A method for preparing a single-rod scale layer of a nuclear power plant fuel assembly and a scale layer deposition device

Abstract

The present application relates to a kind of nuclear power plant fuel assembly single rod scale layer preparation method and scale deposition device, preparation method includes: carrying out pre-oxidation treatment, test rod is placed in heating furnace and is imported carrier gas, control heating furnace temperature rise, steam is imported into heating furnace, after the first set time, complete surface oxide film generation process and remove from heating furnace;Scale layer deposition treatment is carried out, raw material solution is added to reaction kettle, test rod is placed in reaction kettle and adjusts its environmental parameter, control test rod temperature rise and make it occur subcooled boiling, start scale layer deposition process, complete test rod, carry out soaking cleaning.The present application can effectively simulate the generation process of fuel rod surface scale layer under real operating condition by carrying out oxide film preparation and scale layer preparation on the surface of test fuel rod, can obtain test fuel rod with real scale layer structure, provides technical basis for studying fuel assembly single rod scale layer deposition and subsequent study of scale layer removal of fuel rod in actual working condition.

Description

Technical Field

[0001] This invention belongs to the field of coating preparation technology, specifically relating to a method for preparing a single rod of a nuclear power plant fuel assembly scale and a scale deposition apparatus. Background Technology

[0002] Fuel assemblies are the energy source of nuclear power plants. The U-235 chain fission within the fuel rod pellets of the fuel assembly generates energy, but also produces various radioactive nuclides. When the fuel assembly is intact, most of the radioactive nuclides are encased in the fuel rod cladding tubes. These cladding tubes are typically made of zirconium alloy, with an outer diameter of 9.5 mm and a thickness of 0.57 mm. The main component of the zirconium alloy tube is metallic zirconium (Zr), and it also contains elements such as niobium and oxygen, as well as impurities such as iron, chromium, and sulfur.

[0003] The high-temperature, high-pressure water environment at the primary circuit pressure boundary of a pressurized water reactor nuclear power plant causes corrosion damage. Corrosion products are released into the coolant, including metal ions and oxide particles. Since the coolant itself contains iron and nickel ions, these metal ions disperse within it. The surface temperature of the fuel rod cladding tubes exceeds the coolant's saturation temperature, leading to supercooled boiling. In the supercooled boiling region, the solubility of metal ions drops sharply and they rapidly precipitate during the transition from liquid to gas. Under van der Waals forces, the precipitated corrosion products combine with the fuel rod cladding tube surface, forming a loose, porous fouling over time. This fouling is known as corrosion-associated amorphous deposits, primarily originating from iron and nickel ions in the coolant. Its main components are NiFe₂O₄, NiO, and Fe₂O₃, with a high iron content. The thickness of the fouling layer gradually increases over operating time. When the surface of the fuel rod cladding tube is completely covered by the fouling layer, the increased thermal resistance alters the heat transfer characteristics of the cladding tube, significantly increasing the fouling growth rate. The presence of the fouling layer affects the accuracy of eddy current detection for oxide film thickness measurement and interferes with electromagnetic signals during detection at the edge of the fuel waste pool. Therefore, it is necessary to remove the fouling layer from the surface of the fuel rod cladding tube. Thus, simulating the formation of fouling layers during fuel rod operation and providing a technical basis for subsequent research on fouling removal are urgent problems to be solved by those skilled in the art. Summary of the Invention

[0004] One object of the present invention is to provide a method for preparing a single rod fouling layer in a nuclear power plant fuel assembly.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for preparing a single rod fouling layer on a nuclear power plant fuel assembly includes the following steps:

[0007] S1: Pre-oxidation treatment of the test rod, including:

[0008] (1) Place the test bar in the heating furnace, introduce carrier gas into the heating furnace, and then control the heating furnace to heat up to the first set temperature T1;

[0009] (2) Steam is introduced into the heating furnace. After the first set time t1, the test bar completes the process of forming a surface oxide film and is taken out of the heating furnace.

[0010] S2: Scale deposition treatment is performed on the pre-oxidized test rods, including:

[0011] (1) Add the raw material solution to the reaction vessel and place the test rod in the reaction vessel;

[0012] (2) Adjust the environmental parameters inside the reactor to bring the ambient temperature to the set ambient temperature T0 and the ambient pressure to the set ambient pressure P0.

[0013] (3) Control the test bar to heat up to the second set temperature T2, so that it undergoes supercooling boiling and starts the scale deposition process until it stops after the second set time t2.

[0014] (4) Take out the test rod, soak and clean it, and then store it in a vacuum drying environment.

[0015] Preferably, in S1(1), the flow rate of the carrier gas is 10 to 100 L / h; the first set temperature T1 is 500 to 750 °C.

[0016] More preferably, the carrier gas is argon, the flow rate of argon is 10-50 L / h, and the first set temperature T1 is 550-650 °C.

[0017] Preferably, in S1(2), the steam flow rate is 10-100 g / h; the first set time t1 is 5-24 h.

[0018] More preferably, the steam flow rate is 10-50 g / h, and the first set time t1 is 5-12 h.

[0019] Preferably, in S2(1), the raw material solution uses sodium ferric ethylenediaminetetrahexanoate and nickel acetate as ion donors, and deionized water is used to prepare the solution so that the iron ion and nickel ion concentrations in the raw material solution reach a first set concentration m1 and a second set concentration m2, respectively. The first set concentration m1 is 45-100 ppm and the second set concentration m2 is 25-50 ppm.

[0020] More preferably, the first set concentration m1 is 45-55 ppm, and the second set concentration m2 is 25-35 ppm.

[0021] Preferably, in S2(2), the ambient temperature T0 is set to 345℃ and the ambient pressure P0 is set to 15.5~15.8MPa.

[0022] Preferably, in S2(3), the second set temperature T2 is greater than 345℃ and less than or equal to 347℃; the second set time t2 is 24 to 72h.

[0023] More preferably, the second set temperature T2 is 346℃, and the second set time t2 is 48~72h.

[0024] Preferably, in S2(3), during the scale deposition process, heat is continuously supplied to the test rod and its temperature is maintained at the second set temperature T2.

[0025] Preferably, in S2(4), after the test rod is taken out, it is determined whether the scale layer thickness h on the surface of the test rod reaches the set thickness h0. If the scale layer thickness h is less than the set thickness h0, the scale layer deposition process is repeated until the scale layer thickness h is greater than or equal to the set thickness h0, where the set thickness h0 is 10 to 30 μm.

[0026] Preferably, in S2(4), when immersing and cleaning the test rod, the test rod is first immersed in anhydrous ethanol and then cleaned with deionized water.

[0027] More preferably, the test rod is first soaked in anhydrous ethanol for 30-60 minutes, and then rinsed with deionized water 3-5 times.

[0028] Another object of the present invention is to provide a scale deposition apparatus for a method of preparing scale on a single rod of a fuel assembly in a nuclear power plant.

[0029] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0030] A scale deposition apparatus for a method of preparing scale on a single rod of a nuclear power plant fuel assembly includes a raw material tank, a transfer pump, a preheater, a reactor, a cooler, and a waste liquid tank. The outlet of the raw material tank is connected to the inlet of the transfer pump, the outlet of the transfer pump is connected to the inlet of the preheater, the outlet of the preheater is connected to the inlet of the reactor, the outlet of the reactor is connected to the inlet of the cooler, and the outlet of the cooler is connected to the inlet of the waste liquid tank.

[0031] Preferably, the reactor includes a reactor body and a reactor cover. A heating element is provided on the outside of the reactor body. The reactor cover is located on the top of the reactor body and has an opening. A sleeve is provided at the opening and extends into the reactor body. The outer surface of the sleeve is used to install a test rod, and a heating rod is provided inside the sleeve.

[0032] Due to the application of the above-mentioned technical solution, the present invention has the following advantages compared with the prior art:

[0033] This invention prepares an oxide film and a scale layer on the surface of a test fuel rod. The preparation method is simple and can effectively simulate the scale layer formation process on the surface of the fuel rod under real operating conditions. It can obtain a test fuel rod with a real scale layer structure, providing a technical basis for studying the scale layer deposition of a single fuel rod in actual operating conditions and for subsequent research on scale layer removal of fuel rods. The scale layer deposition device has a simple structure, is easy to operate, has low cost, and is practical. Attached Figure Description

[0034] Appendix Figure 1 This is a schematic diagram of the pre-oxidation test apparatus in this embodiment;

[0035] Appendix Figure 2 This is a schematic diagram of the scale deposition device in this embodiment.

[0036] In the attached diagrams above:

[0037] 11. Carrier gas module; 12. Steam generation module; 13. Reaction module;

[0038] 21. Raw material tank; 22. Transfer pump; 23. Preheater; 24. Reactor; 241. Reactor body; 242. Reactor cover; 243. Heating element; 244. Sleeve; 245. Heating rod; 25. Cooler; 26. Waste liquid tank; 271. First switch valve; 272. Second switch valve; 273. Third switch valve; 274. Check valve; 275. Back pressure valve. Detailed Implementation

[0039] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0041] A method for preparing a single rod fouling layer on a nuclear power plant fuel assembly includes the following steps:

[0042] S1: Pre-oxidize the test rod;

[0043] S2: Perform scale deposition treatment on the pre-oxidized test rod.

[0044] Specifically:

[0045] The test rod is made of zirconium alloy tube. In order to effectively simulate the formation process of scale on the surface of fuel rod under real operating conditions and obtain a test rod with a real scale structure, this preparation method mainly includes two stages: pre-oxidation treatment and scale deposition treatment.

[0046] The main reason for performing pre-oxidation treatment in step S1 is that, in the actual nuclear reactor operating environment, thick scale layers usually only form after long-term operation. At this time, under the oxidation of fuel rod cladding tubes by high-temperature and high-pressure water for a long time, a zirconium oxide layer will form on the surface. The crystallized metal ions are adsorbed on the surface of the fuel rod cladding tubes under the action of van der Waals forces. The adsorption rate is affected by factors such as the surface roughness, heat flux density, and dielectric polarizability of the fuel rod cladding tubes. Therefore, the presence of this zirconium oxide layer will affect the deposition rate and effect of the scale layer. In order to better simulate the real scaling environment, it is necessary to pre-oxidize the test rods to form an oxide film with a thickness of 2 to 4 μm on its surface.

[0047] In step S1, the test rod is pre-oxidized using a pre-oxidation test device, specifically: as follows: Figure 1As shown, the pre-oxidation test device includes a carrier gas module 11, a steam generation module 12, and a reaction module 13. The carrier gas module 11 and the steam generation module 12 are respectively connected to the reaction module 13 through pipelines. The carrier gas module 11 is used to store and transport the carrier gas required for the reaction environment to generate the oxide film. The carrier gas includes argon, helium, nitrogen, etc. In this embodiment, the carrier gas module 11 is used to store and transport argon. The steam generation module 12 is used to generate and transport the water vapor required for the reaction environment to generate the oxide film. The reaction module 13 is used to heat the test rod. The reaction module 13 includes a heating furnace, which is specifically a tubular heating furnace.

[0048] In step S1, the pre-oxidation treatment of the test rod specifically includes the following steps:

[0049] (1) Place the test rod in the heating furnace and position it in the outlet cooling section of the heating furnace; start the carrier gas module 11 and introduce argon into the heating furnace. The flow rate of argon is 10-100 L / h, preferably 10-50 L / h; then control the heating furnace to heat up to the first set temperature T1, which is 500-750℃, preferably 550-650℃.

[0050] (2) After the heating furnace is heated to the first set temperature T1, it enters the heat preservation stage, and then the steam generation module 12 is started to introduce steam into the heating furnace. The steam flow rate is 10-100 g / h, preferably 10-50 g / h. After the steam is introduced for 10-60 min, the test rod is transferred to the constant temperature section inside the heating furnace. After the first set time t1, the test rod completes the surface oxide film formation process. The first set time t1 is 5-24 h, preferably 5-12 h. After the test rod cools down, it is taken out of the heating furnace.

[0051] In step S2, a scale deposition device is used to deposit scale on the test rod, specifically: as follows: Figure 2 As shown, the scale deposition device includes a raw material tank 21, a transfer pump 22, a preheater 23, a reactor 24, a cooler 25, and a waste liquid tank 26. The outlet of the raw material tank 21 is connected to the inlet of the transfer pump 22, the outlet of the transfer pump 22 is connected to the inlet of the preheater 23, the outlet of the preheater 23 is connected to the inlet of the reactor 24, the outlet of the reactor 24 is connected to the inlet of the cooler 25, and the outlet of the cooler 25 is connected to the inlet of the waste liquid tank 26.

[0052] The delivery pump 22 is a horizontal flow plunger pump. The flow control accuracy of the horizontal flow plunger pump can reach 0.01 mL / min, and the maximum working pressure is 35 MPa, which can achieve precise flow control.

[0053] The reactor 24 includes a reactor body 241 and a reactor cover 242. A heating element 243 is fitted onto the outside of the reactor body 241 for heat preservation. The reactor cover 242 is detachably mounted on the top of the reactor body 241. The reactor cover 242 has an opening through which a sleeve 244 is installed. The end of the sleeve 244 extends into the reactor body 241. The outer surface of the sleeve 244 is used to mount a test rod, and the inside of the sleeve 244 is used to mount a heating rod 245. That is, the heating rod 245 is inserted into the sleeve 244 from the opening of the reactor cover 242. When the heating rod 245 is heating, it first transfers heat to the sleeve 244 and then to the test rod, achieving supercooled boiling on the surface of the test rod. In some embodiments, a thermocouple temperature sensor can be integrated inside the heating rod 245. The thermocouple temperature sensor, in conjunction with a controller, can achieve temperature display and control.

[0054] In this embodiment: the reactor 24 is made of nickel-based alloy and has an effective volume of 950 mL; the sleeve 244 has a diameter of 18 mm and a length of 140 mm; the heating rod 245 is also made of nickel-based alloy and has a diameter of 18 mm and a length of 140 mm.

[0055] In addition, such as Figure 2 As shown, the scale deposition device also includes a valve assembly, which includes a first switching valve 271, a second switching valve 272, a third switching valve 273, a check valve 274, and a back pressure valve 275. The first switching valve 271 and the check valve 274 are located between the outlet of the preheater 23 and the inlet of the reactor 24. The second switching valve 272, the third switching valve 273, and the back pressure valve 275 are located between the outlet of the cooler 25 and the inlet of the waste liquid tank 26, and the back pressure valve 275 is located between the second switching valve 272 and the third switching valve 273.

[0056] In step S2, the scale deposition treatment of the test rod specifically includes the following steps:

[0057] (1) Add raw material solution to reaction vessel 24. The raw material solution uses sodium ferric ethylenediaminetetrahexanoate and nickel acetate as ion donors and deionized water to prepare the solution so that the iron ion and nickel ion concentrations in the raw material solution reach the first set concentration m1 and the second set concentration m2, respectively. The first set concentration m1 is 45-100 ppm and the second set concentration m2 is 25-50 ppm. The main reason is that the main source of scale is iron and nickel ions in the coolant. Therefore, it is necessary to create a high temperature and high pressure water environment containing iron and nickel ions. Iron and nickel ions will undergo hydrolysis reaction in high temperature and high pressure water, forming corresponding hydroxide precipitates and causing the acidity of the environment to increase, which is not conducive to scale deposition. Therefore, sodium ferric ethylenediaminetetrahexanoate and nickel acetate are used as ion donors. Iron and nickel ions are slowly released into the liquid environment in the water, thereby avoiding the acidic environment caused by strong hydrolysis. In this embodiment, in order to improve the scale deposition rate, the concentration of iron and nickel ions donors is appropriately increased, and the first set concentration m1 is controlled to be 45-55 ppm and the second set concentration m2 is controlled to be 25-35 ppm.

[0058] The test rod is then installed on the sleeve 244 of the reactor 24. Specifically, the test rod is decomposed into multiple tube segments along its extension direction, each tube segment being 20mm in length. Each tube segment is then decomposed into four arc segments along its radial direction. The arc segments are then bound to the outer surface of the sleeve 244 using a nickel-chromium wire with good corrosion resistance, thus achieving a tight connection between the test rod and the sleeve 244 and avoiding errors caused by uneven heat conduction.

[0059] Insert the heating rod 245 into the sleeve 244, and then seal the reactor lid 242 and reactor body 241 together.

[0060] (2) Adjust the environmental parameters inside reactor 24 to bring the ambient temperature to the set ambient temperature T0 and the ambient pressure to the set ambient pressure P0. Since the typical operating pressure of the primary loop of a pressurized water reactor nuclear power plant is 15.5 MPa, and its corresponding saturation temperature is 345°C, the set ambient temperature T0 is 345°C and the set ambient pressure P0 is 15.5–15.8 MPa. Specifically:

[0061] Open the first switch valve 271, the second switch valve 272, and the third switch valve 273, start the transfer pump 22, set the flow rate of the transfer pump 22 to 10-30 mL / min, flush the scale deposition device for 15-30 min, then adjust the pressure through the back pressure valve 275 to make the pressure inside the reactor 24 15.5-15.8 MPa, then control the heating element 243 to work and set the target temperature to 345℃. During the heating process, after the temperature inside the reactor 24 rises to 200℃, start the preheater 23 and set the target temperature to 200℃ until the temperature inside the reactor 24 rises to 345℃ and runs stably for 15-30 min.

[0062] (3) Control the temperature of the test rod to the second set temperature T2, so that it undergoes supercooled boiling and begins the scale deposition process, until it stops after the second set time t2. Specifically:

[0063] The second set temperature T2 is greater than 345°C and less than or equal to 347°C. Because the surface temperature of the fuel rod cladding tube exceeds the saturation temperature of the coolant due to the heating effect of the internal fuel pellets, supercooling boiling will occur. In other words, according to basic thermal hydraulic research, supercooling boiling can occur when the surface temperature of the fuel rod cladding tube is 0.1°C higher than the ambient temperature. Therefore, during scale deposition, the surface temperature of the test rod should be higher than 345°C. However, according to the regulations of the International Atomic Energy Agency, the maximum surface temperature of the test rod must not exceed 347°C. Therefore, the second set temperature T2 is greater than 345°C and less than or equal to 347°C. In this embodiment, the second set temperature T2 is set to 346°C.

[0064] The second set time t2 is 24 to 72 hours. In this embodiment, the second set time t2 is 48 to 72 hours, that is, the scale deposition stops after 48 to 72 hours.

[0065] Furthermore, during the deposition process, heat needs to be continuously supplied to the test rod to maintain its temperature at the second set temperature T2. Specifically, since the vaporization process is endothermic, heat is carried away from the surface of the test rod when supercooled boiling occurs, causing the temperature to drop. To achieve stable supercooled boiling, the surface temperature of the test rod needs to be kept stable above 345°C. Therefore, heating rod 245 needs to continuously supply heat to the test rod. However, since the maximum surface temperature of the test rod must not exceed 347°C, there is a heat flux value for maintaining supercooled boiling, which is calculated using the following formula:

[0066]

[0067]

[0068] in:

[0069] q sc This refers to the heat flux during supercooled boiling, expressed in W / m³. 2 q max Critical heat flux, in W / m³ 2 ,

[0070] μ1 is the fluid viscosity, measured in kg / (m·s), h fg Here, enthalpy of vaporization is expressed in J / kg, and g represents gravity.

[0071] ρ l ρ is the density of the liquid phase. v The density is the gas phase density.

[0072] σ represents the surface tension of the gas-liquid two-phase system, expressed in N / m.

[0073] C pl is the specific heat capacity of the liquid phase, expressed in J / (kg·℃).

[0074] C sf These are experimental constants determined based on the bonding force at the interface between the test rod surface and the fluid.

[0075] Let be the Prandtl constant for the liquid phase, and n be a constant determined based on the fluid properties.

[0076] Based on the above calculation formula, the heat flux required to maintain subcooled boiling is 11 kW / m². 2 At this point, the surface temperature of the test rod can be maintained at the second set temperature T2, satisfying the supercooled boiling condition.

[0077] (4) After the scale deposition is completed, take out the test rod and determine whether the scale thickness h on the surface of the test rod reaches the set thickness h0. If the scale thickness h is less than the set thickness h0, the scale deposition process is repeated until the scale thickness h is greater than or equal to the set thickness h0. The set thickness h0 is 10~30um.

[0078] After soaking and cleaning, the test bar is stored in a vacuum drying environment. Specifically, the test bar is first soaked in anhydrous ethanol for 30-60 minutes, then rinsed with deionized water 3-5 times, and then stored in a vacuum drying oven.

[0079] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a single rod fouling layer on a nuclear power plant fuel assembly, characterized in that: Includes the following steps: S1: Pre-oxidation treatment of the test rod, including: (1) Place the test bar in the heating furnace, introduce carrier gas into the heating furnace, and then control the heating furnace to rise to the first set temperature. The test bar is made of zirconium alloy tube, the flow rate of the carrier gas is 10-100 L / h, and the first set temperature is 500-750℃. (2) Steam is introduced into the heating furnace. After the first set time, the test rod completes the process of forming a surface oxide film and is taken out of the heating furnace. The steam flow rate is 10-100 g / h and the first set time is 5-24 h. An oxide film with a thickness of 2-4 μm is formed on the surface of the test rod. S2: Scale deposition treatment is performed on the pre-oxidized test rods, including: (1) Add the raw material solution to the reaction vessel and place the test rod in the reaction vessel. The raw material solution uses ethylenediaminetetraacetic acid iron sodium and nickel acetate as ion donors and deionized water to prepare the solution so that the iron ion and nickel ion concentrations in the raw material solution reach the first set concentration and the second set concentration, respectively. The first set concentration is 45-100 ppm and the second set concentration is 25-50 ppm. (2) Adjust the environmental parameters inside the reactor to bring the ambient temperature and pressure to the set ambient temperature and pressure. The set ambient temperature is 345℃ and the set ambient pressure is 15.5~15.8MPa. (3) Control the test rod to heat up to the second set temperature so that it undergoes supercooling boiling and starts the scale deposition process until the second set time is reached. The second set temperature is greater than 345℃ and less than or equal to 347℃. The second set time is 24 to 72 hours. During the scale deposition process, heat is continuously input into the test rod and its temperature is maintained at the second set temperature. (4) Take out the test rod and determine whether the scale layer thickness on the surface of the test rod reaches the set thickness. If the scale layer thickness is less than the set thickness, the scale layer deposition process is repeated until the scale layer thickness is greater than or equal to the set thickness. The set thickness is 10-30 μm. Soak and clean the rod, and then store it in a vacuum drying environment.

2. The method for preparing a single rod fouling layer of a nuclear power plant fuel assembly according to claim 1, characterized in that: In S2(4), when immersing and cleaning the test bar, the test bar is first immersed in anhydrous ethanol and then cleaned with deionized water.

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