Method for prefabricating and installing double-layer sodium pipeline

Abstract

The invention belongs to prefabrication and installation methods, and particularly relates to a prefabrication and installation method for a double-layer sodium pipeline. A prefabrication and installation method for a double-layer sodium pipeline comprises the following steps: step (1), a pipe section arrival acceptance stage; (2) a prefabrication stage; (3) an inner pipe welding stage; and (4) an outer pipe welding stage. The method has the remarkable effects that the problems that the cleanliness requirement is high, the pipe penetrating difficulty is large and the assembly precision requirement is high in the stainless steel double-layer sodium pipeline prefabrication and installation process are solved, and the installation quality and the construction safety are improved.

Description

Technical Field

[0001] This invention pertains to prefabrication and installation methods, specifically relating to a method for prefabricating and installing double-layer sodium pipes. Background Technology

[0002] The double-layer sodium pipeline, consisting of inner and outer layers, involves multiple buildings on the nuclear island, with a total pipeline length approaching one million millimeters. Welding gap requirements: 0–5 mm; beveling misalignment: the maximum allowable misalignment on the inner surface should not exceed 10% of the pipe wall thickness and should be ≤2 mm; cleanliness requirements: Class I; construction area cleanliness: Class II work area; welding environment requirements: wind speed ≤2 m / s for gas shielded welding, wind speed ≤8 m / s for arc welding; relative humidity ≤90%; ambient temperature ≥-10℃; weldment temperature >5℃. For inner and outer layer pipelines with diameters of 914 mm / 813 mm, each 2.5-meter section can weigh up to 1.57 tons; for inner and outer layer pipelines with diameters of 711 mm / 610 mm, each 2.5-meter section can weigh up to 1.29 tons. Due to the large pipeline dimensions, high installation requirements, and tight construction schedule, current installation methods for inner and outer layer pipelines do not meet the requirements.

[0003] The conventional installation method for double-layer sodium pipelines involves cutting the outer casing in half and welding it together after the inner casing is installed. However, given the large diameter and thin wall thickness of the double-layer sodium pipelines in the Xiapu project (maximum specification Φ914*15), significant deformation occurs after cutting, making longitudinal weld construction difficult. The longitudinal welds must avoid the inner casing liner, inevitably leading to poor weld positions and compromised weld quality. Furthermore, the extensive cutting of the outer casing results in numerous longitudinal welds, increasing the workload and hindering construction progress and cost control. Summary of the Invention

[0004] This invention addresses the shortcomings of existing technologies by providing a method for the prefabrication and installation of double-layer sodium pipes.

[0005] This invention is achieved as follows: a method for prefabricating and installing double-layer sodium pipes, comprising the following steps:

[0006] Step (1), Pipe section arrival and acceptance stage;

[0007] Step (2), prefabrication stage;

[0008] Step (3), inner tube welding stage;

[0009] Step (4), outer tube welding stage.

[0010] The method for prefabrication and installation of a double-layer sodium pipe as described above, wherein step (1) includes the following:

[0011] Step S1: Pipe section arrival inspection. Clear markings should be present, including the manufacturer's code or trademark, license mark, material, and product number. The product markings should be traceable to the product quality certification documents. Before use, the markings should be checked to ensure they match the drawings. Inspect the pipe sealing for integrity and for scratches, dents, or other defects. The surface should be free of cracks, shrinkage cavities, slag inclusions, folds, overlapping layers, and should not exceed the negative wall thickness deviation. If any defects exist that prevent confirmation of pipe quality, non-conformities should be identified and handled according to the non-conformity management process. Material storage areas should have prominent signage and effective protective measures. Carbon steel materials are strictly prohibited from being placed in the pipe storage area to prevent contamination of the stainless steel.

[0012] The method for prefabrication and installation of double-layer sodium pipes as described above, wherein step (2) includes the following:

[0013] Step S2: Welding of the support plate. After the pipe section passes inspection, the welding of the support plate is completed in the pipe prefabrication workshop. After the weld passes visual inspection, a liquid penetration test is performed. The liquid penetration ratio is 100%. After the test is completed, the pipe is cleaned with a white cloth soaked in anhydrous ethanol.

[0014] Step S3, Pipe Processing: Before beveling, fabricate a sealing device using polyethylene foam board and lint-free white cloth. Install the sealing device approximately 30mm in front of the beveling location to prevent contamination of the pipe's cleanliness during beveling. After beveling, clean the pipe opening. First, use a stainless steel brush and vacuum cleaner to remove debris and metal filings from the inner wall of the pipe and the sealing device. Then, clean the inner wall of the pipe and the sealing device with a white cloth. Finally, remove the sealing device and clean the installation location and pipe opening again with a white cloth soaked in anhydrous ethanol. The processing area should be enclosed to prevent dust and metal filings from cutting and grinding from contaminating other work within the area. After cutting, promptly clean up any debris, dust, and metal filings generated during the process. Perform a 100% visual inspection and a 100% liquid penetration test on the bevel surface. After the inspection is passed, the testing personnel should clean off the test liquid used during the process with a white cloth soaked in anhydrous ethanol.

[0015] Step S4: Pipe cleaning. If burrs, scale, or other contaminants are present on the pipe, clean them using tools such as wire wheels, wire brushes, and grinding discs. All wire wheels and brushes used should be stainless steel-specific tools. If rust, oil stains, or dirt are present on the outer surface of the pipe, wipe it with a white cloth soaked in anhydrous ethanol. After cleaning, wipe away any remaining anhydrous ethanol on the pipe surface with a clean white cloth. For pipes with a diameter ≥ DN600, manual cleaning can be performed inside the pipe. Workers should wear clean protective clothing before entering the pipe to avoid contaminating it. After cleaning, another person should enter the pipe for inspection and verification. For pipes with a diameter ≥ DN400, appropriately sized polyethylene foam boards can be cut as internal supports, wrapped with a clean white cloth, and wipe the inside of the pipe in the same direction using anhydrous ethanol. For pipes with a diameter < DN400, use anhydrous ethanol cloth wiping method for cleaning, combined with dry compressed air to blow the inside of the pipe in sections. After internal cleaning, promptly seal the pipe opening with a pipe cap to prevent secondary contamination of the pipe before welding begins.

[0016] Step S5, Cleanliness Inspection: Visual Inspection: For Level 1 cleanliness, the inner and outer surfaces of the pipes should be clean, free of corrosion and any debris; for Level 2 cleanliness, the inner and outer surfaces of the pipes should be free of debris. Scattered rust spots are allowed on the outer surface of stainless steel pipes, but their cumulative area should be less than 0.5% of the total area; for invisible parts, use an endoscope for surface visual inspection; the visual inspection must be carried out under an illumination of at least 500 lux without causing glare to the operator. White Cloth Inspection: Stainless steel pipes require 100% white cloth inspection. Use a clean, degreased, lint-free white cloth soaked in anhydrous ethanol, drag it three times in the same direction along the pipe, and inspect the white cloth. The white cloth should be free of oxide scale, rust, oil stains, dirt, and other contaminants. After passing the cleanliness inspection, fill in the cleanliness inspection record.

[0017] Step S6: Pipe Insertion. Before insertion, a concealed inspection should be conducted as required, and relevant concealed inspection records should be created. In the prefabrication workshop, the pipe insertion tooling should be arranged and adjusted. The outer pipe should be hoisted into position on an adjustable transport trolley, and the inner pipe should be hoisted into position on a pipe insertion trolley. Four pipe insertion rollers should be installed at the end of the inner pipe. The concentricity of the outer and inner pipes should be achieved by adjusting the transport trolley and the pipe insertion trolley. Then, the pipe insertion rollers on the inner pipe should be pressed tightly against the inner wall of the outer pipe. The operator should slowly insert the inner pipe into the outer pipe to avoid damaging the pipe wall. After insertion, the pipe insertion rollers should be removed from the inner pipe. Stainless steel rubber pads should be used to protect the bevel, and stainless steel clamps and bolts should be used to secure both ends of the pipe. After the insertion is completed, the pipes should be transported to the site and stored in the factory. Before insertion, the cleanliness requirements of both the inner and outer pipes must be met, and any temporary markings on the surface of the inner pipe must be removed.

[0018] Step S7: Modular prefabrication. Based on the drawings, create a 1:1 scale model of the modular prefabricated piping. Import the modular prefabricated piping model into the overall 3D model of the nuclear island. Use BIM technology to simulate the introduction path of the modular prefabricated piping model and verify the feasibility of the installation location after modular prefabrication. If feasible, the inner straight pipes are assembled and welded together in the prefabrication workshop using pipe-threading fixtures. After welding, the outer casing is prefabricated using pipe-threading fixtures.

[0019] Step S8, pipe segment identification, the pipe segment identification content includes pipe segment abbreviation, area number, drawing number, pipe segment number; according to the design document requirements, check the specifications, materials, dimensions, markings, cleanliness, etc. of the prefabricated pipe segments, and fill in the pipe prefabrication inspection record.

[0020] The method for prefabrication and installation of double-layer sodium pipes as described above, wherein step (3) includes the following:

[0021] Step S9: Inner tube assembly. Before assembly, inspect the bevel and inner surface again, with an inspection range ≥20mm. If burrs, rust spots, oxide scale, or other substances harmful to the welding process are found, clean them manually or mechanically. Weld promptly after cleaning. Do not use forceful alignment, adding gaskets, or multiple layers of gaskets to eliminate gaps, misalignment, or non-concentricity defects at the joint end faces. Mechanical connection should be used for assembly and fixation. The bevel gap for butt welding assembly is 0-5mm for manual tungsten inert gas welding or 0-1.1mm for automatic welding. If the local gap of the weld joint assembly is too large, it should be adjusted to the specified size. It is strictly forbidden to add filler material into the gap. When aligning the pipes, the straightness should be measured at 200mm from the center of the joint. When the nominal diameter of the pipe is less than 100mm, the allowable deviation is 1mm; when the nominal diameter of the pipe is greater than or equal to 100mm, the allowable deviation is 2mm; and the allowable deviation for the entire length is 10mm.

[0022] Step S10: Weld Marking. After the pipe sections and fittings are processed and assembled, temporary markings should be made before welding. The pipe section markings should be marked according to the requirements of the isometric drawing, and the marking content should include the unit number, area number, drawing number, and pipe section number. Formal markings should be made after welding is completed.

[0023] Step S11, inner pipe welding: The pipe weld should not be within the pipe support area. The net distance between the circumferential weld and the support should not be less than 50mm. For welds requiring heat treatment, the net distance from the outer side to the edge of the support should be greater than 5 times the weld width and not less than 100mm. Except for standard pipe fittings, the distance between two butt welds on a straight pipe section should not be less than 3 times the thickness of the fitting; when post-weld heat treatment is required, it should not be less than 6 times the thickness of the weldment. When the nominal pipe diameter is less than 150mm, the distance between welds should not be less than the outer diameter of the pipe and not less than 50mm; when the nominal pipe diameter is greater than or equal to 150mm, the distance between welds should not be less than 150mm. Except for standard elbows, the distance between the center of the pipe weld and the bend starting point should not be less than the outer diameter of the pipe and not less than 100mm. The root pass of the pipe weld should be welded using full argon arc welding to prevent slag from being generated during the welding process and contaminating the pipe. When welding two components of different thicknesses, from one... There must be a smooth transition between different thicknesses. The transition area can be achieved through machining or by depositing molten metal onto thinner parts, with a maximum slope not exceeding 1:4. During welding, the interpass temperature should not exceed 150℃. The back of the root weld should be protected with argon or other shielding gas, and the shielding gas should be maintained until the weld metal thickness is not less than 5mm before stopping. When welding on the surface of austenitic stainless steel parts with a thickness less than 5mm, the back should also be protected with argon or other shielding gas. When the wall thickness e≤5mm, the weld reinforcement should be ≤1.5mm; when the wall thickness 5<e≤10mm, the weld reinforcement should be ≤2mm; when the wall thickness e>10mm, the weld reinforcement should be ≤2.5mm. When the wall thickness e≤5mm, the back weld reinforcement should be ≤1.5mm; when the wall thickness e>5mm, the back weld reinforcement should be ≤2.5mm. After the weld is completed, the pipe opening should be sealed with a pipe cap in a timely manner and firmly attached with stainless steel tape.

[0024] Step 12, Non-destructive Testing: After visual inspection of the welds, liquid penetrant testing should be conducted according to the design documents. The liquid penetrant ratio should be 100%. If defects are found on the weld surface, they should be eliminated promptly. After elimination, the inspection should be repeated until it passes. All pipe butt welds should undergo 100% radiographic testing. If the inspection reveals weld defects exceeding the specifications in the design documents, rework is mandatory. After rework, the weld should be inspected according to the original specified method. If weld rework requires cutting the weld joint, the pipe must be reprocessed and cleaned.

[0025] Step S13: Weld surface treatment. After welding, the weld surface of the 316H pipe is cleaned with a stainless steel wire brush to make it uniformly present a silvery-white metallic luster. Pickling and passivation paste is not required. Perform liquid penetration testing with a liquid penetration ratio of 100%. After the test, clean the surface with a white cloth soaked in anhydrous ethanol.

[0026] Step S14: Temporary attachments, such as tack blocks, temporary lifting lugs, temporary pressure testing or debugging plugs, grounding points, or other items that facilitate pipeline handling, installation, and welding and need to be subsequently removed, should be made of the same material as the base material and welded using the same process as the pipeline welds, taking care to prevent material contamination. Temporary attachments must undergo a liquid penetration test after removal and meet the requirements for the base material. Temporary attachments should be removed using methods that do not damage the base material to avoid tearing the pipeline surface. Residual weld spatter should be thoroughly cleaned and ground smooth. If thermal cutting is used to remove temporary attachments, ensure the thermally cut surface is at least 5mm away from the steel pipe surface before switching to mechanical removal.

[0027] Step S15: Inner pipe pressure test. After the inner pipe welding is completed and the non-destructive testing is qualified, the pipeline system is subjected to strength, leakage, and vacuum tests according to the design specifications to inspect the engineering quality of the pipeline system and each connecting component.

[0028] Step S16: Pipe cleaning. After the inner pipe pressure test, the outer wall of the inner pipe and the inner wall of the outer pipe need to be cleaned. The cleaning method is the same as in step S4.

[0029] Step S17, Cleanliness Inspection: Visual Inspection: For Level 1 cleanliness, the inner and outer surfaces of the pipes should be clean and free of corrosion and any debris. For Level 2 cleanliness, the inner and outer surfaces of the pipes should be free of debris. Scattered rust spots are allowed on the outer surface of stainless steel pipes, but their cumulative area should be less than 0.5% of the total area. For invisible parts, use an endoscope for surface visual inspection. The visual inspection must be carried out under an illumination of at least 500 lux without causing glare to the operator. White Cloth Inspection: Stainless steel pipes require 100% white cloth inspection. Use a clean, degreased, lint-free white cloth soaked in anhydrous ethanol and drag it three times in the same direction along the pipe. The white cloth should be free of oxide scale, rust, oil stains, dirt, and other contaminants. After the cleanliness inspection is passed, fill in the cleanliness inspection record.

[0030] 5. The method for prefabrication and installation of a double-layer sodium pipe as described in claim 4, characterized in that: step (4) includes the following:

[0031] Step S18: Outer tube assembly. Before assembling the outer tubes, the concealed inspection and acceptance of the corresponding inner tubes must be qualified. Before assembly, the bevel and inner surface must be inspected again, with an inspection range ≥20mm. If burrs, rust spots, oxide scale, or other substances harmful to the welding process are found, they should be cleaned manually or mechanically. Welding should be carried out promptly after cleaning is qualified. Methods such as forceful alignment, adding gaskets, or clamping multiple layers of gaskets must not be used to eliminate defects such as gaps, misalignment, or non-concentricity at the interface end faces. Mechanical connection should be used for assembly and fixation. For butt welds, the bevel gap should be 0-5mm for manual tungsten inert gas welding or 0-1.1mm for automatic welding. If the gap in the weld joint is too large in any part, it should be adjusted to the specified size. It is strictly forbidden to add filler material into the gap. When aligning pipes, the straightness should be measured 200mm from the center of the joint. When the nominal diameter of the pipe is less than 100mm, the allowable deviation is 1mm; when the nominal diameter of the pipe is greater than or equal to 100mm, the allowable deviation is 2mm; the allowable deviation for the entire length is 10mm.

[0032] Step S19: Weld Marking. After the pipe sections and fittings are processed and assembled, temporary markings should be made before welding. The pipe section markings should be marked according to the requirements of the isometric drawing, and the marking content should include the unit number, area number, drawing number, and pipe section number. Formal markings should be made after welding is completed.

[0033] Step S20, external pipe welding. The pipe weld should not be within the range of the pipe support. The net distance between the circumferential weld and the support and hanger should not be less than 50 mm. For welds that require heat treatment, the net distance between the outside of the weld and the edge of the support and hanger should be preferably greater than 5 times the weld width and not less than 100 mm. Except for standardized pipe fittings, the distance between two butt welds on the straight pipe section should not be less than 3 times the thickness of the pipe fitting. When post-weld heat treatment is required, it should not be less than 6 times the thickness of the welded part. When the nominal diameter of the pipe is less than 150 mm, the distance between the welds should not be less than the outer diameter of the pipe and not less than 50 mm. When the nominal diameter of the pipe is greater than or equal to 150 mm, the distance between the welds should not be less than 150 mm. Except for using standardized elbows, the distance between the center of the pipe weld and the starting point of the bend of the elbow should not be less than the outer diameter of the pipe and not less than 100 mm. The root pass of the pipe weld should be welded by full argon arc welding to prevent the generation of welding slag during the welding process and pollute the pipe. When welding two components with different thicknesses, there must be a smooth transition from one thickness to the other. The transition area can be obtained by machining or surfacing the deposited metal on the thinner component, and its maximum slope should not exceed 1:4. During the welding process, the interpass temperature should not exceed 150 °C. The back of the root weld should be protected by argon gas or other protective gases, and the protective gas can be stopped only when the thickness of the weld metal is not less than 5 mm. When welding on the surface of austenitic stainless steel parts with a thickness less than 5 mm, the back should also be protected by argon gas or other protective gases. When the wall thickness e ≤ 5 mm, the reinforcement of the weld should be ≤ 1.5 mm. When 5 < e ≤ 10 mm, the reinforcement of the weld should be ≤ 2 mm. When e > 10 mm, the reinforcement of the weld should be ≤ 2.5 mm. When e ≤ 5 mm, the reinforcement of the back weld should be ≤ 1.5 mm. When e > 5 mm, the reinforcement of the back weld should be ≤ 2.5 mm. After the welding of the weld joint is completed, the pipe opening should be blocked with a pipe cap in time and firmly pasted with stainless steel tape.

[0034] Step S21, non-destructive testing. After visual inspection of the weld is qualified, liquid penetration inspection should be carried out according to the provisions of the design document. The liquid penetration ratio is 100%. When defects are found on the surface of the weld, they should be eliminated in time. After elimination, re-inspection should be carried out until qualified. All pipe butt welds should be subjected to 100% radiographic inspection. When it is found that the weld defects exceed the provisions of the design document during the inspection, repair must be carried out. After the weld is repaired, inspection should be carried out according to the original specified method. When the weld repair requires cutting the weld joint, pipe processing and pipe cleaning should be carried out again.

[0035] Step S22: Temporary attachments, such as tack blocks, temporary lifting lugs, temporary pressure testing or debugging plugs, grounding points, or other items that facilitate pipeline handling, installation, and welding and need to be subsequently removed, should be made of the same material as the base material and welded using the same process as the pipeline welds, taking care to prevent material contamination. Temporary attachments must undergo a liquid penetration test after removal and meet the requirements for the base material. Temporary attachments should be removed using methods that do not damage the base material to avoid tearing the pipeline surface. Residual weld spatter should be thoroughly cleaned and ground smooth. If thermal cutting is used to remove temporary attachments, ensure the thermally cut surface is at least 5mm away from the steel pipe surface before switching to mechanical removal.

[0036] Step S23, Pressure Test: After the outer pipe is installed and passes non-destructive testing, a pressure test is conducted. Strength, leakage, and vacuum tests are performed on the piping system according to design specifications to verify the engineering quality of the piping system and its connecting components.

[0037] Step S24: Finished Product Protection. Installed pipes must not be bumped, stepped on, or used as lifting points. When other construction activities are carried out in the area, finished pipes and accessories must be properly protected; fragile parts must be reinforced with hard protection. Completed pipes must not be disassembled or moved without authorization. "Item Protection Signs" must be affixed to all installed items, and regular inspections must be conducted as needed to check whether the appearance and environment meet requirements. Any problems found during inspections must be addressed immediately.

[0038] Step S25, Foreign Object Prevention Management: Before use, white cloths used for cleaning the inside of pipes and checking cleanliness, or wooden blocks used for isolating internal and external pipes, etc., should be registered and their introduction and exit should form a control record.

[0039] During construction, tools, small pipe sections, and debris must not be placed at pipe openings; water sources or water-containing tools are prohibited from entering the pipe prefabrication and installation areas, and residual reagents must be cleaned up promptly after liquid seepage is complete; after construction, pipe openings must be sealed with pipe caps and securely taped with stainless steel tape to prevent foreign objects from entering and causing contamination; temporary sealing devices used during pipe grinding and other construction must be complete and reliable, and cleaned and removed after construction to prevent them from being left inside the pipe; when welding pipes, ensure that temporary components such as argon chambers are secure and reliable, and inspect them before and after use to prevent components from being lost inside the pipe; pipes with upward-facing openings must be sealed firmly and securely, and care must be taken to prevent objects from falling from above when opening them; due to pipe weld grinding or pipe interior... When cleaning requires personnel to enter the pipeline, strict control must be exercised. No unrelated items should be brought in except for necessary tools. If foreign objects enter the installed pipeline due to improper operation, inadequate pipeline protection, or other reasons, the name, specifications, quantity, and other detailed information of the foreign object should be promptly reported to the department's management and quality inspectors. The specific location of the foreign object should be confirmed using an endoscope or other methods. Specific removal measures should be developed based on the characteristics of the foreign object. If the foreign object is deep within the pipeline and cannot be removed, technical personnel should initiate an engineering change to add or cut welds on the pipeline to remove the foreign object. The removal of the foreign object should be witnessed by quality inspectors and recorded. Pipeline restoration should be completed strictly according to the above steps.

[0040] In the above-described method for prefabricating and installing double-layer sodium pipes, if any problems exist in step S1 that prevent the quality of the pipes from being confirmed, non-conformities should be identified and handled according to the non-conformity management process. The material storage area should have prominent markings and effective protective measures. Carbon steel materials are strictly prohibited from being placed in the pipe storage area to avoid contamination of the stainless steel.

[0041] In the double-layer sodium pipe prefabrication and installation method described above, in step S2, after the weld passes visual inspection, a liquid penetration test is performed with a liquid penetration ratio of 100%. After the test is completed, the pipe is cleaned with a white cloth soaked in anhydrous ethanol.

[0042] In step S3, the bevel surface undergoes a 100% visual inspection and a 100% liquid penetration test. After the inspection is passed, the testing personnel should clean off the test solution used in the process with a white cloth soaked in anhydrous ethanol.

[0043] In step S4, for pipes with a diameter ≥ DN600, manual cleaning can be performed inside the pipe. Workers should wear clean, one-piece suits before entering to avoid contaminating the pipe. After cleaning, another person should enter the pipe for inspection and verification. For pipes with a diameter ≥ DN400, appropriately sized polyethylene foam boards can be cut as internal supports, and the outside can be wrapped with a clean white cloth. The inside of the pipe should be wiped in the same direction with anhydrous ethanol. For pipes with a diameter < DN400, anhydrous ethanol cloth wiping method should be used for cleaning, combined with segmented blowing of the inside of the pipe with dry compressed air.

[0044] In step S5, after the cleanliness inspection is passed, a cleanliness inspection record is filled out.

[0045] In step S6, a concealed inspection should be performed as required before pipe insertion. Using pipe insertion tools can prevent damage to the pipe wall. After pipe insertion is completed, stainless steel rubber gaskets should be used to protect the bevel, and stainless steel clamps should be used to secure both ends of the pipe with bolts.

[0046] In step S7, modular prefabricated piping is modeled according to the drawings at a 1:1 scale. The modular prefabricated piping model is then imported into the overall 3D model of the nuclear island. BIM technology is used to simulate the introduction path of the modular prefabricated piping model to verify the feasibility of introducing and installing it at the prefabricated location.

[0047] In step S8, the pipe section is marked and its specifications, material, size, markings, cleanliness, etc. are checked, and a pipe prefabrication inspection record is filled out.

[0048] In the double-layer sodium pipe prefabrication and installation method described above, in step S9, when aligning the pipes, the straightness should be measured at a distance of 200mm from the center of the joint. When the nominal diameter of the pipe is less than 100mm, the allowable deviation 'a' is 1mm; when the nominal diameter of the pipe is greater than or equal to 100mm, the allowable deviation 'a' is 2mm; and the allowable deviation for the entire length is 10mm.

[0049] In step S10, after the pipe sections and fittings are processed and assembled, temporary markings should be made before welding.

[0050] In step S11, when the wall thickness e ≤ 5 mm, the weld reinforcement height is ≤ 1.5 mm; when the wall thickness 5 < e ≤ 10 mm, the weld reinforcement height is ≤ 2 mm; when the wall thickness e > 10 mm, the weld reinforcement height is ≤ 2.5 mm; when the wall thickness e ≤ 5 mm, the back weld reinforcement height is ≤ 1.5 mm; when the wall thickness e > 5 mm, the back weld reinforcement height is ≤ 2.5 mm.

[0051] In step S12, after the weld passes visual inspection, a liquid penetrant test should be performed according to the design documents, with a liquid penetrant ratio of 100%. Furthermore, all pipe butt welds should undergo 100% radiographic testing.

[0052] In step S13, after welding, the weld surface of the 316H material pipe is cleaned with a stainless steel wire brush to give it a uniform silvery-white metallic luster. Pickling and passivation with pickling and passivation paste is not required.

[0053] In step S14, temporary attachments must undergo liquid penetration testing after removal and meet the requirements for the base material inspection.

[0054] In step S15, after the inner pipe welding is completed and the non-destructive testing is passed, the pipeline system is subjected to strength, leakage, and vacuum tests according to the design specifications to verify the engineering quality of the pipeline system and its connecting components.

[0055] In step S16, after the inner tube pressure test is completed, the outer wall of the inner tube and the inner wall of the outer tube need to be cleaned.

[0056] In the double-layer sodium pipe prefabrication and installation method described above, in step S18, the concealed inspection and acceptance of the corresponding inner pipe must pass before the outer pipe is assembled.

[0057] In step S23, after the outer pipe is installed and passes non-destructive testing, a pressure test is performed.

[0058] In step S24, "Item Protection Labels" are affixed to the items that have been placed, and regular inspections are conducted as needed to check whether the appearance and environment meet the requirements.

[0059] In step S25, the introduction and exit of tools and equipment used during construction are recorded; water sources or water-containing tools are prohibited from being brought into the pipe prefabrication and installation area, and residual reagents must be cleaned up in time after liquid infiltration is completed; after construction is completed, pipe openings must be sealed with pipe caps and firmly attached with stainless steel tape; pipes with upward openings must be sealed firmly and securely, and care must be taken to prevent objects from falling from above when they are open; if foreign objects enter deep into the pipe and cannot be removed, the technicians shall initiate an engineering change to add or cut welds on the pipe to remove the foreign objects. The removal of foreign objects shall be witnessed by quality inspectors and recorded.

[0060] The significant advantages of this invention are that it solves the problems of high cleanliness requirements, difficult pipe threading, and high assembly accuracy requirements during the prefabrication and installation of stainless steel double-layer sodium pipes, thereby improving installation quality and construction safety. Detailed Implementation

[0061] A method for prefabricating and installing double-layer sodium pipes, the method comprising the following steps:

[0062] Step S1: Pipe section arrival inspection. Clear markings should be present, including the manufacturer's code or trademark, license mark, material (grade, specification, furnace batch number), product number, etc. The product markings should be traceable to the product quality certification documents. Before use, the markings should be checked to ensure they match the drawings. Inspect the pipe sealing for integrity and for scratches, dents, or other defects. The surface should be free of cracks, shrinkage cavities, slag inclusions, folds, overlapping layers, and should not exceed the negative wall thickness deviation. If any defects exist that prevent confirmation of pipe quality, the non-conformity should be identified and handled according to the non-conformity management process. The material storage area should be clearly marked and have effective protective measures. Carbon steel materials are strictly prohibited from being placed in the pipe storage area to avoid contamination of the stainless steel.

[0063] Step S2, support plate welding. After the pipe section passes inspection, the support plate welding is completed in the pipe prefabrication workshop. After the weld passes visual inspection, a liquid penetration test is performed with a liquid penetration ratio of 100%. After the test is completed, the pipe is cleaned with a white cloth soaked in anhydrous ethanol.

[0064] Step S3, Pipe Processing. Before beveling, a sealing device is made using polyethylene foam board and lint-free white cloth. This device is installed approximately 30mm in front of the beveling location to prevent contamination of the pipe's cleanliness during the beveling process. After beveling, the pipe opening is cleaned. First, a stainless steel brush and vacuum cleaner are used to clean debris and metal filings from the inner wall of the pipe and the sealing device. Then, a white cloth is used to clean the debris from the inner wall of the pipe and the sealing device. Finally, the sealing device is removed, and the installation location and pipe opening are cleaned again with a white cloth soaked in anhydrous ethanol. The processing area should be enclosed to prevent dust and metal filings from cutting and grinding from contaminating other work within the area. After cutting and finishing, debris, dust, and metal filings generated during the process should be cleaned promptly. The bevel surface undergoes a 100% visual inspection and a 100% liquid penetration test. After the inspection is passed, the testing personnel should clean the test liquid used during the process with a white cloth soaked in anhydrous ethanol.

[0065] Step S4, Pipe Cleaning. If burrs, scale, or other substances are present on the pipe, clean them using tools such as wire wheels, wire brushes, and grinding discs. All wire wheels and brushes used should be stainless steel-specific tools. If rust, oil stains, or dirt are present on the outer surface of the pipe, wipe it with a white cloth soaked in anhydrous ethanol. After cleaning, wipe away any remaining anhydrous ethanol on the pipe surface with a clean white cloth. For pipes with a diameter ≥ DN600, manual cleaning can be performed inside the pipe. Workers should wear clean protective clothing before entering the pipe to avoid contaminating it. After cleaning, another person should enter the pipe for inspection and verification. For pipes with a diameter ≥ DN400, appropriately sized polyethylene foam boards can be cut as internal supports, wrapped with a clean white cloth, and then wipe the inside of the pipe in the same direction with anhydrous ethanol. For pipes with a diameter < DN400, clean using anhydrous ethanol cloth wiping method, combined with section-by-section blowing of the pipe interior with dry compressed air. After internal cleaning is completed, the pipe opening should be sealed with a pipe cap in a timely manner to ensure that the pipe is not contaminated again before welding begins.

[0066] Step S5, Cleanliness Inspection. Visual Inspection: For Level 1 cleanliness, the inner and outer surfaces of the pipes should be clean, free of corrosion and any debris; for Level 2 cleanliness, the inner and outer surfaces of the pipes should be free of debris. Scattered rust spots are allowed on the outer surface of stainless steel pipes, but their cumulative area should be less than 0.5% of the total area. For invisible areas, use an endoscope for visual inspection. Visual inspection must be conducted under an illumination of at least 500 lux (equivalent to the illumination of a 100-watt light bulb 30cm away from the inspected surface) without causing glare to the operator. White Cloth Inspection: Stainless steel pipes require 100% white cloth inspection. Use a clean, degreased, lint-free white cloth soaked in anhydrous ethanol, drag it three times in the same direction along the pipe, and inspect the cloth. The white cloth should be free of oxide scale, rust, oil stains, and dirt. After passing the cleanliness inspection, fill out the cleanliness inspection record.

[0067] Step S6, Inner and Outer Pipe Insertion. Before insertion, a concealed inspection should be conducted as required, and relevant concealed inspection records should be created. In the prefabrication workshop, the insertion tooling should be arranged and adjusted. The outer pipe should be hoisted and positioned on an adjustable transport trolley, and the inner pipe should be hoisted and positioned on an insertion trolley. Four insertion rollers should be installed at the end of the inner pipe. The concentricity of the outer and inner pipes should be achieved by adjusting the transport trolley and the insertion trolley. Then, the insertion rollers on the inner pipe should be pressed tightly against the inner wall of the outer pipe. The operator should slowly insert the inner pipe into the outer pipe to avoid damaging the pipe wall. After insertion, the insertion rollers should be removed from the inner pipe. Stainless steel rubber pads should be used to protect the bevel, and stainless steel clamps and bolts should be used to secure both ends of the pipe. After insertion, the pipes should be transported to the site and stored in the factory. Before insertion, the cleanliness of both inner and outer pipes must be ensured, and any temporary markings on the surface of the inner pipe must be removed.

[0068] Step S7, Modular Prefabrication. Based on the drawings, complete the modular prefabricated pipe model at a 1:1 scale. Import the modular prefabricated pipe model into the overall 3D model of the nuclear island. Use BIM technology to simulate the introduction path of the modular prefabricated pipe model and verify the feasibility of the installation location after modular prefabrication. If feasible, the inner straight pipes are assembled and welded together in the prefabrication workshop using pipe-threading fixtures. After welding, the outer casing is prefabricated using pipe-threading fixtures.

[0069] Step S8, Pipe Segment Identification. The pipe segment identification includes AE (pipe segment abbreviation), area number, drawing number, and pipe segment number, for example: AE.JA51.232.T1 (the "." in the middle of each segment in the marking can be omitted); according to the design document requirements, check the specifications, materials, dimensions, markings, cleanliness, etc. of the prefabricated pipe segments, and fill in the pipe prefabrication inspection record.

[0070] Step S9, inner tube assembly. Before assembly, inspect the bevel and inner surface again, with an inspection range of ≥20mm. If burrs, rust spots, oxide scale, or other substances harmful to the welding process are found, clean them manually or mechanically. Weld promptly after cleaning. Do not use methods such as forceful alignment, adding gaskets, or clamping multiple layers of gaskets to eliminate defects such as gaps, misalignment, misalignment, or non-concentricity at the joint end faces. Use mechanical connection (or assembly device) for assembly and fixation. The bevel gap for butt welding assembly is 0-5mm (manual tungsten inert gas welding) or 0-1.1mm (automatic welding). If the local gap of the weld joint assembly is too large, it should be adjusted to the specified size. It is strictly forbidden to add filler in the gap. When aligning pipes, the straightness should be measured at 200mm from the center of the joint. When the nominal diameter of the pipe is less than 100mm, the allowable deviation (a) is 1mm. When the nominal diameter of the pipe is greater than or equal to 100mm, the allowable deviation (a) is 2mm. The allowable deviation for the entire length is 10mm.

[0071] Step S10, Weld Marking. After the pipe sections and fittings are processed and assembled, temporary markings should be made before welding. The pipe section markings should be marked according to the requirements of the isometric drawing. The pipe section markings include the unit number, area number, drawing number, and pipe section number. The weld number should be marked as follows: AXXX—prefabricated weld number (XXX represents the weld serial number, represented by 1-999). Formal markings should be made after welding is completed.

[0072] Step S11, Inner Pipe Welding. Pipe welds should not be within the pipe support area. The net distance between the circumferential weld and the support should not be less than 50mm. For welds requiring heat treatment, the net distance from the outer edge of the support should be greater than 5 times the weld width and not less than 100mm. Except for standardized pipe fittings, the distance between two butt welds on a straight pipe section should not be less than 3 times the thickness of the fitting; when post-weld heat treatment is required, it should be not less than 6 times the thickness of the weldment. When the nominal pipe diameter is less than 150mm, the distance between welds should not be less than the outer diameter of the pipe and not less than 50mm; when the nominal pipe diameter is greater than or equal to 150mm, the distance between welds should not be less than 150mm. Except for standardized elbows, the distance between the center of the pipe weld and the bend's starting point should not be less than the outer diameter of the pipe and not less than 100mm. The root pass of the pipe weld should be welded using full argon arc welding to prevent slag from being generated during the welding process and contaminating the pipe. When welding two components of different thicknesses, from one thickness to the other... A thickness must have a smooth transition. The transition area can be obtained by machining or by depositing molten metal on thinner parts. The maximum slope should not exceed 1:4. During welding, the interpass temperature should not exceed 150℃. The back of the root weld should be protected with argon or other protective gas, and the protective gas should be stopped only when the weld metal thickness is not less than 5mm. When welding on the surface of austenitic stainless steel parts with a thickness of less than 5mm, the back should also be protected with argon or other protective gas. When the wall thickness e≤5mm, the weld reinforcement height should be ≤1.5mm; when the wall thickness 5<e≤10mm, the weld reinforcement height should be ≤2mm; when the wall thickness e>10mm, the weld reinforcement height should be ≤2.5mm. When the wall thickness e≤5mm, the back weld reinforcement height should be ≤1.5mm; when the wall thickness e>5mm, the back weld reinforcement height should be ≤2.5mm. After the weld is completed, the pipe opening should be sealed with a pipe cap in time and firmly attached with stainless steel tape.

[0073] Step 12, Non-destructive Testing. After visual inspection of the welds, a liquid penetrant test should be performed according to the design documents, with a liquid penetrant ratio of 100%. If defects are found on the weld surface, they should be eliminated promptly. After elimination, the test should be repeated until the welds pass. All pipe butt welds should undergo 100% radiographic inspection. If weld defects exceed the specifications in the design documents, repair is mandatory. After repair, the weld should be inspected according to the original method. If weld repair requires cutting the weld joint, the pipe must be re-processed and cleaned.

[0074] Step S13, Weld Surface Treatment. Due to the special nature of its material, after welding, the weld surface of 316H pipes is cleaned with a stainless steel wire brush to give it a uniform silvery-white metallic luster. Pickling and passivation with pickling and passivation paste is not required. If further cleaning is needed after weld surface treatment, follow the cleaning procedures in Step S2.

[0075] Step S14, Temporary Accessories. Temporary accessories or temporary welds, such as tack blocks, temporary lifting lugs, temporary plugs for pressure testing or debugging, grounding points, or other items that facilitate pipeline handling, installation, welding, and subsequent removal, should be made of the same materials as the base material and welded using the same process as the pipeline welds, taking care to prevent material contamination. For temporary accessories, a liquid penetration test must be performed after removal, and the results must meet the inspection requirements of the base material. Temporary accessories should be removed in a manner that does not damage the base material to avoid tearing the pipeline surface material. Residual weld scars should be thoroughly cleaned and ground smooth. If thermal cutting is used to remove temporary accessories, ensure that the thermally cut surface is more than 5mm away from the steel pipe surface before switching to mechanical removal.

[0076] Step S15, Inner Pipe Pressure Test. After the inner pipe welding is completed and the non-destructive testing is qualified, the pipeline system is subjected to strength, leakage and vacuum tests according to the design specifications to inspect the engineering quality of the pipeline system and each connecting component.

[0077] Step S16, Pipeline Cleaning. After the inner pipe pressure test is completed, the outer wall of the inner pipe and the inner wall of the outer pipe need to be cleaned. The cleaning method is the same as in step S4.

[0078] Step S17, Cleanliness check. The cleanliness check is the same as step S5.

[0079] Step S18, outer pipe assembly. Before assembling the outer pipe, the corresponding inner pipe must pass the concealed inspection and acceptance. The assembly work is the same as step S9.

[0080] Step S19, weld marking. The outer pipe weld marking is the same as in step S10.

[0081] Step S20: Welding of the outer tube. The welding of the outer tube is the same as in step S11.

[0082] Step S21, Non-destructive testing. Visual inspection and liquid penetrant testing of the outer pipe weld are performed in the same way as in step S12.

[0083] Step S22, Temporary Annex. The requirements for the temporary annex are the same as in step S12.

[0084] Step S23, Pressure Test. After the outer pipe is installed and passes the non-destructive testing, a pressure test is performed, as in step 15.

[0085] Step S24, Finished Product Protection. Installed pipes must not be bumped, stepped on, or used as lifting points. When other construction activities are carried out in the area, finished pipes and accessories must be protected, and fragile parts must be given additional hard protection. Completed pipes must not be disassembled or moved without authorization. "Item Protection Signs" should be affixed to items in place, and regular inspections should be conducted as needed to check whether the appearance and environment meet the requirements. Any problems found during inspections should be dealt with immediately.

[0086] Step S25, Foreign Object Prevention Management. Before use, white cloths used for cleaning the inside of pipes and checking cleanliness, or wooden blocks used for isolating internal and external pipes, should be registered, and their introduction and exit recorded to form a control record.

[0087] During construction, tools, small pipe sections, and debris must not be placed at pipe openings; water sources or water-containing tools are prohibited from entering the pipe prefabrication and installation areas, and residual reagents must be cleaned up promptly after liquid seepage is complete; after construction, pipe openings (including open areas between inner and outer pipe layers) must be sealed with pipe caps and securely taped with stainless steel tape to prevent foreign objects from entering and causing contamination; temporary sealing devices used during pipe grinding and other construction should be complete and reliable, and cleaned and removed after construction to prevent them from being left inside the pipe; when welding pipes, ensure that temporary components such as argon chambers are secure and reliable, and check them before and after use to prevent components from being lost inside the pipe; pipes with upward-facing openings must be sealed firmly and securely, and when openings are necessary, ensure that there are no objects above them. Foreign objects falling into pipes; when personnel need to enter the pipe interior for welding or cleaning, strict control must be exercised, and no unrelated items should be brought in except for necessary tools; if foreign objects enter the installed pipes due to improper operation, inadequate pipe protection, or other reasons, the name, specifications, quantity, and other detailed information of the foreign object should be reported to the department's management and quality inspection personnel in a timely manner. The specific location of the foreign object should be confirmed using an endoscope or other methods, and specific removal measures should be formulated based on the characteristics of the foreign object. If the foreign object is deep in the pipe and cannot be removed, technical personnel should initiate an engineering change to add or cut welds to the pipe to remove the foreign object. The removal of the foreign object should be witnessed by quality inspection personnel and recorded. Pipeline restoration should be completed strictly according to the above steps.

[0088] In step S1, if there are any problems that make it impossible to confirm the quality of the pipeline, non-conformity items should be initiated and handled in accordance with the non-conformity management process; the material storage area should have conspicuous signs and effective protective measures, and carbon steel materials are strictly prohibited from being placed in the pipeline storage area to avoid contamination of stainless steel.

[0089] In step S2, after the weld passes visual inspection, a liquid penetration test is performed with a liquid penetration ratio of 100%. After the test is completed, the weld is cleaned with a white cloth soaked in anhydrous ethanol.

[0090] In step S3, the bevel surface undergoes 100% visual inspection and 100% liquid penetration inspection. After the inspection is passed, the inspector should clean the test liquid used in the process with a white cloth soaked in anhydrous ethanol.

[0091] In step S4, for pipes with a diameter ≥ DN600, manual cleaning can be performed inside the pipe. Before entering the pipe, the construction personnel should wear clean coveralls to avoid contaminating the pipe. After cleaning, another person should be arranged to enter the pipe for inspection and verification. For pipes with a diameter ≥ DN400, appropriately sized polyethylene foam boards can be cut as internal supports, and the outside can be wrapped with a clean white cloth. The inside of the pipe can be wiped in the same direction with anhydrous ethanol. For pipes with a diameter < DN400, anhydrous ethanol cloth swab washing method can be used for cleaning, combined with dry compressed air to blow the inside of the pipe in sections.

[0092] In step S5, after the cleanliness inspection is passed, a cleanliness inspection record is filled out.

[0093] In step S6, a concealed inspection should be carried out as required before pipe insertion. Using pipe insertion tools can avoid damage to the pipe wall. After pipe insertion is completed, stainless steel rubber pads are used to protect the bevel, and stainless steel clamps are used to fix the pipe ends with bolts.

[0094] In step S7, modular prefabricated pipe modeling is completed according to the drawings at a 1:1 scale. The modular prefabricated pipe model is imported into the overall three-dimensional model of the nuclear island. BIM technology is used to simulate the introduction path of the modular prefabricated pipe model and verify the feasibility of introducing and installing the modular prefabricated pipe model at the installation location.

[0095] In step S8, the pipe section is marked and its specifications, material, size, markings, cleanliness, etc. are checked, and a pipe prefabrication inspection record is filled out.

[0096] In step S9, when aligning the pipes, the straightness should be measured at a distance of 200mm from the center of the joint. When the nominal diameter of the pipe is less than 100mm, the allowable deviation (a) is 1mm. When the nominal diameter of the pipe is greater than or equal to 100mm, the allowable deviation (a) is 2mm. The allowable deviation for the entire length is 10mm.

[0097] In step S10, after the pipe sections and fittings are processed and assembled, temporary markings should be made before welding.

[0098] In step S11, when the wall thickness e ≤ 5 mm, the weld reinforcement height is ≤ 1.5 mm; when the wall thickness 5 < e ≤ 10 mm, the weld reinforcement height is ≤ 2 mm; when the wall thickness e > 10 mm, the weld reinforcement height is ≤ 2.5 mm; when the wall thickness e ≤ 5 mm, the back weld reinforcement height is ≤ 1.5 mm; when the wall thickness e > 5 mm, the back weld reinforcement height is ≤ 2.5 mm.

[0099] In step S12, after the weld passes visual inspection, a liquid penetration test shall be carried out in accordance with the design documents. The liquid penetration ratio shall be 100%, and all pipe butt welds shall be subject to 100% radiographic testing.

[0100] In step S13, after the 316H material pipe is welded, the weld surface is cleaned with a stainless steel wire brush to make it uniformly present a silvery-white metallic luster, without the need for pickling and passivation paste.

[0101] In step S14, the temporary attachments must undergo liquid penetration testing after removal and meet the requirements of the base material inspection.

[0102] In step S15, after the inner tube welding is completed and the non-destructive testing is qualified, the pipeline system is subjected to strength, leakage and vacuum tests according to the design specifications to inspect the engineering quality of the pipeline system and each connecting component.

[0103] In step S16, after the inner tube pressure test is completed, the outer wall of the inner tube and the inner wall of the outer tube need to be cleaned.

[0104] In step S18, the concealed inspection and acceptance of the corresponding inner pipe must be qualified before the outer pipe is assembled.

[0105] In step S23, after the outer tube is installed and passes the non-destructive testing, a pressure test is performed.

[0106] In step S24, “item protection tag” is affixed to the items that have been placed, and regular inspections are carried out as needed to check whether the appearance and environment meet the requirements.

[0107] In step S25, the introduction and exit of tools and equipment used during construction are recorded; water sources or water-containing tools are prohibited from being brought into the pipe prefabrication and installation area, and residual reagents must be cleaned up in time after liquid infiltration is completed; after construction is completed, pipe openings must be sealed with pipe caps and firmly attached with stainless steel tape; pipes with upward openings must be sealed firmly and securely, and care must be taken to prevent objects from falling from above when they are open; if foreign objects enter deep into the pipe and cannot be removed, the technicians shall initiate an engineering change to add or cut welds on the pipe to remove the foreign objects. The removal of foreign objects shall be witnessed by quality inspectors and recorded.

[0108] Here is a specific example.

[0109] Step S1: Pipe Section Arrival and Acceptance. The pipe section surface should have clear markings, including the manufacturer's code or trademark, license mark, material (grade, specification, furnace batch number), and product number. The product markings should be traceable to the product quality certification documents. Before use, the markings should be checked to ensure they match the drawings. Inspect the pipe sealing for integrity and for any scratches, dents, or other defects. The surface should be free of cracks, shrinkage cavities, slag inclusions, folds, overlapping layers, and should not exceed the negative wall thickness deviation. If any defects exist that prevent confirmation of pipe quality, the non-conformity should be identified and handled according to the non-conformity management process. The material storage area should have prominent signage and effective protective measures. Carbon steel materials are strictly prohibited from being placed in the pipe storage area to avoid contamination of the stainless steel.

[0110] Step S2, support plate welding. After the pipe section passes inspection, the support plate welding on the outer surface of the inner pipe is completed in the pipe prefabrication workshop. After the weld passes visual inspection, a liquid penetration test is performed. The liquid penetration ratio is 100%. After the test is completed, the test liquid is cleaned with a white cloth soaked in anhydrous ethanol.

[0111] Step S3, Pipe Processing. Before beveling, a sealing device is made using polyethylene foam board and lint-free white cloth. This device is installed approximately 30mm in front of the beveling location to prevent contamination of the pipe's cleanliness during the beveling process. After beveling, the pipe opening is cleaned. First, a stainless steel brush and vacuum cleaner are used to remove debris and metal filings from the inner wall of the pipe and the sealing device. Then, a white cloth is used to clean the debris from the inner wall of the pipe and the sealing device. Finally, the sealing device is removed, and the installation location and pipe opening are cleaned again with a white cloth soaked in anhydrous ethanol. The beveling area should be enclosed to prevent dust and metal filings from cutting and grinding from contaminating other work within the area. After cutting, debris, dust, and metal filings generated during construction should be cleaned promptly. The bevel surface undergoes a 100% visual inspection and a 100% liquid penetration test. After the inspection is passed, the testing personnel should clean the test solution used during the process with a white cloth soaked in anhydrous ethanol.

[0112] Step S4, Pipe Cleaning. If burrs, scale, or other substances are present on the pipe, clean them using stainless steel wire wheels, wire brushes, grinding discs, etc. If rust, oil stains, or dirt are present on the outer surface of the pipe, wipe it with a white cloth soaked in anhydrous ethanol. After cleaning, wipe away any remaining anhydrous ethanol on the pipe surface with a clean white cloth. For pipes with a diameter ≥ DN600, manual cleaning can be performed inside the pipe. Workers should wear clean protective clothing before entering the pipe to avoid contaminating it. After cleaning, another person should enter the pipe to inspect and verify the cleaning. For pipes with a diameter ≥ DN400, appropriately sized polyethylene foam boards can be cut as internal supports, wrapped with a clean white cloth, and then wiped in the same direction with anhydrous ethanol. For pipes with a diameter < DN400, use anhydrous ethanol cloth wiping method for cleaning, combined with dry compressed air to blow the inside of the pipe in sections. After internal cleaning, promptly seal the pipe openings with pipe caps to prevent secondary contamination of the pipe interior.

[0113] Step S5, Cleanliness Inspection. Visual Inspection: For Level 1 cleanliness, the inner and outer surfaces of the pipes should be clean, free of corrosion and any debris; for Level 2 cleanliness, the inner and outer surfaces of the pipes should be free of debris. Scattered rust spots are allowed on the outer surface of stainless steel pipes, but their cumulative area should be less than 0.5% of the total area. For invisible areas, use an endoscope for visual inspection. Visual inspection must be conducted under an illumination of at least 500 lux (equivalent to the illumination of a 100-watt light bulb 30cm away from the inspected surface) without causing glare to the operator. White Cloth Inspection: Stainless steel pipes require 100% white cloth inspection. Use a clean, degreased, lint-free white cloth soaked in anhydrous ethanol, drag it three times in the same direction along the pipe, and inspect the cloth. The white cloth should be free of oxide scale, rust, oil stains, and dirt. After passing the cleanliness inspection, fill out the cleanliness inspection record.

[0114] Step S6, Internal and External Pipe Installation. Before installation, a concealed inspection should be conducted as required. Temporary markings on the inner pipe surface must be removed, and relevant concealed inspection records should be created. In the prefabrication workshop, the installation tooling should be arranged and adjusted. The outer pipe should be hoisted and positioned on an adjustable transport trolley, and the inner pipe should be hoisted and positioned on an installation trolley. Four installation rollers should be installed at the end of the inner pipe. The concentricity of the outer and inner pipes should be achieved by adjusting the transport trolley and the installation trolley. Then, the installation rollers on the inner pipe should be pressed tightly against the inner wall of the outer pipe. The operator should slowly push the inner pipe into the outer pipe. The installation device can prevent damage to the inner and outer walls of the pipe. After installation, the installation rollers should be removed from the inner pipe. Stainless steel rubber pads should be used to protect the bevel, and stainless steel clamps should be used to secure both ends of the pipe with bolts. Stainless steel rubber rings should be added inside the stainless steel clamps. After the installation is completed, the pipes should be transported to the site and stored in the factory.

[0115] Step S7, Modular Prefabrication. Based on the drawings, complete the modular prefabricated pipe model at a 1:1 scale. Import the modular prefabricated pipe model into the overall 3D model of the nuclear island. Use BIM technology to simulate the introduction path of the modular prefabricated pipe model and verify the feasibility of the installation location after modular prefabrication. If feasible, the inner straight pipes are assembled and welded together in the prefabrication workshop using pipe-threading fixtures. After welding, the outer casing is prefabricated using pipe-threading fixtures.

[0116] Step S8: Identify the pipe segments. The pipe segment identification includes AE (pipe segment abbreviation), area number, drawing number, and pipe segment number, for example: AE.JA51.232.T1; according to the design document requirements, check the specifications, materials, dimensions, markings, cleanliness, etc. of the prefabricated pipe segments, and fill in the pipe prefabrication inspection record.

[0117] Step S9, inner tube assembly. Before on-site installation, the joints must be assembled. Before assembly, the bevel and inner surface must be inspected again, with an inspection range of ≥20mm. If burrs, rust spots, oxide scale, or other substances harmful to the welding process are found, they should be cleaned manually or mechanically. Welding should be carried out promptly after the cleaning is qualified. It is not allowed to use methods such as strong alignment, adding shims, or clamping multiple layers of shims to eliminate defects such as gaps, misalignment, misalignment, or non-concentricity of the interface end face. Mechanical connection (or assembly device) should be used for assembly and fixation. The gap of the butt weld bevel should be 0-5mm (manual tungsten inert gas welding) or 0-1.1mm (automatic welding). If the local gap of the weld joint is too large, it should be adjusted to the specified size. It is strictly forbidden to add filler in the gap. When the pipes are aligned, the straightness should be measured at 200mm from the center of the interface. When the nominal diameter of the pipe is less than 100mm, the allowable deviation (a) is 1mm. When the nominal diameter of the pipe is greater than or equal to 100mm, the allowable deviation (a) is 2mm. The allowable deviation for the entire length is 10mm.

[0118] Step S10, Weld Marking. After the pipe sections and fittings are processed and assembled, temporary markings should be made before welding. The pipe section markings should be marked according to the requirements of the isometric drawing. The pipe section markings include the unit number, area number, drawing number, and pipe section number. The weld number should be marked as follows: AXXX—prefabricated weld number (XXX represents the weld serial number, represented by 1-999). Formal markings should be made after welding is completed.

[0119] Step S11, Inner Pipe Welding. Pipe welds should not be within the pipe support area. The net distance between the circumferential weld and the support should not be less than 50mm. For welds requiring heat treatment, the net distance from the outer edge of the support should be greater than 5 times the weld width and not less than 100mm. Except for standardized pipe fittings, the distance between two butt welds on a straight pipe section should not be less than 3 times the thickness of the fitting; when post-weld heat treatment is required, it should be not less than 6 times the thickness of the weldment. When the nominal pipe diameter is less than 150mm, the distance between welds should not be less than the outer diameter of the pipe and not less than 50mm; when the nominal pipe diameter is greater than or equal to 150mm, the distance between welds should not be less than 150mm. Except for standardized elbows, the distance between the center of the pipe weld and the bend's starting point should not be less than the outer diameter of the pipe and not less than 100mm. The root pass of the pipe weld should be welded using full argon arc welding to prevent slag from being generated during the welding process and contaminating the pipe. When welding two components of different thicknesses, from one thickness to the other... A thickness must have a smooth transition. The transition area can be obtained by machining or by depositing molten metal on thinner parts. The maximum slope should not exceed 1:4. During welding, the interpass temperature should not exceed 150℃. The back of the root weld should be protected with argon or other protective gas, and the protective gas should be stopped only when the weld metal thickness is not less than 5mm. When welding on the surface of austenitic stainless steel parts with a thickness of less than 5mm, the back should also be protected with argon or other protective gas. When the wall thickness e≤5mm, the weld reinforcement height should be ≤1.5mm; when the wall thickness 5<e≤10mm, the weld reinforcement height should be ≤2mm; when the wall thickness e>10mm, the weld reinforcement height should be ≤2.5mm. When the wall thickness e≤5mm, the back weld reinforcement height should be ≤1.5mm; when the wall thickness e>5mm, the back weld reinforcement height should be ≤2.5mm. After the weld is completed, the pipe opening should be sealed with a pipe cap in time and firmly attached with stainless steel tape.

[0120] Step 12, Non-destructive Testing. After visual inspection of the inner pipe welds, a liquid penetrant test should be performed according to the design documents, with a liquid penetrant ratio of 100%. If defects are found on the weld surface, they should be eliminated promptly. After elimination, the test should be repeated until the welds pass. All pipe butt welds should undergo 100% radiographic inspection. If weld defects exceed the design document specifications, repair is mandatory. After repair, the weld should be inspected according to the original method. If weld repair requires cutting the weld joint, the pipe beveling and cleaning must be performed again.

[0121] Step S13, weld surface treatment. Due to the special nature of its material, after welding, the weld surface of the 316H pipe is cleaned with a stainless steel wire brush to give it a uniform silvery-white metallic luster, without the need for pickling or passivation. Cleaning after weld surface treatment is performed according to the cleaning procedures in step S2.

[0122] Step S14, Temporary Accessory Management. Temporary accessories or temporary welds, such as tack blocks, temporary lifting lugs, temporary plugs for pressure testing or debugging, grounding points, or other items that facilitate pipeline handling, installation, welding, and subsequent removal, should use materials identical to the base material and be welded using the same process as the pipeline welds, taking care to prevent material contamination. For temporary accessories, a liquid penetration test must be performed after removal, and the results must meet the inspection requirements of the base material. Temporary accessories should be removed using methods that do not damage the base material to avoid tearing the pipeline surface material. Residual weld spatter should be thoroughly cleaned and ground smooth. If thermal cutting is used to remove temporary accessories, ensure that the thermally cut surface is more than 5mm away from the steel pipe surface before switching to mechanical removal methods.

[0123] Step S15, Inner Pipe Pressure Test. After the inner pipe welding is completed and the non-destructive testing is qualified, the pipeline system is subjected to strength, leakage and vacuum tests according to the design specifications to inspect the engineering quality of the pipeline system and each connecting component.

[0124] Step S16, Pipeline Cleaning. After the inner pipe pressure test is completed, the outer wall of the inner pipe and the inner wall of the outer pipe need to be cleaned. The cleaning method is the same as in step S4.

[0125] Step S17, Cleanliness check. The cleanliness check is the same as step S5.

[0126] Step S18, outer pipe assembly. Before assembling the outer pipe, the corresponding inner pipe must pass the concealed inspection and acceptance. The assembly work is the same as step S9.

[0127] Step S19, weld marking. The outer pipe weld marking is the same as in step S10.

[0128] Step S20: Welding of the outer tube. The welding of the outer tube is the same as in step S11.

[0129] Step S21, Non-destructive testing. Visual inspection and liquid penetrant testing of the outer pipe weld are performed in the same way as in step S12.

[0130] Step S22, Temporary Annex. The requirements for the temporary annex are the same as in step S12.

[0131] Step S23, Pressure Test. After the outer pipe is installed and passes the non-destructive testing, a pressure test is performed, as in step 15.

[0132] Step S24, Finished Product Protection. Installed pipes must not be bumped, stepped on, or used as lifting points; when other construction activities are carried out in the area, finished pipes and accessories must be properly protected; completed pipes must not be disassembled or moved without authorization; "Item Protection Signs" should be affixed to items in place, and regular inspections should be conducted as needed to check whether the appearance and environment meet the requirements; any problems found during inspections should be dealt with immediately.

[0133] Step S25, Foreign Object Prevention Management. Before use, white cloths used for cleaning the inside of pipes and checking cleanliness, or wooden blocks used for isolating internal and external pipes, should be registered, and their introduction and exit recorded to form a control record.

[0134] During construction, tools, small pipe sections, and debris must not be placed at pipe openings; water sources or water-containing tools are prohibited from entering the pipe prefabrication and installation areas, and residual reagents must be cleaned up promptly after liquid seepage is complete; after construction, pipe openings (including open areas between inner and outer pipe layers) must be sealed with pipe caps and securely taped with stainless steel tape to prevent foreign objects from entering and causing contamination; temporary sealing devices used during pipe grinding and other construction should be complete and reliable, and cleaned and removed after construction to prevent them from being left inside the pipe; when welding pipes, ensure that temporary components such as argon chambers are secure and reliable, and check them before and after use to prevent components from being lost inside the pipe; pipes with upward-facing openings must be sealed firmly and securely, and when openings are necessary, ensure that there are no objects above them. Foreign objects falling into pipes; when personnel need to enter the pipe interior for welding or cleaning, strict control must be exercised, and no unrelated items should be brought in except for necessary tools; if foreign objects enter the installed pipes due to improper operation, inadequate pipe protection, or other reasons, the name, specifications, quantity, and other detailed information of the foreign object should be reported to the department's management and quality inspection personnel in a timely manner. The specific location of the foreign object should be confirmed using an endoscope or other methods, and specific removal measures should be formulated based on the characteristics of the foreign object. If the foreign object is deep in the pipe and cannot be removed, technical personnel should initiate an engineering change to add or cut welds to the pipe to remove the foreign object. The removal of the foreign object should be witnessed by quality inspection personnel and recorded. Pipeline restoration should be completed strictly according to the above steps.

Claims

1. A method of prefabrication and installation of a double-layered sodium pipe, characterized in that, The method comprises the following steps: Step (1), pipe segment arrival acceptance stage; Step (2), prefabrication stage; Step (3), inner pipe welding stage; Step (4), outer pipe welding stage.

2. A method of pre-fabrication and installation of a double-layered sodium pipe as claimed in claim 1, wherein: The step (1) comprises the following contents, Step S1, pipe segment arrival acceptance, clear marks should be provided, including manufacturer code or trademark, license mark, material, product number, and product marks should be traceable to product quality certificate, and the marks should be checked before use to be consistent with the drawings; check whether the pipe plugging is complete, whether the surface has scratches, depressions and other defects, and require that the surface should be free of cracks, shrinkage, slag, folding, heavy skin, and wall thickness negative deviation and other defects, if there are problems that cannot confirm the quality of the pipe, the non-conformance item should be opened, and the non-conformance item management process is handled; the material storage area should have clear signs and effective protection measures, and carbon steel materials should be strictly prohibited in the pipe storage area to avoid contamination of stainless steel.

3. A method of pre-fabrication and installation of a double-layered sodium pipe as claimed in claim 2, wherein: The step (2) comprises the following contents, Step S2, support plate welding, after the pipe segment is checked and qualified, support plate welding is completed in the pipe prefabrication workshop, after the welding seam visual inspection is qualified, liquid penetration inspection is carried out, the liquid penetration ratio is 100%, after the detection work is completed, the white cloth soaked with anhydrous ethanol is used for cleaning, Step S3, pipe processing, before beveling, the sealing device is made of polyethylene foam board and non-linting white cloth material, the sealing device is installed about 30mm in front of the beveling position to prevent pollution to the cleanliness of the pipe during beveling, after beveling is completed, the pipe opening part is cleaned, first, the stainless steel brush and dust collector are used to clean the impurities and iron filings on the inner wall of the pipe and the sealing device, then the white cloth is used to clean the impurities on the inner wall of the pipe and the sealing device, finally, the sealing device is taken out, and the sealing device installation position and the pipe opening are cleaned again with the white cloth soaked with anhydrous ethanol; the processing construction area should be ensured to be closed to prevent dust and iron filings generated by cutting and grinding from polluting other work in the area, after cutting and blanking are completed, the impurities, dust and iron filings generated during construction are cleaned in time, the bevel surface is subjected to 100% visual inspection and 100% liquid penetration inspection, and the detection personnel should clean the detection liquid used during the process with the white cloth soaked with anhydrous ethanol after the inspection is qualified, Step S4, pipeline cleaning, when burrs, scale and other substances exist on the pipeline, use wire wheel, wire brush, abrasive disc and other tools for cleaning, the wire wheel and wire brush used should be stainless steel special tools; when rust, oil stains and dirt and other impurities exist on the outer surface of the pipeline, use white cloth dipped in anhydrous ethanol to wipe, after cleaning, use clean white cloth to wipe the residual anhydrous ethanol on the surface of the pipeline; for pipelines with a pipe diameter ≥DN600, personnel can enter the pipeline for manual cleaning, before entering the pipeline, the construction personnel should wear clean one-piece clothing to avoid contaminating the pipeline, after cleaning, another person should enter the pipeline to check and verify; for pipelines with a pipe diameter ≥DN400, cut polyethylene foam board of appropriate size as internal support, wrap the outside with clean white cloth, and wipe the inside of the pipeline in the same direction with anhydrous ethanol; for pipelines with a pipe diameter <DN400, use anhydrous ethanol cloth bundle to clean and combine with dry compressed air to blow the inside of the pipeline in sections, after internal cleaning, promptly seal the pipe opening with a pipe cap to ensure that the pipeline is not contaminated again during the welding period, Step S5, cleanliness inspection, visual inspection: the inner and outer surfaces of the pipeline with first-class cleanliness should be clean, without corrosion and any impurities; the inner and outer surfaces of the pipeline with second-class cleanliness should not have impurities, and the outer surface of the stainless steel pipe may have scattered rust spots, but the cumulative area should be less than 0.5% of the total area; the invisible part is inspected visually using an endoscope; under the premise of not causing the operator's dizziness, visual inspection must be carried out under an illumination of at least 500 lux, white cloth inspection: the stainless steel pipeline requires 100% white cloth inspection, immerse the clean, degreased and lint-free white cloth in anhydrous ethanol, pull and drag the pipeline in the same direction for 3 times, inspect the white cloth, and there should be no scale, rust, oil stains and dirt and other impurities on the white cloth, after the cleanliness inspection is passed, fill in the cleanliness inspection record, Step S6, inner and outer pipe threading, before threading, hidden inspection should be carried out according to the requirements, and relevant hidden records should be formed; in the prefabrication workshop, complete the arrangement and adjustment of the pipe threading tool, hoist the outer sleeve into position on the adjustable transport trolley, hoist the inner pipe into position on the pipe threading trolley, install 4 pipe threading roller devices on the end of the inner pipe, adjust the concentricity of the outer sleeve and the inner pipe through the adjustable transport trolley and the pipe threading trolley, then tightly attach the pipe threading roller devices on the inner pipe to the inner wall of the outer sleeve, and slowly thread the inner pipe into the outer sleeve by the operator, which can avoid damaging the pipe wall; after completing the pipe threading, remove the pipe threading roller devices from the inner pipe, use a stainless steel rubber pad to protect the groove, and use a stainless steel clamp and bolts to fasten and fix the two pipe openings, after the pipe threading is completed, transport it to the site and introduce it into the plant for storage, before pipe threading, ensure the cleanliness requirements of the inner and outer pipes, and remove the temporary markings on the surface of the inner pipe, Step S7, modular prefabrication, according to the drawing 1:1 scale complete modular prefabricated pipe modeling, the modular prefabricated pipe model into the nuclear island whole three-dimensional model, using BIM technology simulation modular prefabricated pipe model introduction path, verify the feasibility of the installation location after modular prefabrication; if feasible, the inner straight pipe and the inner straight pipe are matched and welded in the prefabrication workshop by the pipe penetrating tool, and the outer sleeve pipe is pre-fabricated by the pipe penetrating tool after the welding is completed, Step S8, pipe segment identification, pipe segment identification content includes pipe segment abbreviation, area number, drawing number, pipe segment number; according to the design document requirements, check the specification, material, size, identification, cleanliness of the prefabricated pipe segment, and fill in the pipe prefabrication inspection record.

4. A method of pre-fabrication and installation of a double-layered sodium pipe as claimed in claim 3, wherein: The step (3) comprises the following contents, Step S9, inner tube matching, check the bevel and inner surface again before matching, the inspection range is ≥20mm, if burrs, rust spots, scales and other harmful substances to the welding process are found, manual or mechanical cleaning should be taken, and welding should be carried out in time after the cleaning is qualified; the interface end face gap, deflection, misalignment or eccentricity and other defects should be eliminated by mechanical connection, and the interface end face gap, deflection, misalignment or eccentricity and other defects should be eliminated by mechanical connection; the gap between the butt welding matching bevel is 0-5mm for manual tungsten argon arc welding or 0-1.1mm for automatic welding, when the local gap of the welding opening matching is too large, it should be trimmed to the specified size, and it is strictly forbidden to add filler in the gap; when the pipe is matched, the flatness should be measured at a distance of 200mm from the interface center, when the nominal diameter of the pipe is less than 100mm, the allowable deviation is 1mm, when the nominal diameter of the pipe is greater than or equal to 100mm, the allowable deviation is 2mm, and the full length allowable deviation is 10mm, Step S10, weld identification, after the pipe segment, pipe fitting and other processes and matching are completed, temporary identification should be done before welding, the pipe segment identification should be marked according to the requirements of the axonometric drawing, the pipe segment identification content includes unit number, area number, drawing number, pipe segment number, and the formal identification is carried out after the welding is completed, Step S11, inner tube welding, pipe weld is not suitable in the scope of pipe support, ring weld distance from the net distance of support hanger should not be less than 50 mm, need heat treatment of weld, outside from the edge of support hanger net distance should be greater than 5 times the width of weld, and not less than 100 mm; Except for the shaping pipe fittings, the distance between the two straight pipe sections should not be less than 3 times the thickness of the pipe fittings, and not less than 6 times the thickness of the weld when post-weld heat treatment is required, the distance between the welds should not be less than the outer diameter of the pipe when the nominal diameter of the pipe is less than 150 mm, and not less than 50 mm, the distance between the welds should not be less than 150 mm when the nominal diameter of the pipe is greater than or equal to 150 mm; Except for the use of shaping elbow, the center of the pipe weld and the bending point of the elbow should not be less than the outer diameter of the pipe, and not less than 100 mm; The backing layer of the pipe weld uses full argon arc welding to prevent slag from being generated during welding and to contaminate the pipe; When welding two parts with different thicknesses, there must be a smooth transition from one thickness to another. The transition area can be obtained by machining or building up metal on the thinner part, and the maximum slope cannot exceed 1:4; During welding, the interpass temperature should not exceed 150℃; The back of the root weld should be protected by argon or other protective gas, and the protection gas should be stopped when the weld metal thickness is not less than 5 mm. When welding on the surface of austenitic stainless steel parts with a thickness of less than 5 mm, the back should also be protected by argon or other protective gas; When the wall thickness e≤5mm, the weld reinforcement is ≤1.5mm, when the wall thickness 5 Step 12, non-destructive testing, after the weld inspection is qualified, liquid penetrant testing should be carried out according to the design document, the liquid penetrant ratio is 100%, when defects are found on the surface of the weld, they should be eliminated in time, and the inspection should be carried out again after elimination until it is qualified, all pipe butt welds should be 100% radiographic tested, when the test finds that the weld defects exceed the design document, they must be repaired, and the weld repair should be tested according to the original method, when the weld repair needs to cut the weld, the pipe processing and cleaning should be carried out again, Step S13, weld surface treatment, after the welding of 316H material pipe is completed, the weld surface is cleaned with stainless steel wire brush to make it evenly silver white, acid pickling and passivation are not needed, liquid penetrant testing is carried out, the liquid penetrant ratio is 100%, after the detection work is completed, it is cleaned with white cloth soaked with anhydrous ethanol, Step S14, temporary accessories, temporary accessories or temporary welding points such as spot solid block, temporary lug, pressure test or debugging temporary blocking plate, ground connection or other items that facilitate pipe handling, installation, welding and need to be removed later, the material used should be completely the same as the base material, and the same process as the pipe welding is used for welding, and attention should be paid to prevent material pollution; for temporary accessories, liquid permeation inspection should be carried out after removal and meet the inspection requirements of the base material, and attention should be paid to the removal of the temporary accessories without damaging the base material to avoid causing tearing of the pipe surface material, and the remaining accessory welding scar should be cleaned and polished round; if hot cutting is used to remove the temporary accessory, the hot cutting surface should be more than 5 mm away from the steel pipe surface before mechanical removal, Step S15, inner pipe pressure test, after the inner pipe welding is completed and the non-destructive testing is qualified, the pipe system is subjected to strength, leakage and vacuum test according to the design requirements, and the engineering quality of the pipe system and each connecting part is tested, Step S16, pipe cleaning, after the inner pipe pressure test is completed, the inner pipe outer wall and the outer pipe inner wall need to be cleaned, and the cleaning method is the same as step S4, Step S17, cleanliness inspection, visual inspection: the inner and outer surfaces of the first-class cleanliness pipe should be clean, without corrosion and any impurities; the inner and outer surfaces of the second-class cleanliness pipe should not have impurities, and the outer surface of the stainless steel pipe material is allowed to have scattered rust spots, but the cumulative area should be less than 0.5% of the total area; the surface visual inspection is carried out using an endoscope for invisible parts; under the premise of not causing the operator's dizziness, the visual inspection must be carried out under the illumination of at least 500 lux, white cloth inspection: the stainless steel pipe requires 100% white cloth inspection, the white cloth is soaked in anhydrous ethanol, and the pipe is pulled in the same direction for 3 times, the white cloth is inspected, and there should be no oxidation scale, rust, oil stains and dust on the white cloth, after the cleanliness inspection is qualified, the cleanliness inspection record is filled in.

5. A method of pre-fabrication and installation of a double-layered sodium pipe as claimed in claim 4, wherein: The step (4) comprises the following contents, Step S18, outer pipe assembly, before the outer pipe assembly, the corresponding inner pipe hidden inspection and acceptance should be qualified, the bevel and inner surface are inspected again before assembly, the inspection range is ≥20 mm, if burrs, rust spots, oxidation scale and other substances harmful to the welding process are found, manual or mechanical cleaning is adopted, and the cleaning should be carried out in time after the cleaning is qualified; the interface end face gap, deflection, misalignment or eccentricity and other defects should be eliminated by mechanical connection, and the interface end face gap, deflection, misalignment or eccentricity and other defects should be eliminated by mechanical connection; the butt welding assembly gap is 0-5 mm for manual tungsten argon arc welding or 0-1.1 mm for automatic welding, when the local gap of the welding opening assembly is too large, it should be trimmed to the specified size, and it is strictly forbidden to add filler in the gap; when the pipe is assembled, the straightness should be measured at a distance of 200 mm from the interface center, when the nominal diameter of the pipe is less than 100 mm, the allowable deviation is 1 mm, when the nominal diameter of the pipe is greater than or equal to 100 mm, the allowable deviation is 2 mm, and the full length allowable deviation is 10 mm, Step S19, weld identification, after the pipe segment, pipe fittings and other processing and group pair are completed, temporary identification should be done before welding, pipe segment identification should be marked according to the requirements of the axial drawing, and the pipe segment identification content includes unit number, area number, drawing number and pipe segment number, and formal identification is carried out after welding, Step S20, outer tube welding, pipe welding is not suitable in the range of pipe support, the net distance of girth weld from support hanger should not be less than 50mm, the net distance of outer side from the edge of support hanger should be greater than 5 times the width of weld and not less than 100mm; except for the shaped pipe fittings, the distance between the two butt welds on the straight pipe segment should not be less than 3 times the thickness of the pipe fittings, and not less than 6 times the thickness of the weld when post-weld heat treatment is required, the distance between the welds should not be less than the outer diameter of the pipe and not less than 50mm when the nominal diameter of the pipe is less than 150mm, and the distance between the welds should not be less than 150mm when the nominal diameter of the pipe is greater than or equal to 150mm; except for the use of shaped elbows, the distance between the center of the pipe weld and the bending point of the elbow should not be less than the outer diameter of the pipe and not less than 100mm; the backing layer of the pipe weld adopts full argon arc welding to prevent the generation of welding slag and pollution of the pipe during the welding process; when welding two parts with different thicknesses, there must be a smooth transition from one thickness to another, the transition area can be obtained by machining or building up metal on the thinner part, and the maximum slope cannot exceed 1:4; during the welding process, the interpass temperature should not exceed 150℃; the back of the root weld should be protected by argon or other protective gas, and the protection gas should be stopped when the weld metal thickness is not less than 5mm, and the back of the austenitic stainless steel part with a thickness of less than 5mm should also be protected by argon or other protective gas; when the wall thickness e≤5mm, the weld reinforcement is ≤1.5mm, when the wall thickness 5 Step S21, non-destructive testing, after the weld visual inspection is qualified, liquid penetration inspection should be carried out according to the provisions of the design document, the liquid penetration ratio is 100%, when defects are found on the surface of the weld, they should be eliminated in time, and the inspection should be carried out again after elimination until qualified, all pipe butt welds should be subjected to 100% radiographic inspection, when the inspection finds that the weld defects exceed the provisions of the design document, the welds must be repaired, and the welds should be inspected according to the original method after repair, and the pipe should be processed and cleaned again when the welds need to be cut off, Step S22, temporary accessories, temporary accessories or temporary welding points such as spot solid blocks, temporary lugs, pressure test or debugging temporary blocking plates, grounding points or other items that facilitate pipe handling, installation, welding and need to be removed later, the materials used should be completely the same as the base material, and the same process as the pipe weld is used for welding, and attention should be paid to prevent material contamination; For temporary accessories, liquid penetrant inspection should be carried out after removal and meet the base material inspection requirements, and attention should be paid to the removal of temporary accessories without damaging the base material to avoid tearing the pipe surface material, and the remaining accessory weld scar should be cleaned and polished round; If hot cutting is used to remove the temporary accessory, the hot cutting surface should be more than 5mm away from the steel pipe surface before using mechanical methods to remove it, Step S23, pressure test, after the outer pipe is installed and the non-destructive testing is qualified, the pressure test is carried out, the strength, leakage and vacuum test of the pipe system is carried out according to the design requirements, and the engineering quality of the pipe system and each connecting part is tested, Step S24, product protection, the installed pipe is strictly prohibited from being collided, stepped on or used as a lifting point; When other construction activities are carried out in the area, the finished pipe and accessories should be well protected, and hard protection should be added to the fragile parts; The completed pipe shall not be dismantled or moved arbitrarily; The "object protection signboard" is hung on the installed object, and regular inspection is carried out according to the actual needs, the appearance and environment are checked to see if they meet the requirements, and the problems found in the inspection are immediately treated, Step S25, foreign matter management, white cloth for pipe internal cleaning and cleanliness inspection, or wooden bar for isolation between inner and outer pipes, etc. should be registered before use, and control records should be formed for introduction and exit; During construction, tools, small pipe sections and sundries shall not be put into the pipe opening; Water sources or tools containing water shall not be brought into the pipe pre-fabrication and installation area, and the residual reagent should be cleaned up in time after liquid penetration is completed; After construction is completed, the pipe opening should be sealed with pipe cap and pasted firmly with stainless steel tape to prevent foreign matter from entering and contaminating; Temporary blocking devices used during pipe polishing and other construction should be complete and reliable, and should be cleaned up and removed after construction is completed to prevent being left in the pipe; During pipe welding, ensure that the temporary parts such as argon chamber are firm and reliable, and should be checked before and after use to prevent parts from being left in the pipe; The pipe blocking should be firm and firm when the opening is upward, and attention should be paid to the upper part when it needs to be opened; When it is necessary for a person to enter the pipe due to pipe weld polishing or pipe internal cleaning, strict control should be carried out, and no irrelevant articles should be brought in except the necessary tools; If foreign matter enters the installed pipe due to improper operation, improper pipe protection or other reasons, the name, specification, quantity and other detailed information of the foreign matter entering the pipe should be reported to the department managers and quality inspectors in time, the specific position of the foreign matter is confirmed by endoscope or other methods, specific foreign matter removal measures are developed according to the characteristics of the foreign matter, if the foreign matter enters the deep part of the pipe and cannot be removed, the technical personnel should initiate engineering change, increase or cut the weld on the pipe to remove the foreign matter, the foreign matter removal should be witnessed by quality inspectors and records should be formed, and the pipe restoration should be strictly carried out according to the above steps.

6. A method of pre-fabrication and installation of a double-layered sodium pipe as claimed in claim 5, wherein: If there is any problem that makes the pipe quality unconfirmed, the nonconformity item should be opened and handled according to the nonconformity management process in step S1. The material storage area should have clear identification and effective protection measures, and no carbon steel materials should be placed in the pipe storage area to avoid contamination of the stainless steel.

7. A method of pre-fabrication and installation of a double-layered sodium pipe as claimed in claim 6, wherein: In step S2, after the visual inspection of the weld is qualified, liquid penetrant inspection is performed, and the liquid penetrant ratio is 100%. After the detection work is completed, the white cloth soaked with anhydrous ethanol is used to clean it up, In step S3, 100% visual inspection and 100% liquid penetrant inspection are performed on the groove surface. After the inspection is qualified, the detection personnel should use the white cloth soaked with anhydrous ethanol to clean up the detection liquid used during the process, In step S4, for pipes with a pipe diameter ≥DN600, manual cleaning can be performed inside the pipe. Before entering the pipe, the construction personnel should wear clean one-piece clothes to avoid contaminating the pipe. After cleaning is completed, another person should enter the pipe to check and verify. For pipes with a pipe diameter size DN≥400, appropriate-sized polyethylene foam boards can be cut and used as internal supports. The outside is wrapped with clean white cloth, and the inside is scrubbed in the same direction with anhydrous ethanol. For pipes with a pipe diameter size <DN400, the inside is cleaned with anhydrous ethanol and cleaned with dry compressed air for segmented blowing, In step S5, after the cleanliness inspection is qualified, the cleanliness inspection record is filled out, In step S6, before pipe threading, hidden inspection should be performed according to the requirements. The pipe threading tool can avoid damaging the pipe wall. After pipe threading is completed, a stainless steel rubber pad is used to protect the groove, and a stainless steel clamp is used to fix the two pipe ends with bolts, In step S7, the modular prefabricated pipe model is completed according to the 1:1 ratio of the drawing. The modular prefabricated pipe model is imported into the nuclear island overall three-dimensional model. The BIM technology is used to simulate the introduction path of the modular prefabricated pipe model. The feasibility of the installation position after the modular prefabrication is verified, In step S8, the pipe section is identified and checked for specifications, materials, sizes, identification, cleanliness, etc. At the same time, the pipe prefabrication inspection record is filled out.

8. A method of pre-fabrication and installation of a double-layered sodium pipe as claimed in claim 7, wherein: In step S9, when the pipe is aligned, the straightness should be measured 200mm away from the center of the interface. When the nominal diameter of the pipe is less than 100mm, the allowable deviation a is 1mm. When the nominal diameter of the pipe is greater than or equal to 100mm, the allowable deviation a is 2mm. The full-length allowable deviation is 10mm, In step S10, after the pipe section, pipe fitting, etc. are processed and aligned, temporary identification should be done before welding, In step S11, when the wall thickness e ≤5mm, the weld excess height is ≤1.5mm. When the wall thickness 5 <e ≤10mm, the weld excess height is ≤2mm. When the wall thickness e >10mm, the weld excess height is ≤2.5mm. When the wall thickness e ≤5mm, the back weld excess height is ≤1.5mm. When the wall thickness e >5mm, the back weld excess height is ≤2.5mm, In step S12, after the visual inspection of the weld is qualified, liquid penetrant inspection should be performed according to the design document. The liquid penetrant ratio is 100%, and 100% radiographic inspection should be performed on all pipe butt welds, In step S13, the 316H material pipe is cleaned with a stainless steel wire brush after welding to make the weld surface uniform and silver-white, without the need for pickling and passivation, In step S14, the temporary accessories are removed and liquid penetration inspection is performed to meet the requirements of the base material inspection, In step S15, after the inner pipe is welded and non-destructive testing is passed, strength, leakage and vacuum tests are performed on the pipe system according to design requirements to check the engineering quality of the pipe system and each connecting component, In step S16, after the inner pipe pressure test is completed, the outer wall of the inner pipe and the inner wall of the outer pipe need to be cleaned.

9. A method of pre-fabrication and installation of a double-layered sodium pipe as claimed in claim 8, wherein: In step S18, the corresponding inner pipe hidden inspection and acceptance need to be qualified before the outer pipe is assembled, In step S23, after the outer pipe is installed and non-destructive testing is passed, pressure testing is performed, In step S24, the "object protection signboard" is hung on the object that has been placed, and regular inspection is performed according to actual needs to check whether the appearance and environment meet the requirements, In step S25, the tools and equipment used during construction are introduced and removed to form a control record; water sources or water-containing tools are prohibited from being brought into the pipe precast and installation area; after liquid penetration is completed, the residual reagent needs to be cleaned in a timely manner; after construction is completed, the pipe opening needs to be sealed with a pipe cap and firmly pasted with stainless steel tape; the sealing of upward-opening pipes needs to be solid and firm, and attention should be paid to the fact that no objects should fall from above when the pipe is open; if foreign matter enters the deep part of the pipe and cannot be removed, the technical personnel will initiate an engineering change, increase or cut the weld on the pipe, and remove the foreign matter; the removal of foreign matter should be witnessed by quality inspection personnel and a record should be formed.