A method for welding an inner hole of a connecting pipe of a molten salt heat exchanger to a connecting pipe of a stack container and a welding system thereof

Abstract

The application discloses a molten salt heat exchanger pipe and a hole welding method and a welding system thereof, pipe butt joint gap and radial edge offset amount are strictly controlled, full-process welding visual monitoring of a molten pool is realized, segmented welding is performed by using inner hole tungsten argon arc welding, a finite element simulation deformation benchmark database is established in advance, radial shrinkage and angular deformation of the pipe are collected in real time after single-segment welding, the arc offset position and welding process parameters of the subsequent welding segment are dynamically corrected, the reverse tensile stress generated by the cooling shrinkage of the subsequent weld is used to offset the accumulated deformation in the early stage, and the welding operation is completed through nondestructive testing, axiality verification and sealing test after welding. The application discloses a molten salt heat exchanger pipe and a hole welding method and a welding device thereof, pipe butt joint gap and radial edge offset amount are strictly controlled, full-process welding visual control of a molten pool is realized, segmented welding is performed by using inner hole tungsten argun argun arc welding, a finite element simulation deformation benchmark database is established in advance, radial shrinkage and the angular deformation of the pipe are collected in real time after single-segment welding, the arc offset location and welding process parameters of the subsequent welding segment are dynamically corrected, the reverse tensile stress generated through the cooling shrinkage of the subsequent weld is used to offset the accumulated deformation in the early stage.

Description

Technical Field

[0001] This invention relates to the field of nuclear reactor equipment welding, and more specifically to a method and system for welding the inner bore of a molten salt heat exchanger nozzle and a reactor vessel nozzle. Background Technology

[0002] As one of the core reactor types of the fourth-generation advanced nuclear energy system, the thorium-based molten salt reactor (such as TMSR-LF1) has a high degree of equipment integration. The molten salt-molten salt heat exchanger is built into the reactor vessel cavity, and the inlet and outlet pipes at both ends of the heat exchanger need to be welded to the reserved pipes in the reactor vessel. This scenario has severe structural limitations: the reactor vessel cavity is completely enclosed and the space is small, making it impossible for personnel and large tooling to enter the welding area; the cavity is densely packed with nuclear safety level 1 equipment such as graphite components, metal pressure-bearing components, and fuel salt loops, making the operating space extremely limited. It is a closed blind welding condition, and on-site welding has long suffered from three major core defects that are difficult to solve.

[0003] First, the sealed cavity lacks external observation channels; the welding torch can only be inserted through a narrow tube. Operators cannot directly observe the molten pool flow, bevel fusion, and weld back formation. Relying solely on manual experience for welding easily leads to fatal defects such as incomplete fusion, burn-through, and root inclusions, which compromise nuclear safety barriers. Although some existing internal welding equipment is equipped with cameras, the camera elements are positioned far from the rear end of the connecting rod, resulting in a large distance from the molten pool, easily obstructed viewing angles, poor image clarity and synchronization, and an inability to achieve precise, continuous monitoring of the molten pool.

[0004] Secondly, the assembly dimensions for nuclear-grade pressure-bearing thick-walled nozzles are extremely demanding. Excessive gaps or radial misalignment can easily lead to defects such as back-side collapse, incomplete penetration, and stress concentration; while insufficient gaps can result in inadequate fusion. Existing welding solutions mostly involve welding the inner bores of heat exchange tubes to the tube sheet (tube-to-plate connection), which is far less difficult than butt welding the inner bores of nozzles to each other (tube-to-tube connection) and cannot be directly adopted.

[0005] Third, the overall axial alignment of the heat exchanger after welding must not exceed 2mm. Excessive deformation will disrupt the internal molten salt flow field and may even cause physical interference with graphite and metal components within the reactor core, leading to equipment failure. Currently, the industry's common deformation control methods are all passive, including pre-setting anti-deformation before welding, post-weld mechanical correction, and adding rigid tooling to the welding area. These methods have inherent shortcomings: the heat input and heat dissipation from the cavity during welding are constantly changing dynamically, and the pre-set constraint dimensions cannot match the real-time deformation trend. As deformation accumulates, it can easily exceed the allowable axial alignment threshold. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the defects of existing blind welding of closed cavities, which cannot observe the molten pool in real time, make it difficult to meet the assembly accuracy of the nozzles, and make it easy to cause excessive deviation of the heat exchanger axis by relying on passive shape control methods. The present invention provides a welding method and welding system for the inner hole welding of the nozzles of molten salt heat exchangers and the nozzles of the reactor vessel.

[0007] The method for welding the inner bore of the molten salt heat exchanger nozzle and the reactor vessel nozzle according to the present invention includes the following steps: 1) Performing pipe-to-pipe butt joint assembly of the built-in molten salt heat exchanger nozzle and the reactor vessel nozzle, controlling the butt joint gap ≤0.5mm and the radial misalignment ≤0.5mm, and fixing the position after the assembly is qualified; 2) Arranging multiple sets of laser displacement sensors circumferentially around the outer periphery of the nozzle, and rigidly fixing a miniature endoscope to the side of the welding torch end, so that the endoscope probe maintains a fixed observation distance with the molten pool and moves synchronously with the welding torch throughout the entire range; 3) Using internal tungsten inert gas welding to perform segmented welding of the circumferential weld, relying on the pre-established finite element simulation deformation reference database, collecting the real-time radial shrinkage and angular deformation of the nozzle after each segment of arc extinguishing welding is completed, dynamically correcting the arc start offset position and welding process parameters of the next welding segment, using the reverse tensile stress of the subsequent weld cooling shrinkage to offset the cumulative deformation of the welded segment, and completing the welding with full-process visual monitoring by the endoscope; 4) Completing non-destructive testing and axis verification, and carrying out a sealing test after passing the test to complete the welding operation. This invention addresses the challenges of enclosed and confined working conditions for integrated molten salt reactors by employing an active deformation offsetting welding process that combines single-sided welding of the inner bore with measured deformation feedback and simulation pre-calibration. This process overcomes the technical bottlenecks of traditional passive shape control, poor blind welding formation, and excessive deformation accumulation. It achieves high-precision forming of nuclear-grade tube-to-tube butt welds in confined spaces without external operating space, effectively controls the deformation of the welding axis of the nozzle, and significantly improves the quality and consistency of welding formation.

[0008] In a preferred embodiment, in step 1), a tooling device with radial and axial bidirectional adjustment functions is used to achieve high-precision assembly in the confined space within the reactor. If the assembly exceeds tolerances, repeated adjustments are made until the accuracy requirements are met, and at least three evenly distributed locating welds are used to fix the docking position. This invention adapts to the limited working conditions within the reactor where there is no external adjustment space through bidirectional adjustable tooling, achieving high-precision assembly and stable locking of the docking state. It can significantly reduce weld misalignment and uneven gap defects from the welding source, providing a precise assembly basis for single-sided welding and double-sided forming, and avoiding the root defect problem of nuclear-grade pressure-bearing welds.

[0009] In a preferred embodiment, in step 3), the circumferential weld is evenly divided into independent welding sections of 60°~90°, and a symmetrical alternating segmented welding mode is adopted to complete the welding of the root pass, filler layer, and cap coat layer layer by layer. This invention, by limiting reasonable segmented angle ranges and combining them with a symmetrical alternating welding strategy, can evenly distribute the welding heat input, avoid localized thermal stress concentration, effectively reduce the accumulation of circumferential deformation of the circumferential weld, and provide a stable welding foundation for subsequent dynamic deformation compensation.

[0010] In a preferred embodiment, in step 3), after a single segment of welding is completed, deformation data is synchronously collected by a laser displacement sensor and uploaded to the industrial control host. The industrial control host's built-in compensation control system, combined with simulation benchmark comparison calculations, outputs the optimal arc-starting offset within the 0-30mm range and matches and fine-tunes the welding current and welding speed. This invention, through a compensation mechanism that combines segmented arc extinguishing with measured deformation, simulation benchmark comparison, and coordinated dynamic adjustment of arc-starting position and heat input, can accurately match the real-time deformation state, achieve refined dynamic correction of welding deformation, adapt to dynamic thermal deformation fluctuations during the welding process, and significantly improve deformation control accuracy.

[0011] In a preferred embodiment, in step 3), 70% of the allowable value of axial deformation is set as the layered deformation warning threshold. When the cumulative deformation of a single layer of welding reaches the warning threshold, the arc-starting position of the subsequent weld layer is actively shifted to implement directional correction and suppress deformation accumulation. By setting a layered deformation warning and a proactive correction mechanism, this invention can intervene in the deformation accumulation trend in advance, avoid deformation exceeding the tolerance after multi-layer welding, achieve layer-by-layer convergence of welding deformation, and ensure that the overall axiality of the connector is stable and meets the standards.

[0012] In a preferred embodiment, in step 2), the endoscope probe maintains a constant observation distance of 5-10 mm from the molten pool, synchronously feeding and rotating with the welding torch throughout the process, without obstruction of the welding line of sight, and simultaneously monitoring the molten pool morphology on the front side and the forming state on the back side of the weld. This invention, through its integrated welding torch-mounted endoscopic visualization monitoring structure, completely solves the problem of lack of real-time observation conditions in blind welding within the weld pool, monitoring the molten pool morphology and the forming of the weld on both sides throughout the process, allowing for timely intervention in welding abnormalities, and effectively preventing forming defects such as incomplete fusion, burn-through, and weld beads.

[0013] In a preferred embodiment, the finite element simulation deformation benchmark database is constructed as follows: a three-dimensional model of the pipe butt joint is established, temperature-related nonlinear material thermophysical parameters are assigned, a double ellipsoidal heat source model is used to match the welding heat input, tooling constraints and heat transfer boundary conditions are applied, the welding sequence is iteratively optimized, theoretical deformation data of different sections and different weld layers are extracted to establish a benchmark database, and the data is pre-stored in the compensation control system for real-time comparison and compensation on site. This invention, through offline simulation pre-optimization of the welding process and the establishment of a standardized theoretical deformation benchmark database, can avoid the random errors of purely empirical welding, provide accurate reference for real-time deformation compensation on site, and improve the accuracy and reliability of dynamic correction.

[0014] In a preferred embodiment, the welding process parameters are as follows: welding current 80~120A, welding voltage 10~12V, welding speed 60~100mm / min, wire feed speed 200~400mm / min, and argon flow rate 8~12L / min. This invention, by optimizing and adapting the welding parameters for the narrow-range annular butt joint of thick-walled nozzles, can stably control the heat input and deposition efficiency of the molten pool, ensuring uniform and stable formation of multi-layer, multi-pass welds, balancing weld mechanical properties and sealing performance, and meeting the welding quality requirements of nuclear-grade equipment.

[0015] The welding system according to the present invention, applied to the above-mentioned internal hole welding method, includes an automatic internal hole welding torch assembly, a welding-in-place endoscope visualization assembly, a circumferential deformation sensing and acquisition assembly, and an external industrial control and display assembly. These four components are connected via optical fibers and signal lines to form a real-time visualization-deformation acquisition-algorithm compensation system. A closed-loop control system for dynamic welding adjustment; the automatic welding torch assembly integrates positioning, extension, circumferential rotation, tungsten electrode oscillation, bevel trajectory tracking, uniform wire feeding, and multi-channel gas supply functions, adapting to the welding environment inside the nozzle cavity, and realizing layered and segmented internal hole welding with double-sided argon gas protection; the miniature camera probe of the welding endoscope visualization assembly is rigidly fixed to the side of the welding torch end, moving synchronously with the welding torch, realizing unobstructed full-process visual monitoring of the welding area; multiple sets of laser displacement sensors of the circumferential deformation sensing and acquisition assembly are evenly arranged around the outer wall of the nozzle in the circumferential direction, used to synchronously acquire welding radial shrinkage and angular deformation data in segments; the external industrial control display assembly has a built-in compensation control system, which calculates welding compensation parameters by combining simulation benchmarks and measured deformation data, and synchronously drives and controls all actuators of the welding torch to realize dynamic active correction of welding point position and heat input. This invention utilizes a dedicated closed-loop system that integrates visual monitoring, deformation acquisition, algorithm compensation, and dynamic welding adjustment. This system is adaptable to welding conditions in narrow, enclosed internal holes within the reactor core, enabling automated, visual, and precise intelligent shape control throughout the welding process. It overcomes the drawbacks of traditional manual welding, such as low precision and poor controllability.

[0016] In a preferred embodiment, the welding torch tracking mechanism follows the bevel trajectory offset in real time to maintain precise alignment between the tungsten electrode and the weld center, while the welding torch oscillation mechanism widens the weld pool width through small lateral oscillations of the tungsten electrode. This invention, by adapting to the welding trajectory offset through real-time bevel tracking and combining it with small oscillations to widen the weld pool, can continuously ensure weld alignment accuracy, improve bevel fusion effect, and effectively address the problems of uneven fusion and narrow weld formation in narrow bevel butt welds.

[0017] This invention employs a combination of high-precision assembly and root pass of in-core tubes, integrated welding torch with real-time visual monitoring during welding, segmented welding combined with offline pre-built deformation benchmarks using finite element analysis, segmented real-time deformation acquisition and dynamic collaborative compensation of arc initiation position and heat input, and an active shape control strategy that relies on subsequent weld shrinkage to counteract the deformation of the previous welding. This completely abandons the passive and lagging deformation control methods of traditional pre-deformation, rigid constraints, and post-weld correction. It effectively adapts to the dynamic thermal deformation fluctuation characteristics of the entire welding process, suppresses and converges the cumulative welding deformation layer by layer, and significantly improves the uniformity of weld formation, centering accuracy, and overall axis control accuracy of single-sided welding with double-sided forming in confined spaces within the reactor core. It solves the industry pain points of traditional blind welding, such as numerous defects, uncontrollable deformation, and low finished product qualification rate. It realizes visualized, precise, and closed-loop intelligent welding of the pressure-bearing nozzles of molten salt reactor cores, greatly improving the mechanical properties and sealing reliability of the welded joints, and ensuring the welding assembly accuracy and long-term operational safety of the main equipment nozzles of integrated molten salt reactors. Attached Figure Description

[0018] Figure 1 is a flowchart of the overall process for welding the inner hole of the molten salt heat exchanger nozzle and the reactor vessel nozzle of the present invention.

[0019] Figure 2 is a schematic diagram of the cross-section for precision assembly control of the pipe in this invention.

[0020] Figure 3 is a schematic diagram of the overall structure of the automatic GTAW welding and deformation compensation system for internal holes of the present invention.

[0021] Figure 4 is a schematic diagram of the active deformation compensation control principle of the present invention.

[0022] Figure 5 is a flowchart of the finite element simulation pre-control process for welding deformation according to the present invention. Detailed Implementation

[0023] The present invention will now be fully described in conjunction with the accompanying drawings and preferred embodiments.

[0024] As shown in Figure 1, the process flow according to the present invention includes:

[0025] 1) Perform precision assembly process:

[0026] The heat exchanger nozzles and reactor vessel nozzles are assembled using tooling.

[0027] This invention focuses on the development of pipe-to-pipe butt welding for the molten salt-to-molten salt heat exchanger in the TMSR-LF1 integrated molten salt reactor, which differs from the heat exchange tube-to-tube sheet welding in the existing technology, laying the foundation for the successful installation of the main equipment of the thorium-based molten salt reactor.

[0028] As shown in Figure 2, the heat exchanger nozzle and the reactor vessel nozzle form a butt joint bevel structure, where G is the butt joint gap and M is the radial misalignment. The butt joint gap G is controlled to be ≤0.5mm and the radial misalignment M to be ≤0.5mm. If the dimensions do not meet the standards, the tooling is readjusted and reassembled; if the dimensions are acceptable, the process proceeds to the next step. This invention employs tooling with both radial and axial bidirectional adjustment functions, achieving ultra-high assembly accuracy of G≤0.5mm and M≤0.5mm stably in the closed environment of the reactor vessel where there is no external adjustment space, eliminating fatal defects in pressure-bearing welds from the source of processing and assembly.

[0029] This invention uses precision assembly technology to control the assembly gap and misalignment to ≤0.5mm. This precision is achieved within the confined space inside the reactor vessel, providing a reliable assembly basis for high-quality single-sided welding and double-sided forming.

[0030] 2) After the assembly is completed, proceed with the pre-welding equipment layout.

[0031] At least four sets of laser displacement sensors are evenly distributed around the outer perimeter of the pipe, with a sampling frequency of not less than 10Hz.

[0032] The miniature endoscope is rigidly fixed to the side of the welding torch tip, with the probe and the molten pool kept 5-10 mm apart, and the endoscope and welding torch move synchronously.

[0033] This invention fixes a miniature endoscope to the end of the welding torch and moves with the welding process. Compared with the prior art, the endoscope can always be kept at a distance of 5~10mm from the molten pool with the end of the welding torch, realizing real-time high-precision visual monitoring of the molten pool morphology on the front side and the formation on the back side of the weld, overcoming the technical bottleneck of traditional blind welding being invisible.

[0034] 3) After the equipment is installed, two operations are performed simultaneously: welding and active dynamic deformation compensation control, and real-time endoscope monitoring.

[0035] Existing technologies all employ passive strategies for welding deformation control—namely, pre-setting anti-deformation before welding, post-weld correction, or rigid constraints. The essential flaw of these passive methods is their inability to adapt to the dynamic fluctuations in deformation during the welding process. This invention is the first to adopt an active dynamic deformation compensation strategy in butt welding of nozzle-to-nozzle inner hole: (1) By real-time monitoring of the deformation of the welded parts, quantitative perception of welding deformation is achieved; (2) Based on measured data, the arc-starting position of subsequent welding is dynamically adjusted, and the reverse deformation generated by the cooling shrinkage of subsequent welding is used to actively compensate for the deformation that has already occurred; (3) The shrinkage deformation generated by each weld segment cancels each other out over the entire circumference, fundamentally avoiding deformation accumulation.

[0036] This invention determines the initial optimal solution through pre-welding finite element simulation and dynamically corrects it based on real-time monitoring data during welding, organically combining simulation prediction with measured feedback. Compared to solutions that rely solely on simulation or experience-based adjustments, the pre-optimization + real-time feedback synergistic mechanism significantly improves the accuracy and reliability of deformation control.

[0037] During the welding stage, the circumferential weld is welded in sections. The arc starting position of each section is dynamically adjusted by combining the simulation benchmark and real-time deformation data. The shrinkage of the later section weld offsets the early deformation, and the deformation of the heat exchanger axis is controlled to ≤2mm.

[0038] Operators can observe the molten pool and weld formation in real time through an external display terminal, and make timely adjustments to the welding current and welding speed if any abnormalities occur.

[0039] The welding process parameters are as follows: welding current 80~120A, welding voltage 10~12V, welding speed 60~100mm / min, wire feeding speed 200~400mm / min, and argon flow rate 8~12L / min.

[0040] 4) After all segmented welding is completed, 100% non-destructive testing is carried out, including radiographic testing, ultrasonic testing, and penetrant testing. At the same time, the overall axial alignment of the heat exchanger is measured using a laser tracker. If defects exceeding the standard are found, the defective areas are ground and removed before re-inspection. After all indicators pass the test, the air pressure sealing test is carried out, and a 0.85MPa air pressure holding test is conducted for 30 minutes. The entire process is completed after the pressure test is completed.

[0041] To achieve the above-mentioned process flow, this invention provides a supporting welding system, as shown in Figure 3. The system includes an automatic welding torch assembly for internal holes, a welding-in-place endoscope visualization assembly, a circumferential deformation sensing and acquisition assembly, and an external industrial control and display assembly. These four components form a closed-loop control system of "real-time visualization - deformation acquisition - algorithm compensation - dynamic welding adjustment" through optical fibers and signal lines, realizing visual blind welding and active deformation correction.

[0042] The automatic welding torch assembly for internal bores includes a torch positioning mechanism, a torch advance / retract mechanism, a torch rotation mechanism, a torch oscillation mechanism, a torch tracking mechanism, a tungsten electrode, a torch wire feeding mechanism, and a multi-channel gas supply assembly. The torch positioning mechanism serves as the base, supporting all the torch's actuators. The torch advance / retract mechanism forms a telescopic torch rod, adapting to different nozzle cavity depths. The torch rotation mechanism drives the entire torch body to rotate 360° circumferentially at a uniform speed. The torch oscillation mechanism controls the tungsten electrode to oscillate slightly laterally, widening the weld pool. The torch tracking mechanism follows the bevel trajectory in real time, ensuring the tungsten electrode is aligned with the weld center. The torch wire feeding mechanism uses a wire feed nozzle to uniformly deliver filler wire matching the base material. The multi-channel gas supply assembly supplies high-purity argon shielding gas to the front and back sides respectively, isolating the high-temperature weld from oxidation. A side grinding head, an optional auxiliary accessory, can clean the weld oxide layer and weld beads; it is not shown in the figure. The entire welding torch assembly, in conjunction with multiple drive mechanisms, completes layered and segmented internal bore tungsten inert gas welding, achieving single-sided welding with double-sided forming.

[0043] The welding endoscope visualization component includes a miniature camera probe, a rigid clamp, and an optical fiber transmission line. The miniature camera probe is fixed to the side of the welding torch tip using the rigid clamp. The probe maintains a stable observation distance of 5-10 mm from the molten pool. The probe has a diameter of 3 mm and a resolution of 1920×1080. The image is transmitted without delay via optical fiber to the weld pool observation window on the high-definition display of the external industrial control display component, and is fed and rotated synchronously with the welding torch throughout the process.

[0044] The circumferential deformation sensing and acquisition component includes at least four sets of laser displacement sensors, a data acquisition module, and signal transmission cables. The six sets of laser displacement sensors shown in the figure are evenly distributed circumferentially along the outer wall of the pipe, surrounding the annular weld. Each sensor has a measurement accuracy of ±0.01mm and a sampling frequency of ≥10Hz. The annular weld is divided into multiple independent welding sections with arc lengths of 60° to 90°. After each weld section is completed and the arc is extinguished, multiple sets of laser displacement sensors synchronously acquire the current radial shrinkage Δr and angular deformation Δα of the pipe, and upload the measurement data to the industrial control host of the external industrial control display component.

[0045] The external industrial control display component includes a high-definition monitor, an industrial control host, and a drive adjustment module. The high-definition monitor interface features partitioned settings including a weld pool observation window, sensor data monitoring tables, welding parameter tables, and a real-time welding machine parameter adjustment panel. The weld pool observation window outputs endoscopic images in real time. The industrial control host has a built-in compensation control system. This system uses pre-established finite element simulation deformation results as a benchmark, comparing the measured deformation values ​​collected by multiple sensors with the simulation benchmark to automatically calculate the compensation welding parameters required for the next welding section (including the arc start position offset δ, welding current fine-tuning ΔI, and welding speed fine-tuning Δv). The drive adjustment module receives control commands from the industrial control host and synchronously controls all mechanisms of the welding torch's advance, retreat, rotation, oscillation, tracking, and wire feeding, adjusting the welding point position and heat input.

[0046] like Figure 4 As shown, four sets of displacement sensors are used for deformation monitoring. The circumferential butt weld is evenly divided into four zones (each arc segment is 90°). Four displacement sensors are arranged on the outer circumference of the nozzle to collect deformation data in segments and transmit it to the compensation control system. The compensation control system has a built-in deformation signal processing and roundness calculation, circumferential compensation calculation model. It dynamically adjusts the arc starting position of each segment according to the deformation value. The arc starting position is offset from the simulated optimized position (the offset δ is calculated in real time by the control system based on the measured deformation, and the value of δ ranges from 0 to 30 mm). Specifically, when the preceding weld segment causes the nozzle to shift to one side and deform, the compensation control system adjusts the arc-starting position of the next welding segment so that the cooling and shrinkage direction of the subsequent weld is exactly opposite to the direction of the deformation that has occurred. The shrinkage deformation of the subsequent weld offsets the deformation that has already occurred. After each layer and segment is welded, the cumulative deformation of the layer is calculated by comparing it with the initial state without deformation. This invention uses 70% of the maximum allowable axial deformation of 2mm as the layer warning threshold, corresponding to a warning value of 1.4mm. When the cumulative deformation in a certain direction approaches the warning value, the arc-starting position of the next layer actively shifts to that direction, using the shrinkage in the opposite direction of the subsequent weld to correct the deviation. Through alternating welding of 4 to 6 segments around the circumference and real-time dynamic adjustment of the arc-starting position of each segment, the shrinkage deformation generated by each weld segment cancels each other out over the entire circumference, controlling the final axial deviation change to ≤2mm.

[0047] The benchmark data used by the compensation control system for deformation comparison calculations is pre-established based on offline finite element simulation, as shown in Figure 5. First, geometric modeling is performed, and the completed geometric model is imported into the finite element analysis software for preprocessing. During the preprocessing stage, several settings are completed simultaneously: importing loading material parameters, introducing a double ellipsoidal heat source model, inputting welding parameters, and applying boundary conditions. After preprocessing, the solution calculation is executed. The solution calculation outputs the welding temperature field and welding stress-strain field in parallel. On the one hand, the welding temperature field is used to verify the effectiveness of the heat source model; on the other hand, the welding sequence is optimized based on the welding stress-strain field results. The optimized welding sequence is then fed back to the preprocessing stage for iterative calculation. After multiple rounds of iterative simulation analysis, the optimal welding scheme is output. During the on-site implementation stage, welding operations are carried out according to the optimal welding scheme. During the welding process, deformation data of the nozzle is collected through real-time monitoring and fed back to the compensation control system for dynamic correction, forming a complete closed loop of "offline simulation pre-optimization and online real-time monitoring dynamic compensation". The theoretical stress and deformation data obtained from finite element simulation for different welding sections and layers are pre-stored in the compensation control system of the industrial control host as a simulation benchmark database for comparison and calculation with measured deformation data at the welding site. The double ellipsoidal heat source model is a general model for welding thermo-mechanical coupling, and those skilled in the art can complete model calibration and reproduction by combining it with the parameters of this working condition.

[0048] Example 1

[0049] This embodiment takes the butt welding of the inlet and outlet pipes (material GH3535, outer diameter Φ89mm, wall thickness 6mm) of the molten salt-molten salt heat exchanger secondary loop in the TMSR-LF1 integrated molten salt reactor as an example.

[0050] 1) Precision assembly

[0051] Precision assembly of the secondary loop inlet / outlet pipes and the reactor vessel pipes was performed using assembly fixtures (including 4 sets of radial adjusting bolts and 2 sets of axial positioning blocks). Before assembly, the pipe end faces were precision machined to ensure flatness ≤0.05mm and perpendicularity ≤0.05mm. During assembly, a dial indicator (accuracy 0.01mm) was used to monitor radial misalignment, and a feeler gauge (accuracy 0.02mm) was used to monitor the assembly gap, adjusting point by point until the gap and misalignment were ≤0.5mm. After assembly, the relative positions of the pipes were fixed using three-point evenly distributed tack welding (weld length 5~8mm).

[0052] To verify the criticality of a gap ≤0.5mm, comparative tests were conducted under the same conditions: when the gap was 1.0mm, significant collapse and weld beads appeared on the back side of the single-sided weld with double-sided forming, and RT testing revealed a root non-fusion defect rate of approximately 12%; when the gap was 1.5mm, the burn-through rate increased significantly, and the welding pass rate dropped to approximately 78%. Therefore, a gap control of ≤0.5mm can ensure the quality of single-sided weld with double-sided forming.

[0053] 2) Deployment of the endoscopic real-time visual monitoring system during welding

[0054] Four displacement sensors (laser displacement sensors with an accuracy of ±0.01mm) are evenly distributed along the circumference of the pipe to monitor the deformation and radial shrinkage in each direction in real time, with a data sampling frequency of 10Hz.

[0055] The miniature endoscope probe (3mm in diameter, 1920×1080 resolution) is fixed to the side of the welding torch tip using a rigid clamp, with the distance from the molten pool controlled within the range of 5~10mm. It moves synchronously with the welding torch, and the endoscope's field of view covers the weld formation area in front of and behind the molten pool. The endoscope's video signal is transmitted to a high-definition monitor via fiber optic cable, allowing the operator to observe the molten pool status and the back weld formation in real time. If any abnormalities occur, the welding current or welding speed can be adjusted promptly.

[0056] 3) Finite element simulation pre-optimization of welding deformation

[0057] Before welding, a three-dimensional finite element model of the nozzle-stack vessel joint was established using SYSWELD finite element software. The model underwent mesh refinement in the weld region, with a minimum mesh size of 0.5 mm and a total mesh size of approximately 150,000 elements. The thermophysical properties of the material (thermal conductivity, specific heat capacity, density, coefficient of thermal expansion, elastic modulus, and yield strength) were obtained using nonlinear data of GH3535 alloy as a function of temperature. A double-ellipsoidal moving heat source model was used to simulate the GTAW heat input. The heat source model parameters (front hemisphere length 4 mm, rear hemisphere length 6 mm, width 3 mm, depth 2 mm) were obtained through calibration using the actual welding temperature field. The temperature and deformation fields under four different welding sequence schemes were calculated. The initial optimal welding sequence was determined with the minimum radial shrinkage and axial angular deformation as the optimization objectives. Simulation results showed that the symmetrical alternating welding scheme had the minimum radial shrinkage (simulated value 1.6 mm), a 58% reduction compared to the sequential welding scheme (simulated value 3.8 mm). The initial optimal welding sequence is determined as follows: the entire circumference is divided into 4 segments for symmetrical alternating welding (0~90°→180~270°→90~180°→270~360°), and the base layer, filler layer and cover layer are completed alternately in each segment.

[0058] 4) Automatic GTAW welding of inner holes and active dynamic deformation compensation control

[0059] Insert the internal automatic tungsten inert gas (TIG) welding torch into the internal inlet of the reactor vessel, aligning the torch axis with the inlet axis. Perform single-sided welding with double-sided forming in the horizontal welding position. The basic welding process parameters are: welding current 100A, welding voltage 11V, welding speed 80mm / min, wire feed speed 300mm / min, argon flow rate 10L / min, tungsten electrode extension length 3~4mm, and arc length 2~3mm.

[0060] The welding adopts a symmetrical alternating welding scheme, and the entire circumference is divided into 4 sections (each section is 90°), in the following order: Section 1 0~90° → Section 2 180~270° → Section 3 90~180° → Section 4 270~360°.

[0061] During the welding process, active dynamic deformation compensation control should be performed according to the following steps:

[0062] 41) After each section (90°) of welding is completed, four displacement sensors automatically collect the angular deformation Δα and radial shrinkage Δr of the welded section.

[0063] 42) The control system compares the measured data with the finite element simulation prediction values ​​and calculates the deviation between the actual deformation and the predicted deformation;

[0064] 43) Based on the measured deviation, the control system calculates the arc-starting position offset δ of the next segment according to a predetermined algorithm. For example, when the measured radial contraction is 0.2 mm larger than the predicted value, the control system calculates that the arc-starting position of the next segment will be offset by approximately 10~15 mm towards the maximum radial contraction direction;

[0065] 44) When welding the second section, the arc starting position is offset by 12mm from the maximum radial shrinkage position of the first section based on the simulated optimized position, so that the cooling shrinkage direction of the second section weld is opposite to the deformation direction of the first section.

[0066] 45) The third and fourth segments follow the same pattern, with the starting position of each segment dynamically adjusted based on the measured deformation of the previous segment;

[0067] 46) After each layer of welding is completed, calculate the cumulative deformation of that layer and compare it with the initial state without deformation. If the cumulative deformation in a certain direction reaches the warning value (take 1.4mm, which is 70% of the allowable value of 2mm), then the arc starting position of the next layer in that direction will be actively shifted to that direction by 15~20mm, and the correction will be made by using the shrinkage in the opposite direction of the subsequent welding.

[0068] 47) By alternating welding in four segments around the circumference and dynamically adjusting the arc-starting position of each segment in real time, the shrinkage deformation of each weld segment cancels each other out over the entire circumference.

[0069] To verify the effectiveness of active dynamic deformation compensation control, the applicant conducted comparative tests under the same conditions:

[0070] Control Group 1 (Traditional symmetrical alternating welding, without dynamic compensation): A symmetrical alternating welding sequence was used, but the arc initiation position was not dynamically adjusted. The measured change in axis axiality after welding was 2.8 mm, exceeding the target value (2 mm).

[0071] Control Group 2 (Finite Element Simulation Optimization + Dynamic Compensation): The initial scheme was determined by simulation optimization, and the arc initiation position of each segment during welding was dynamically adjusted based on the measured deformation. After welding, the measured change in axiality was 1.5 mm, meeting the target requirement of ≤2 mm, which is about 46% lower than that of Control Group 1.

[0072] Comparative analysis shows that the synergistic strategy of finite element pre-optimization and real-time dynamic compensation is significantly better than the passive approach of simply using symmetrical alternating welding.

[0073] 5) Post-weld inspection

[0074] The welds were inspected by 100% radiographic testing (RT) according to NB / T47013.2 standard, and were qualified at Class I; 100% ultrasonic testing (UT) according to NB / T47013.3 standard, and were qualified at Class I; 100% penetrant testing (PT) according to NB / T47013.5 standard, and were qualified at Class I.

[0075] A laser tracker was used to detect the overall axial misalignment of the heat exchanger. Eight measuring points were evenly distributed along the circumference of the nozzle to measure the axial position deviation before and after welding, and the overall axial misalignment change was calculated. The test results showed that the maximum axial misalignment change was 1.5 mm (<2 mm target value), which meets the design requirements.

[0076] 6) Air pressure test

[0077] After all welding and testing were completed, a 0.85MPa air pressure test was conducted on the secondary side of the heat exchanger. The pressure was maintained for 30 minutes, and the pressure drop was ≤0.02MPa. There was no leakage in any weld, which verified the sealing performance and strength of the joint.

[0078] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any simple equivalent substitutions or improvements made in accordance with the technical solutions described in the specification and claims of the present invention shall fall within the scope of protection of the present invention. The processes and equipment structures not described in detail in the present invention are all conventional and well-known technologies in the field.

Claims

1. A method for welding the inner bore of a molten salt heat exchanger nozzle to a reactor vessel nozzle, characterized in that, Includes the following steps: 1) Perform pipe-to-pipe assembly of the built-in molten salt heat exchanger nozzle and the reactor vessel nozzle, controlling the connection gap ≤0.5mm and radial misalignment ≤0.5mm. After the assembly is qualified, fix it in position. 2) Multiple sets of laser displacement sensors are arranged circumferentially around the outside of the nozzle, and a miniature endoscope is rigidly fixed to the side of the welding torch tip so that the endoscope probe maintains a fixed observation distance from the molten pool and moves synchronously with the welding torch throughout the entire range. 3) The circumferential weld is welded in segments using internal tungsten inert gas welding. Based on the pre-established finite element simulation deformation reference database, the real-time radial shrinkage and angular deformation of the pipe are collected after each segment of arc extinguishing welding is completed. The arc starting offset position and welding process parameters of the next welding segment are dynamically corrected. The reverse tensile stress of the subsequent weld cooling shrinkage is used to offset the cumulative deformation of the welded segment. The welding is completed by combining the endoscope with full-process visual monitoring. 4) After completing non-destructive testing and axis alignment verification, and passing the test, conduct a sealing test to complete the welding operation.

2. The method for welding the inner bore of the molten salt heat exchanger nozzle and the reactor vessel nozzle according to claim 1, characterized in that, In step 1), tooling with radial and axial bidirectional adjustment functions is used to achieve high-precision assembly in the closed space inside the stack. When the assembly exceeds the tolerance, it is repeatedly adjusted until the accuracy requirements are met, and the docking position is fixed by at least three evenly distributed positioning welds.

3. The method for welding the inner bore of the molten salt heat exchanger nozzle and the reactor vessel nozzle according to claim 2, characterized in that, In step 3), the circumferential weld is evenly divided into independent welding sections of 60°~90°, and a symmetrical alternating segmented welding mode is adopted to complete the welding of the root layer, fill layer and cover layer layer one by one.

4. The method for welding the inner bore of the molten salt heat exchanger nozzle and the reactor vessel nozzle according to claim 3, characterized in that, In step 3), after a single segment of welding is completed, deformation data is synchronously collected by a laser displacement sensor and uploaded to the industrial control host. The industrial control host has a built-in compensation control system that combines simulation benchmark comparison calculation to output the optimal arc offset in the range of 0~30mm, and matches and fine-tunes the welding current and welding speed.

5. The method for welding the inner bore of the molten salt heat exchanger nozzle and the reactor vessel nozzle according to claim 1, characterized in that, In step 3), 70% of the allowable value of axial deformation is set as the early warning threshold for layered deformation. When the cumulative deformation of a single layer of welding reaches the early warning threshold, the arc starting position of the subsequent weld layer is actively shifted to implement directional correction and suppress the accumulation of deformation.

6. The method for welding the inner bore of the molten salt heat exchanger nozzle and the reactor vessel nozzle according to claim 1, characterized in that, In step 2), the endoscope probe maintains a constant observation distance of 5-10 mm with the molten pool, and feeds and rotates synchronously with the welding torch throughout the process. There is no obstruction of the welding line of sight, and the morphology of the molten pool on the front side and the forming state on the back side of the weld are monitored simultaneously.

7. The method for welding the inner bore of the molten salt heat exchanger nozzle and the reactor vessel nozzle according to claim 1, characterized in that, The finite element simulation deformation benchmark database is constructed as follows: a three-dimensional model of the pipe butt joint is established, temperature-related nonlinear material thermophysical parameters are assigned, a double ellipsoidal heat source model is used to match the welding heat input, tooling constraints and heat transfer boundary conditions are applied, the welding sequence is iteratively optimized, theoretical deformation data of different sections and different weld layers are extracted to establish a benchmark database, and the data are pre-stored in the compensation control system for real-time comparison and compensation on site.

8. The method for welding the inner bore of the molten salt heat exchanger nozzle and the reactor vessel nozzle according to claim 1, characterized in that, The welding process parameters are as follows: welding current 80~120A, welding voltage 10~12V, welding speed 60~100mm / min, wire feed speed 200~400mm / min, and argon flow rate 8~12L / min.

9. A welding system applied to the internal hole welding method according to any one of claims 1-8, characterized in that, The system includes an automatic welding torch assembly for internal holes, a welding endoscope visualization assembly, a circumferential deformation sensing and acquisition assembly, and an external industrial control and display assembly. These four components are connected by optical fibers and signal lines to form a closed-loop control system that integrates real-time visualization, deformation acquisition, algorithm compensation, and dynamic welding adjustment. The automatic welding torch assembly for internal holes integrates positioning, telescopic, circumferential rotation, tungsten electrode oscillation, bevel trajectory tracking, uniform wire feeding, and multi-channel gas supply functions. It is adapted to the welding environment of the inner cavity of the pipe and realizes layered and segmented internal hole welding with double-sided argon gas protection. The miniature camera probe of the welding endoscope visualization component is rigidly fixed to the side of the welding torch end and moves synchronously with the welding torch to achieve unobstructed full-process visual monitoring of the welding area. The circumferential deformation sensing and acquisition component has multiple sets of laser displacement sensors evenly arranged around the outer wall of the pipe in the circumferential direction, which are used to collect welding radial shrinkage and angular deformation data in segments and synchronously. The external industrial control display component has a built-in compensation control system. It calculates welding compensation parameters by combining simulation benchmarks and measured deformation data, and synchronously drives and controls all actuators of the welding torch to achieve dynamic and active correction of welding point position and heat input.

10. The welding system according to claim 9, characterized in that, The welding torch tracking mechanism follows the bevel trajectory deviation in real time to keep the tungsten electrode and the weld center precisely aligned. The welding torch oscillation mechanism widens the molten pool by slightly oscillating the tungsten electrode laterally.

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