers indentation to measure hardness before and after steam exposure.
[0017] Another aspect of the present disclosure relates to a Zircaloy-4 and SS316 substrates coated with a chromium (Cr) layer for improved accident tolerance. In an aspect, the chromium layer is deposited using a High-Velocity Oxygen Fuel (HVOF) spraying process, wherein the chromium layer has a particle size distribution ranging from 5 to 45 µm. In an aspect, the Cr-coated substrate has a surface roughness of 6±2 µm achieved by grit blasting before coating. In an aspect, the Cr-coated substrate is characterized by enhanced oxidation resistance and mechanical integrity following exposure to steam at 1200°C for 1 hour.
[0018] In an aspect, the Cr coating adheres to the substrate with an adhesion strength measured according to ASTM C-633 standard.
[0019] In an aspect, the surface roughness of the Cr-coated substrate is measured at five random locations according to ISO 4287:2015 standard.
[0020] In an aspect, the Cr-coated substrate is further characterized by X-ray diffraction (XRD) analysis to identify phases formed during steam oxidation.
[0021] In an aspect, the coating thickness and porosity are quantified using metallurgical image analysis as per ASTM B-276.
[0022] Another aspect of the present disclosure relates to a coating method for the substrate of a component, such as a zirconium alloy cladding tube, for use in a water-based nuclear reactor under normal operating conditions and high-temperature oxidation conditions. The method includes a pressurized high-velocity carrier flame of a temperature between 1500° C and 2700° C, adding chromium or chromium-based alloy powder particles having an average diameter of 30 microns or less to the pressurized high-velocity carrier flame, and spraying the particles onto the substrate at a high velocity, preferably from 400 to 700 meters/sec, to form a chromium and/or chromium-based alloy coating on the substrate to a desired thickness.
[0023] Accordingly, one advantage of the present invention is that it uses high-velocity oxy-fuel (HVOF) thermal spray methods for depositing chromium and/or chromium-based alloy on a zirconium alloy flat, cylindrical, or tubular substrate.
[0024] These features and advantages of the present disclosure may be appreciated by reviewing the following description of the present disclosure, along with the accompanying figures wherein like reference numerals refer to like parts.
BRIEF DESCRIPTION OF DRAWINGS
[0025] The accompanying drawings illustrate the embodiments of systems, methods, and other aspects of the disclosure. Any person with ordinary skills in the art will appreciate that the illustrated element boundaries (e.g., boxes, groups of boxes, or other shapes) in the figures represent an example of the boundaries. In some examples, one element may be designed as multiple elements, or multiple elements may be designed as one element. In some examples, an element shown as an internal component of one element may be implemented as an external component in another and vice versa. Furthermore, the elements may not be drawn to scale.
[0026] Various embodiments will hereinafter be described in accordance with the appended drawings, which are provided to illustrate, not limit, the scope, wherein similar designations denote similar elements, and in which:
[0027] FIG. 1 illustrates a perspective view of a scanning electron microscopy (SEM) of the feedstock chrome powder, in accordance with an embodiment of the present subject matter.
[0028] FIG. 2 illustrates a perspective view of an experimental setup for high-temperature steam testing, in accordance with an embodiment of the present subject matter.
[0029] FIG. 3 illustrates perspective views of SEM images of the top surface of as-sprayed Cr coating, in accordance with an embodiment of the present subject matter.
[0030] FIG. 4 illustrates perspective views of cross-sectional SEM images of as-sprayed Cr-coated Zircaloy-4/SS316 (a) Uniform thickness 60-70 microns, (b) EDS analysis of as-sprayed coating (c) Higher magnified image of Cr coating, in accordance with an embodiment of the present subject matter.
[0031] FIG. 5 illustrates a perspective view of XRD patterns of Cr powder and as-sprayed Cr coating, in accordance with an embodiment of the present subject matter.
[0032] FIG. 6 illustrates perspective views of the SEM image of HVOF Cr coating after the steam oxidation test, in accordance with an embodiment of the present subject matter.
[0033] FIG. 7 illustrates a flow chart of a method for enhancing the accident tolerance of Zircaloy-4 and SS316 based nuclear fuel claddings, in accordance with an embodiment of the present subject matter.
DETAILED DESCRIPTION
[0034] The present disclosure is best understood with reference to the detailed figures and description set forth herein. Various embodiments have been discussed with reference to the figures. However, those skilled in the art will readily appreciate that the detailed descriptions provided herein with respect to the figures are merely for explanatory purposes, as the methods and systems may extend beyond the described embodiments. For instance, the teachings presented and the needs of a particular application may yield multiple alternative and suitable approaches to implement the functionality of any detail described herein. Therefore, any approach may extend beyond certain implementation choices in the following embodiments.
[0035] References to “one embodiment,” “at least one embodiment,” “an embodiment,” “one example,” “an example,” “for example,” and so on indicate that the embodiment(s) or example(s) may include a particular feature, structure, characteristic, property, element, or limitation but that not every embodiment or example necessarily includes that particular feature, structure, characteristic, property, element, or limitation. Further, repeated use of the phrase “in an embodiment” does not necessarily refer to the same embodiment.
[0036] Methods of the present invention may be implemented by performing or completing manually, automatically, or a combination thereof, selected steps or tasks. The term “method” refers to manners, means, techniques, and procedures for accomplishing a given task including, but not limited to, those manners, means, techniques, and procedures either known to or readily developed from known manners, means, techniques, and procedures by practitioners of the art to which the invention belongs. The descriptions, examples, methods, and materials presented in the claims and the specification are not to be construed as limiting but rather as illustrative only. Those skilled in the art will envision many other possible variations within the scope of the technology described herein.
[0037] An improved HVOF spray method has been developed to deposit chromium (Cr) or chromium-based alloys onto the surface of a substrate, including tubular surfaces. Of particular interest are substrates used as components in nuclear reactors, specifically zirconium alloy substrates, such as fuel rod cladding tubes used in water-cooled nuclear reactors.
[0038] 'Pure Cr' or 'pure chromium,' as used herein, refers to 100% metallic chromium that may include trace amounts of unintended impurities that do not serve any metallurgical function. For example, pure Cr may contain a few parts per million (ppm) of oxygen. 'Cr-alloy,' 'chromium alloy,' 'Cr-based alloy,' or 'chromium-based alloy,' as used herein, refers to alloys where Cr is the dominant or majority element, along with small but purposeful amounts of other elements that serve a specific function. The Cr alloy may comprise 80% to 99% atom percentage of chromium. 'Cr2O3,' 'chromium oxide,' 'Cr2O3-based alloys,' or 'chromium oxide-based alloys,' as used herein, refer to ceramic-based compositions. The chromium oxide-based alloys may comprise 100% ceramic chromium oxide.
[0039] 'Cr-containing alloys' or 'chromium-containing alloys' are those in which Cr is added in smaller quantities than the majority element or elements. For example, 316 stainless steel, which is an iron-based alloy containing 16% to 18% Cr and 10% to 14% Ni, would be classified as Cr-containing, but not Cr-based.
[0040] The improved coating method enhances the integrity of the fuel cladding pipe under high-temperature accident conditions, and equally importantly, under normal operating conditions. Even under normal operating conditions, hydrogen may form due to Zr oxidation or may be present in water. This hydrogen diffuses into the Zr cladding (a process called hydriding) and causes brittleness in the cladding. The improved Cr-coated fuel cladding pipe will be less susceptible to hydriding of the Zr cladding, contributing to increased cycle length and thereby improving the economics of operating the reactor. The Cr-coated cladding tube is also expected to resist delayed hydride cracking, leading to better performance during subsequent dry cask storage.
[0041] The Cr or Cr/Cr2O3-based alloy coating provided by the method described herein will reduce hydriding by minimizing oxidation and by acting as a diffusion barrier to hydrogen in the water from entering the cladding. There are significant benefits to having such a Cr coating even under normal conditions, but the role of the Cr or Cr/Cr2O3-based coating becomes indispensable during high-temperature accident conditions.
[0042] This specification addresses the critical need for improved accident tolerance in fuel cladding materials, particularly in the context of severe nuclear accidents. For the first time, pure Chromium (Cr) coatings, applied through HVOF spraying, are being investigated for their potential to enhance oxidation resistance under high-temperature steam conditions representative of loss-of-coolant accident scenarios. The specification focuses on the microstructural analysis, oxidation kinetics, and mechanical properties of Cr-coated Zircaloy-4 and SS316 substrates subjected to steam oxidation at 1200°C. Preliminary results indicate that Cr coatings significantly reduce oxidation rates compared to uncoated Zircaloy-4 and SS316, thereby mitigating the risks associated with hydrogen generation and cladding embrittlement. Moreover, the Cr coatings maintain structural integrity and adhesion under prolonged exposure to high-temperature steam, suggesting their viability as a robust protective layer. In an embodiment, SS316 substrates are based on stainless steel 316 substrates.
[0043] The substrate materials used for this study were Zircaloy-4 and SS316. The specimens were prepared as cylindrical tubes and flat with dimensions of 50 mm in length and 9.5 mm in diameter and 50X50 mm, respectively. Before coating, the samples were grit blasted using Al2O3 grit 16 mesh followed by ultrasonic cleaning in acetone to remove any surface contaminants. Surface roughness after blasting was 6±2 µm. Commercial grade Cr powder (purity=99.9 wt.%), by MEC, India was used to form the coating layers, as shown in FIG. 1. FIG. 1 illustrates a perspective view 100 of a scanning electron microscopy (SEM) of the feedstock chrome powder, in accordance with an embodiment of the present subject matter.
[0044] The Cr coatings were deposited using the High Velocity Oxygen Fuel (HVOF) spraying process. A MEC HIPOJET-2700 HVOF gun with special hardware was utilized to apply the coatings. The chromium powder used for the coating had a particle size distribution ranging from 5 to 45 µm. The HVOF process parameters were optimized to achieve a coating thickness of approximately 50-60 µm. The spraying parameters are listed in Table 1.
[0045] Table 1: HVOF Spray parameters
Parameter Value
Oxygen flow 250 slpm
LPG flow 50-55 slpm
Air flow 550 slpm
Powder Feed rate 40 g/min
Spray distance 178 mm
[0046] After the HVOF coating process, the samples were allowed to cool naturally in the air. The coated samples were then cross-sectioned, polished, and characterized using scanning electron microscopy (SEM) to evaluate the microstructure and thickness of the Cr coatings.
[0047] The high-temperature steam-oxidation tests were conducted in a tube furnace equipped with a controlled steam atmosphere. The coated and uncoated Zircaloy-4 and SS316 samples were exposed to steam at a temperature of 1200°C for 1 hr. The steam was generated by passing distilled water through a pre-heated evaporator and introduced into the furnace at a flow rate of 10 ml/min. When the temperature reached 500°C, flowing steam was supplied into the chamber by opening the valve; steam flow velocity of 2 m/s at atmospheric pressure. A laboratory test setup was designed as shown in FIG. 2. FIG. 2 illustrates a perspective view 200 of an experimental setup for high-temperature steam testing, in accordance with an embodiment of the present subject matter.
[0048] During the oxidation tests, the mass gain of the samples was recorded using a high-precision balance to assess the extent of oxidation. The oxidation kinetics were analyzed by plotting the mass gain per unit area as a function of exposure time.
[0001] Microstructural and Mechanical Characterization: Pree and post-exposure, the as-sprayed and oxidized samples were examined using SEM (Carl Zeiss Evo18, Germany) equipped with energy-dispersive X-ray spectroscopy (EDS) to study the oxide layer's morphology and composition. Cross-sectional analyses were performed to measure the thickness of the oxide layer and to evaluate the integrity of the Cr coating after exposure to high-temperature steam.
[0002] Additionally, X-ray diffraction (XRD) (PANalytical Empyrean Series 2, Netherlands) analysis was conducted to identify the phases formed during oxidation. The mechanical integrity of the Cr coatings was assessed using micro-Vickers indentation to measure hardness before and after steam exposure. A Vickers micro hardness instrument (SHIMADZU HMV-G-21ST, Japan) was used to measure the microhardness of the coatings at a load of 300 g for all coatings. Five readings were taken for each coating and an average value was calculated. Metallurgical Image Analysis System (Make: M/s. Qualitech Systems, India) was used to quantify the porosity present in coatings as per ASTM B-276. The surface roughness of the as-sprayed coatings was measured using a Surface Roughness Tester (Mitutoyo Model- SJ-210, Japan). Each coated sample was measured at five random locations with the average and standard deviation of the Ra values being quoted as per the ISO 4287:2015 standard. Adhesion tests were also carried out to evaluate the bonding strength of the Cr coating to the Zircaloy-4 and SS316. For coating adhesion strength measurement, three cylindrical samples 25.4 mm in diameter and 50 mm in length were prepared (thickness of 380 µm) as per the ASTM C-633 standard. The adhesion strength test of the coated samples was measured using a Universal Testing Machine (Model: Instron, Digital tensile bond testing machine 50 kN, USA) having a tensile load with transverse speed ranging from 0.8 to 1.3 mm/minute.
[0003] Results and discussion
[0004] As-sprayed coating
[0005] FIG. 3 illustrates perspective views 300 of SEM images of the top surface of as-sprayed Cr coating, in accordance with an embodiment of the present subject matter. FIG. 3(a) shows the top surface of Cr coating represents the good melting of Cr during spraying. Thus, the flame temperature was sufficient for Cr melting. In traditional HVOF the flame temperature is around 1500-1800 °C. New hardware was introduced to the HIPOJET gun by changing its nozzle design and geometry to get a sufficient temperature to get Cr melting. Cr has a melting point of 1907 °C (3465 °F), which is relatively low compared to the majority of transition metals. The new nozzle produces a flame temperature is around 1950-2000 °C, which is sufficient to melt Cr, as shown in FIG. 3(b) a fully melted Cr splat. However, some partially melted zones were also observed as shown in FIG. 3(a), further it is confirmed by cross-sectional analysis.
[0006] FIG. 4 illustrates perspective views 400 of cross-sectional SEM images of as-sprayed Cr-coated Zircaloy-4/SS316 (a) Uniform thickness 60-70 microns, (b) EDS analysis of as-sprayed coating (c) Higher magnified image of Cr coating, in accordance with an embodiment of the present subject matter. The thickness of the Cr coating was uniform and about 60-70 µm as shown in FIG. 4(a). To obtain more detailed information on the as-sprayed coatings, the microstructure was analyzed at higher magnification. SEM image shown in FIG. 4(c), there were some pores in the as-sprayed coatings indicated in circles. A few unmelted particles were also observed in the microstructure. Furthermore, EDS analysis shows that some oxides (<5%) were formed during the spraying but others had no impurity phase. Dark grey areas SEM images represent the oxide phases, and the light grey areas correspond to well-deformed lamellae with less oxygen.
[0007] FIG. 5 illustrates a perspective view 500 of XRD patterns of Cr powder and as-sprayed Cr coating, in accordance with an embodiment of the present subject matter. It can be confirmed by the XRD pattern that HVOF coating has the cubic phase of Cr as its initial powder has (PDF card index: 06-0694). Due to the lower amount of oxide phase formed in the coating, no oxides were detected in XRD. The coating was also subjected to mechanical testing which is summarized in Table 2.
[0008] Table 2: Mechanical properties of as-sprayed Cr coating
Properties Micro Vickers Hardness HV0.3 Porosity–[%] Adhesion Strength (Mpa) Surface Roughness
Values 630 ± 5 <1% 70.36±7 9±2 µm
[0009] High-temperature steam oxidation testing
[0010] Coated samples 202 were hung in a steam chamber 204 as shown in FIG. 2. To monitor steam flow there were two flow meters (F1 and F2) were installed in the setup. For temperature monitoring, four thermocouples were installed viz. T1, T2, T3 and T4. When the temperature reached 1200°C (recorded by T1 data), flowing steam was supplied into the steam chamber 204 by opening the valve 206. FIG. 2 further depicts a safety switch 208, a steam generator 210, heaters 212, and furnace 214.
[0011] Flat samples (one side) and tubes (outer surface) were coated on one side only to investigate the oxidation behavior between uncoated and coated surfaces. It was identified that the Cr-coated layer was maintained without spallation or severe oxidation, whereas on the same sample, the uncoated surface got oxidized and a layer of about 300 µm was formed after one hr steam-oxidation test.
[0012] FIG. 6 illustrates perspective views 600 of the SEM image of HVOF Cr coating after the steam oxidation test, in accordance with an embodiment of the present subject matter. After the steam exposer, the top surface is shown in FIG. 6(a) and its corresponding EDS analysis in FIG. 6(b). The Cr-coated surface has shown very high corrosion resistance. In addition, the oxide layer was not formed at the interface between the Zircaloy-4/SS316 and Cr-coated layer, as confirmed by the SEM micrograph as shown in FIG. 6(c). It can be concluded that the oxygen diffusion through the Cr-coated layer was effectively prevented during the high-temperature steam oxidation test. At the top surface of the Cr coating, Cr got oxidized and formed a very thin Cr2O3 layer, but the layer was not uniform and very thin, unable to be seen in the cross-sectional SEM micrographs. However, at some locations, Cr2O3 formation can be seen as shown in FIG. 6(d) marked with a circle. By EDS analysis of the top surface, it was confirmed by getting oxygen peaks. To obtain more detailed information on the as-sprayed coatings, the microstructure was analyzed at higher magnification. After the steam oxidation, FIG. 6(e) shows more oxide strings than the as-sprayed coating shown in FIG. 4(c). After the steam oxidation test, the adhesion property of the Cr-coated layer on Zircaloy-4 and SS316 substrates was reasonable to resist the spalling problem due to thermal expansion and oxidation in the LOCA condition.
[0013] FIG. 7 illustrates a flow chart of method 700 for enhancing the accident tolerance of Zircaloy-4/SS316 based nuclear fuel claddings, in accordance with an embodiment of the present subject matter. The method 700 includes a step 702 of preparing a Zircaloy-4 and SS316 substrates in the form of cylindrical tubes or flat samples. The method 700 includes a step 704 of grit blasting the substrate using Aluminium oxide (Al2O3) grit to achieve a surface roughness of 6±2 µm. The method 700 includes a step 706 of depositing a chromium (Cr) coating onto the prepared Zircaloy-4 and SS316 substrates using a High-Velocity Oxygen Fuel (HVOF) spraying process. The Cr powder used has a particle size distribution ranging from 5 to 45 µm. The method 700 includes a step 708 of allowing the coated substrate to cool naturally in the air. The method 700 includes a step 710 of conducting a high-temperature steam-oxidation test at 1200°C for 1 hour in a controlled steam atmosphere to assess the oxidation resistance and mechanical integrity of the Cr coating. The method 700 includes a step 712 of analyzing the oxidation kinetics by measuring the mass gain per unit area of the coated substrate during exposure to steam at 1200°C. In an embodiment, the grit blasting is followed by ultrasonic cleaning in acetone to remove surface contaminants from the Zircaloy-4 and SS316 substrates. In an embodiment, the chromium coating is characterized by a microstructure evaluated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). In an embodiment, the mechanical integrity of the Cr coating is assessed using micro-Vickers indentation to measure hardness before and after steam exposure.
[0014] The method described herein addresses the issue of potential steam reactions with zirconium fuel cladding in a nuclear reactor. This method provides a corrosion-resistant coating that acts as a barrier on the zirconium fuel cladding.
[0015] In various aspects, a method of applying a coating onto a zirconium fuel cladding substrate of a component for use in a water-cooled nuclear reactor is provided. The method includes using a high-velocity flame at a temperature of 1800°C, adding powder particles with a particle size ranging from 5 to 30 microns to the high-velocity flame, and spraying the particles onto a substrate at a velocity of 200 to 800 meters per second to form a coating on the substrate with a desired thickness, for example, up to 100 microns or more. The particles are selected from pure chromium particles, chromium oxide particles, chromium-based alloys, and combinations thereof.
[0016] In one aspect, the invention provides a pure chromium-coated zirconium alloy substrate.
[0017] In another aspect, the invention also provides a multilayered coating. A first coating is deposited on the zirconium substrate to form a first coating layer, and a second coating composition is deposited on the first coating layer to form a second coating layer. The first coating composition includes elemental pure chromium metal, and the second coating composition includes chromium oxide.
[0018] The coating elements should have a melting point greater than a predetermined temperature. In certain embodiments, the melting point is greater than 1200°C.
[0019] The substrate can be a fuel element for a nuclear water reactor. In certain embodiments, the substrate is a zirconium fuel cladding.
[0020] The substrate may have any shape associated with the component to be coated. For example, the substrate may be cylindrical, curved, or flat.
[0021] The method described herein also provides a cladding tube formed from a zirconium alloy with a coating deposited on it. The coating is selected from pure chromium, chromium oxide, chromium-based alloys, and combinations thereof. The coating may have a desired thickness, typically ranging from about 50 to 200 microns or more. HVOF spray coatings can be built up to several hundred microns thick.
[0022] The first and second coating compositions can each be deposited using a process selected from the group consisting of thermal spray methods, such as HVOF, plasma spray, flame spray, and Hybrid-Low Velocity Oxy-Fuel (H-LVOF) processes.
[0023] Thus, pure Cr coating was successfully deposited on Zircaloy-4 and SS316 substrates using the HVOF process for the first time. A uniform Cr coating thickness was achieved. The as-sprayed pure Cr coating exhibits a body-centered cubic phase and possesses good mechanical properties. The Cr-coated surface demonstrated very high corrosion resistance under high-temperature steam exposure during a 1-hour oxidation test. No spallation or coating failure was observed after the high-temperature steam exposure oxidation test. Therefore, the adhesion properties of the Cr-coated layer on Zircaloy-4 and SS316 were sufficient to resist spalling due to thermal expansion and oxidation in LOCA conditions. These findings highlight the potential of HVOF-sprayed Cr coatings to enhance the accident tolerance of nuclear fuel cladding.
[0024] As used herein, and unless the context dictates otherwise, the term “configured to” or “coupled to” is intended to include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms “configured to”, “configured with”, “coupled to” and “coupled with” are used synonymously. Within the context of this document terms “configured to”, “coupled to” and “coupled with” are also used euphemistically to mean “communicatively coupled with” over a network, where two or more devices can exchange data with each other over the network, possibly via one or more intermediary device.
[0025] It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.
[0026] No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
[0027] It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. There is no intention to limit the invention to the specific form or forms enclosed. On the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention, as defined in the appended claims. Thus, it is intended that the present invention cover the modifications and variations of this invention, provided they are within the scope of the appended claims and their equivalents.

CLAIMS
I/We claim:
1. A method for enhancing the accident tolerance of Zircaloy-4 and SS316 based nuclear fuel claddings, the method comprising:
preparing a Zircaloy-4 and SS316 substrates in the form of cylindrical tubes or flat samples;
grit blasting the substrate using Aluminium oxide (Al2O3) grit to achieve a surface roughness of 6±2 µm;
depositing a chromium (Cr) coating onto the prepared Zircaloy-4 and SS316 substrates using a High-Velocity Oxygen Fuel (HVOF) spraying process, wherein the Cr powder used has a particle size distribution ranging from 5 to 45 µm;
allowing the coated substrate to cool naturally in air; and
conducting a high-temperature steam-oxidation test at 1200°C for 1 hour in a controlled steam atmosphere to assess the oxidation resistance and mechanical integrity of the Cr coating.

2. The method as claimed in claim 1, comprises analyzing the oxidation kinetics by measuring the mass gain per unit area of the coated substrate during exposure to steam at 1200°C.

3. The method as claimed in claim 1, wherein the grit blasting is followed by ultrasonic cleaning in acetone to remove surface contaminants from the Zircaloy-4 and SS316 substrates.

4. The method as claimed in claim 1, wherein the chromium coating is characterized by a microstructure evaluated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS).
5. The method as claimed in claim 1, wherein the mechanical integrity of the Cr coating is assessed using micro-Vickers indentation to measure hardness before and after steam exposure.

6. A Zircaloy-4 and SS316 substrates coated with a chromium (Cr) layer for improved accident tolerance, wherein the chromium layer is deposited using a High-Velocity Oxygen Fuel (HVOF) spraying process, wherein the chromium layer has a particle size distribution ranging from 5 to 45 µm, wherein the Cr-coated substrate has a surface roughness of 6±2 µm achieved by grit blasting before coating, wherein the Cr-coated substrate is characterized by enhanced oxidation resistance and mechanical integrity following exposure to steam at 1200°C for 1 hour.

7. The Zircaloy-4 and SS316 substrates as claimed in claim 6, wherein the Cr coating adheres to the substrate with an adhesion strength measured according to ASTM C-633 standard.

8. The Zircaloy-4 and SS316 substrates as claimed in claim 6, wherein the surface roughness of the Cr-coated substrate is measured at five random locations according to ISO 4287:2015 standard.

9. The Zircaloy-4 and SS316 substrates as claimed in claim 6, wherein the Cr-coated substrate is further characterized by X-ray diffraction (XRD) analysis to identify phases formed during steam oxidation.

10. The Zircaloy-4 and SS316 substrates as claimed in claim 6, wherein the coating thickness and porosity are quantified using metallurgical image analysis as per ASTM B-276.

Dated August 29, 2024
Digitally Signed by
Jyoti Chauhan
Agent for Applicant
IN-PA-1684

ABSTRACT

HIGH-VELOCITY OXYGEN FUEL (HVOF) SPRAYED CHROMIUM COATING FOR NUCLEAR FUEL CLADDING TUBE

Disclosed is a method for improving the accident tolerance of Zircaloy-4 and SS316 based nuclear fuel claddings that involves preparing the substrate by grit blasting with Aluminium oxide to achieve a specified surface roughness. A chromium (Cr) coating is then deposited onto the prepared substrate using a High-Velocity Oxygen Fuel (HVOF) spraying process, utilizing Cr powder with a particle size distribution of 5 to 45 µm. The coated substrate is allowed to cool naturally in the air. The method includes conducting a high-temperature steam-oxidation test at 1200°C for 1 hour to evaluate the Cr coating's oxidation resistance and mechanical integrity.

The most illustrative drawing: FIG. 1
, Claims:I/We claim:
1. A method for enhancing the accident tolerance of Zircaloy-4 and SS316 based nuclear fuel claddings, the method comprising:
preparing a Zircaloy-4 and SS316 substrates in the form of cylindrical tubes or flat samples;
grit blasting the substrate using Aluminium oxide (Al2O3) grit to achieve a surface roughness of 6±2 µm;
depositing a chromium (Cr) coating onto the prepared Zircaloy-4 and SS316 substrates using a High-Velocity Oxygen Fuel (HVOF) spraying process, wherein the Cr powder used has a particle size distribution ranging from 5 to 45 µm;
allowing the coated substrate to cool naturally in air; and
conducting a high-temperature steam-oxidation test at 1200°C for 1 hour in a controlled steam atmosphere to assess the oxidation resistance and mechanical integrity of the Cr coating.

2. The method as claimed in claim 1, comprises analyzing the oxidation kinetics by measuring the mass gain per unit area of the coated substrate during exposure to steam at 1200°C.

3. The method as claimed in claim 1, wherein the grit blasting is followed by ultrasonic cleaning in acetone to remove surface contaminants from the Zircaloy-4 and SS316 substrates.

4. The method as claimed in claim 1, wherein the chromium coating is characterized by a microstructure evaluated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS).
5. The method as claimed in claim 1, wherein the mechanical integrity of the Cr coating is assessed using micro-Vickers indentation to measure hardness before and after steam exposure.

6. A Zircaloy-4 and SS316 substrates coated with a chromium (Cr) layer for improved accident tolerance, wherein the chromium layer is deposited using a High-Velocity Oxygen Fuel (HVOF) spraying process, wherein the chromium layer has a particle size distribution ranging from 5 to 45 µm, wherein the Cr-coated substrate has a surface roughness of 6±2 µm achieved by grit blasting before coating, wherein the Cr-coated substrate is characterized by enhanced oxidation resistance and mechanical integrity following exposure to steam at 1200°C for 1 hour.

7. The Zircaloy-4 and SS316 substrates as claimed in claim 6, wherein the Cr coating adheres to the substrate with an adhesion strength measured according to ASTM C-633 standard.

8. The Zircaloy-4 and SS316 substrates as claimed in claim 6, wherein the surface roughness of the Cr-coated substrate is measured at five random locations according to ISO 4287:2015 standard.

9. The Zircaloy-4 and SS316 substrates as claimed in claim 6, wherein the Cr-coated substrate is further characterized by X-ray diffraction (XRD) analysis to identify phases formed during steam oxidation.

10. The Zircaloy-4 and SS316 substrates as claimed in claim 6, wherein the coating thickness and porosity are quantified using metallurgical image analysis as per ASTM B-276.

Dated August 29, 2024
Digitally Signed by
Jyoti Chauhan
Agent for Applicant
IN-PA-1684