Nuclear Power Plant Positioning Pin Local Dry-Underwater TIG Welding Process
1. Definition and Technical Principles
Local dry underwater welding is an advanced underwater welding technique in which a confined dry environment is created around the weld zone beneath the water surface, allowing conventional gas-shielded arc welding (typically TIG/GTAW) to be performed under conditions closely approximating above-water welding. The "local dry" method involves the construction of a temporary sealed enclosure—such as a caisson, welding chamber, or buoyancy-assisted containment structure—around the joint to be welded, followed by the removal of water from within this enclosure to create a dry, gas-filled atmosphere suitable for arc stability and metallurgical control.
In the specific context of nuclear power plant (NPP) positioning pin welding, this technique is applied to the fabrication and repair of dowel pins, alignment pins, and structural locator hardware that are integral to the assembly of reactor vessel internals, containment structures, and support frameworks. These positioning pins must maintain dimensional accuracy, structural integrity, and radiation-tolerant material properties throughout the operational lifetime of the nuclear facility, typically spanning 40–60 years.
The fundamental principle relies on three key engineering requirements:
- Hydrostatic pressure management: The welding enclosure must withstand the differential pressure between the external water column and the internal atmospheric (or slightly pressurized) environment to prevent water ingress during the welding operation.
- Atmospheric control: Once the enclosure is sealed and dewatered, an inert or controlled shielding gas atmosphere (typically argon or argon-helium mixtures) is maintained to prevent oxidation and nitrogen pickup in the weld metal.
- Thermal management: The surrounding water acts as a heat sink, creating non-uniform thermal gradients that must be managed through interpass temperature control, preheating strategies, and careful sequencing of weld passes.
2. Category and Business Positioning
Within the broader capability portfolio of Cladding Technology Shanxi Co., Ltd., this technical entry falls under the TIG/MIG Weld Overlay and Specialized Welding technology route, specifically within the nuclear-grade underwater welding sub-discipline. This positions the company at the intersection of:
- Nuclear-grade fabrication: Meeting the stringent quality requirements of nuclear regulatory bodies (NNSA in China, NRC in the United States, ONR in Russia) for structural welds in safety-related components.
- Specialized underwater operations: Providing a unique capability for underwater welding in submerged construction, repair, and maintenance scenarios where conventional above-water methods are infeasible.
- Nuclear power plant lifecycle support: Serving both new-build NPP projects (during submerged component fabrication) and in-service inspection/repair (ISI) programs for operating plants.
The business positioning of this capability is particularly strong in markets where:
- Submerged structural components require precision welding during installation (e.g., underwater reactor support structures, submerged cooling system piping)
- In-service repair of submerged containment structures or piping systems is required without full de-watering of the containment vessel
- Emergency response welding for underwater structural components following accident scenarios
- Repair of positioning pins that have experienced fatigue cracking, stress corrosion cracking, or mechanical damage during plant operation
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The local dry underwater TIG welding process for positioning pins in nuclear power plants serves several critical technical objectives:
- Structural integrity restoration: Repairing or fabricating positioning pins that maintain the precise alignment of nuclear reactor internals, ensuring gap tolerances within specified limits (typically ±0.5 mm for reactor vessel internals)
- Metallurgical compatibility: Achieving weld metal properties that match or exceed the base material requirements for radiation resistance, thermal fatigue resistance, and stress corrosion resistance
- Non-destructive verifiability: Producing welds that are fully inspectable by ultrasonic testing (UT), radiographic testing (RT), and visual testing (VT) to nuclear acceptance criteria
- Regulatory compliance: Meeting all applicable nuclear codes and standards for qualification, execution, and inspection of nuclear-grade welds
3.2 Value Contribution
This capability provides significant value across the nuclear power plant lifecycle:
- Extended plant availability: Enables in-situ repair of submerged components without requiring full plant shutdown and de-watering, reducing outage duration by an estimated 30–50% compared to conventional methods
- Cost reduction: Eliminates the need for large-scale dewatering operations, temporary cofferdams, or component removal/replacement, reducing total repair costs by 40–60% in typical scenarios
- Reduced radiation exposure: Minimizes worker dose by enabling remote or semi-automated welding operations within contained underwater environments
- Technical differentiation: Establishes a unique competitive advantage in the nuclear repair market, where few companies possess both nuclear-grade welding qualifications and specialized underwater welding expertise
4. Key Process and Implementation Points
4.1 Process Overview
The local dry underwater TIG welding process for positioning pins follows a structured sequence of operations:
- Pre-assessment and planning: Detailed examination of the joint geometry, base material condition, water depth, and environmental conditions; development of a Welding Procedure Specification (WPS) and Procedure Qualification Record (PQR)
- Enclosure design and fabrication: Design of the local dry welding chamber with appropriate pressure rating, access provisions, and gas management systems
- Site preparation: Surface cleaning, fit-up verification, and alignment confirmation of the positioning pin joint
- Enclosure installation and dewatering: Placement of the welding chamber, sealing against the substrate, and controlled removal of water
- Atmosphere preparation: Establishment of the shielding gas environment within the enclosure with oxygen and moisture levels verified
- Welding execution: TIG welding performed according to the qualified WPS with real-time monitoring of parameters
- Post-weld treatment: Controlled cooling, stress relief (if required), and surface finishing
- Inspection and qualification: Full NDT suite applied per nuclear code requirements
- Documentation: Complete welding log, NDT reports, and quality records compiled for regulatory submission
4.2 Key Process Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Welding Current (DC) | 80–180 A | Depends on pin diameter and joint configuration |
| Travel Speed | 30–80 mm/min | Adjusted for penetration depth and bead profile |
| Shielding Gas | Argon (99.995%) or Ar/He mix | Oxygen content <20 ppm; moisture <5 ppm |
| Gas Flow Rate | 8–15 L/min | Adjusted for enclosure volume and turbulence |
| Preheat Temperature | 50–150°C | Material-dependent; critical for low-alloy steels |
| Interpass Temperature | ≤250°C (max) | Monitored with thermal indicators; critical for HAZ control |
| Water Depth | 0–30 m (typical) | Pressure rating of enclosure must exceed depth pressure + margin |
| Enclosure Internal Pressure | 0.1–0.3 MPa gauge | Slightly above atmospheric to prevent water ingress |
| Electrode Material | ER308L, ER316L, or per WPS | Matched to base material and service conditions |
| Tungsten Electrode | Thorium-free (LaB₆ or ZrO₂), 1.6–2.4 mm | Pre-ground to sharp cone; replaced per qualification |
4.3 Critical Implementation Considerations
4.3.1 Enclosure Design and Pressure Management
The local dry welding enclosure is the defining element of this process. Key design considerations include:
- Pressure rating: Must withstand the hydrostatic pressure at the operating depth plus a safety margin of at least 1.5×. For a water depth of 10 m, the external pressure is approximately 0.1 MPa; the enclosure must be rated for at least 0.15 MPa gauge differential pressure.
- Sealing integrity: Gasket materials must be compatible with both the base material surface and the shielding gas atmosphere. Viton or PTFE-based seals are typically specified for nuclear applications.
- Access provisions: The enclosure must provide adequate access for the welder's hands, torch, and inspection equipment. Hand ports with appropriate seals are standard.
- Gas management: Ventilation and gas replenishment systems must maintain shielding gas purity throughout the welding operation, accounting for consumption by arc oxidation and any micro-leaks.
- Emergency depressurization: Safety systems must allow controlled release of internal pressure in the event of emergency evacuation.
4.3.2 Thermal Management in Submerged Environments
The surrounding water creates unique thermal challenges that must be addressed:
- Heat dissipation asymmetry: The water-cooled side of the weldment experiences significantly faster cooling rates than the air-exposed side, leading to asymmetric residual stress and potential distortion.
- Preheat strategy: Localized preheating of the joint area (typically using induction heating or electric resistance heating) is essential to compensate for water-side heat loss and ensure adequate weld penetration.
- Interpass monitoring: Infrared thermography or embedded thermocouples are used to monitor interpass temperatures, particularly on the water-cooled side where temperature may drop below the required minimum between passes.
- Post-weld cooling control: After welding, the enclosure is maintained at controlled pressure while the weld cools slowly to avoid thermal shock from the surrounding water. Controlled depressurization follows once the weld has cooled below the stress relief temperature.
4.3.3 Welding Technique and Parameter Control
TIG welding in the local dry environment requires precise parameter control:
- Arc stability: Maintained through consistent gas flow, proper torch angle (typically 10–15° from vertical), and stable travel speed. Any disruption to the shielding gas envelope can introduce porosity.
- Penetration control: Achieved through careful balance of current, travel speed, and torch angle. Insufficient penetration leads to lack of fusion; excessive penetration causes backside spatter and potential distortion.
- Bead profile: The weld bead must exhibit uniform width, convexity, and transition to the base metal. Underwater conditions can cause slight variations that must be monitored and corrected.
- Multi-pass strategy: For thicker positioning pins, a multi-pass sequence is employed with the first pass (root pass) providing full penetration and subsequent passes building up to the required profile.
4.3.4 Post-Weld Inspection in the Local Dry Environment
NDT is typically performed within the local dry enclosure before depressurization:
- Visual Testing (VT): 100% visual examination of all weld surfaces for defects including cracks, porosity, undercut, and excessive reinforcement.
- Ultrasonic Testing (UT): Phased array ultrasonic testing (PAUT) or conventional UT per applicable nuclear code. The dry environment allows for full coupling and reliable signal transmission.
- Radiographic Testing (RT): Digital radiography or film radiography performed within the enclosure, with lead shielding and remote handling for radiation safety.
- Penetrant Testing (PT): Applied to accessible surfaces for detection of surface-breaking defects, particularly in the HAZ and weld toe regions.
5. Applicable Standards and Acceptance Criteria
5.1 Nuclear Code Requirements
The welding process, materials, and acceptance criteria must comply with the following nuclear standards:
| Standard | Scope | Key Requirements |
|---|---|---|
| NB/T 20002.2-2019 | Welding procedure qualification for nuclear power plant components | PQR/WPS qualification, essential variables, impact testing |
| NB/T 20002.3-2019 | Welder qualification for nuclear power plant | Welder certification, skill maintenance, periodic requalification |
| NB/T 20002.5-2019 | Acceptance criteria for nuclear-grade welds | NDT acceptance levels, defect size limits |
| ASME BPV Section III, Div. 1 | Rules for construction of nuclear power plant components | Welding procedure, welder qualification, NDT requirements |
| ASME BPV Section III, Div. 2 | Rules for construction of nuclear power plant components (metric) | Metric equivalents of Div. 1 requirements |
| ASME BPV Section XI | Rules for inspection and testing of nuclear power plant components | In-service inspection, repair acceptance criteria |
| ASME BPV Section VIII, Div. 3 | Rules for construction of nuclear power plant components (high temperature) | High-temperature service requirements |
| GB/T 19522-2009 | Acceptance criteria for non-destructive testing of welded joints in nuclear power plant components | UT and RT acceptance levels for nuclear welds |
| GB/T 150.4-2011 | Non-destructive testing of welded joints in pressure vessels | RT and UT acceptance criteria |
| ISO 17635:2020 | Non-destructive testing of welds – Recommended methods | General NDT methodology and acceptance principles |
5.2 Welding Procedure and Qualification Standards
- NB/T 20002.2-2019: Requires full PQR qualification for each welding process variant, including the local dry underwater condition as an essential variable. Impact testing (Charpy V-notch) at service temperature or one temperature below is mandatory for materials above specified thickness thresholds.
- ASME BPV Section III, Appendix X: Defines essential variables for welding procedure qualification, including welding position, preheat range, interpass temperature, and electrode classification. The underwater local dry condition is treated as a distinct qualification environment.
- NB/T 20002.3-2019: Welder qualification requires demonstration of skill on a test coupon under conditions representative of the actual welding environment, including the local dry enclosure setup.
5.3 Material Standards
- ASTM A213/A213M: Stainless steel tubing for reactor internals
- ASTM A312/A312M: Seamless austenitic stainless steel pipe and tube
- SAE-AMS 5528: Inconel 625 for high-temperature positioning pins
- GB/T 12771-2019: Stainless steel welded tubes for general purposes (reference)
- ASTM A743/A743M: Castings for pressure-containing parts (for cast positioning pins)
5.4 Acceptance Criteria Summary
Typical acceptance criteria for nuclear-grade positioning pin welds include:
- RT Acceptance: No indication of cracks, lack of fusion, or slag inclusions. Porosity limited to individual indications ≤1.5 mm and clustered porosity ≤3% of weld area (per NB/T 20002.5 and ASME BPV Section III, Div. 1, UW-51).
- UT Acceptance: No indication exceeding the threshold for planar defects (cracks, lack of fusion). Volumetric indications (porosity, slag) limited per code-specific tables.
- VT Acceptance: No surface cracks, undercut exceeding 0.25 mm, or excessive reinforcement (≤1.5 mm or 25% of weld width, whichever is less).
- Dimensional Tolerances: Pin diameter within ±0.1 mm of nominal; length within ±0.5 mm; perpendicularity within 0.1 mm/m.
- Hardness: Weld metal and HAZ hardness within specified limits (typically ≤30 HRC for austenitic stainless steels per ASME BPV Section III, Table 3181.1).
6. Common Risks and Controls
6.1 Technical Risks
| Risk | Cause | Control Measures |
|---|---|---|
| Porosity in weld metal | Shielding gas contamination, moisture ingress, insufficient gas flow | Gas purity verification (<20 ppm O₂), enclosure leak testing, flow rate monitoring |
| Lack of fusion | Inadequate heat input, poor fit-up, excessive travel speed | Fit-up verification, parameter monitoring, root pass inspection before subsequent passes |
| Cracking (hot or cold) | High restraint, hydrogen pickup, excessive interpass temperature | Low-hydrogen procedures, preheat/interpass temperature control, post-weld stress relief |
| Distortion and misalignment | Asymmetric thermal gradients, inadequate fixturing | Weld sequencing optimization, symmetric welding pattern, rigid fixturing |
| Enclosure failure | Pressure differential exceeding design limit, seal degradation | Pressure monitoring with alarms, regular seal inspection, pressure relief valves |
| Insufficient penetration | Inadequate current, incorrect torch angle, poor joint preparation | Parameter verification, joint preparation inspection, backing bar usage |
| Corrosion of weld or HAZ | Intergranular sensitization, improper heat treatment | Solution heat treatment where required, low-carbon filler metal selection (L grades) |
6.2 Operational and Safety Risks
- Hydrostatic pressure hazard: If the enclosure seal fails, rapid water ingress can create a dangerous environment for the welder. Control: redundant sealing, continuous pressure monitoring, emergency escape provisions, and welder training in emergency procedures.
- Gas asphyxiation: Loss of shielding gas supply or oxygen depletion within the enclosure. Control: gas supply redundancy, oxygen monitoring with alarms, and emergency ventilation systems.
- Electrical hazards: TIG welding equipment operating in a confined, potentially wet environment. Control: proper grounding, insulated equipment, voltage-rated components, and regular electrical inspection.
- Radiation exposure (in-service repairs): Welding near radioactive components. Control: dose assessment, shielding, remote welding where feasible, and strict adherence to ALARA principles.
- Underwater structural hazards: Enclosure instability, unexpected water movement, or structural collapse. Control: thorough pre-assessment, structural analysis of the enclosure, and environmental monitoring.
6.3 Quality Assurance Risks
- NDT reliability in confined space: Performing UT or RT within a confined enclosure may limit probe access or film positioning. Control: pre-planned NDT approach, use of phased array UT for improved access, and digital radiography for flexibility.
- Documentation completeness: Nuclear-grade welding requires comprehensive documentation. Control: real-time welding log recording, automated parameter capture, and independent quality assurance review.
- Welder skill degradation: Infrequent performance of underwater welding can lead to skill loss. Control: periodic skill assessment, simulation training, and mandatory requalification at defined intervals.
7. Application Scenarios Across the Company's Technology Routes
7.1 TIG/MIG Weld Overlay Route
The local dry underwater TIG welding capability for positioning pins is a specialized extension of the company's core TIG/MIG weld overlay technology. Key integration points include:
- WPS development and qualification: The same WPS qualification framework used for conventional weld overlay is adapted for the underwater local dry environment, with additional essential variables related to enclosure pressure, water depth, and gas management.
- Material expertise: The metallurgical knowledge base developed through years of weld overlay work (including stainless steel, nickel alloys, and duplex steels) directly supports the selection of filler metals and heat treatment strategies for underwater positioning pin welds.
- NDT capability: The company's established NDT infrastructure and qualified personnel can be deployed for underwater welding inspection with minimal additional training, as the local dry environment allows conventional NDT techniques to be applied.
- Quality management system: The existing nuclear-grade quality management system (QMS) is extended to cover underwater welding operations, ensuring regulatory compliance and customer confidence.
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding is primarily used for solid-state cladding of large flat surfaces, the local dry underwater welding capability provides a complementary service for:
- Repair of cladded components: When hydraulic explosively bonded cladding is damaged (e.g., during handling, installation, or in-service), localized weld repair can be performed underwater using the local dry TIG technique to restore cladding integrity.
- Transition layer welding: For cladded components that require a weld overlay transition layer (e.g., when joining a cladded plate to a non-cladded component), the underwater local dry TIG technique can be used to deposit the transition layer in submerged conditions.
- Post-bonding weld repairs: After hydraulic explosive bonding, any defects identified in the bond interface can be addressed with localized weld repair, including underwater scenarios where the bonded component is already installed.
7.3 Explosion Welding Route
Explosion welding is used for producing clad plates and pipes with high-quality metallurgical bonds. The local dry underwater welding capability supports this route through:
- Post-explosion welding repair: Expulsion welding can produce localized defects (e.g., unmelted regions, porosity) that require weld repair. For components installed in submerged environments, the local dry TIG technique enables in-situ repair without component removal.
- Welding of explosion-welded joints: When explosion-welded clad plates are joined to form larger assemblies, the welds must be deposited with appropriate filler metals to maintain metallurgical compatibility. The local dry underwater technique extends this capability to submerged assembly operations.
- Repair of explosion-welded piping: Explosion-welded clad pipes used in nuclear service may require repair at weld joints or defect sites. The local dry underwater TIG technique provides a means to perform such repairs in submerged conditions.
8. Qualification Building and Customer Value
8.1 Qualification Building
The development and mastery of the local dry underwater TIG welding process for nuclear positioning pins contributes significantly to the company's qualification portfolio:
- Process qualification: Successful PQR qualification of the underwater local dry TIG process establishes a qualified WPS that can be used for future projects, reducing qualification time and cost for subsequent jobs.
- Welder certification: Training and certifying welders in underwater local dry TIG welding builds a specialized workforce that can support nuclear underwater welding projects, a capability held by very few companies globally.
- Equipment qualification: Development and qualification of the local dry welding enclosure, gas management systems, and monitoring equipment establishes a repeatable and auditable process infrastructure.
- NDT qualification: Extending NDT capabilities to underwater local dry environments (including underwater UT, RT, and VT) builds additional qualification depth.
8.2 Product Delivery Enhancement
This capability enhances the company's product delivery capabilities in several ways:
- Integrated service offering: The company can offer a complete package from clad plate/pipe fabrication (via explosion welding or hydraulic explosive bonding) through to in-service repair (via underwater local dry TIG welding), providing a single-source solution for nuclear customers.
- Reduced project risk: The ability to perform underwater repairs eliminates the need for component removal, de-watering, and reinstallation, reducing project schedule and cost risk for the customer.
- Regulatory acceptance: A qualified and documented underwater welding process provides regulatory authorities with confidence that repairs will meet nuclear safety requirements, facilitating approval of repair plans.
- Extended service life: By enabling repair of submerged components that would otherwise require replacement, the company helps customers extend the operational life of their nuclear power plant components.
8.3 Customer Value
The local dry underwater TIG welding capability delivers measurable value to nuclear power plant customers:
- Cost savings: Typical underwater repair projects using this technique save 40–60% compared to conventional methods requiring full de-watering or component replacement.
- Schedule reduction: Outage duration is reduced by 30–50%, translating to significant revenue savings for the plant operator (estimated at $50,000–$200,000 per day of avoided outage for a large NPP).
- Reduced radiation dose: Minimized worker exposure to radioactive environments during repair operations, supporting the plant's radiation safety program and regulatory dose limits.
- Technical assurance: A qualified, documented, and auditable welding process provides the customer with confidence that repairs will meet nuclear safety requirements and regulatory expectations.
- Environmental benefit: Reduced consumption of resources (water, energy, materials) compared to component replacement, supporting the customer's environmental, social, and governance (ESG) objectives.
9. Conclusion
The local dry underwater TIG welding process for nuclear power plant positioning pins represents a highly specialized and technically demanding capability that sits at the intersection of nuclear-grade welding, underwater operations, and quality management. Its development and mastery by Cladding Technology Shanxi Co., Ltd. establishes a unique competitive position in the nuclear repair market, where demand for specialized underwater welding services is growing as nuclear plants approach their extended operational lifetimes and require increasingly sophisticated in-service repair capabilities.
By integrating this capability with the company's existing expertise in TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding, the company can offer a comprehensive, single-source solution for nuclear cladding and repair needs, from new component fabrication through to in-service maintenance and repair. This integrated approach, supported by a robust quality management system and regulatory compliance framework, provides nuclear customers with the technical assurance, cost efficiency, and schedule reliability required for safe and successful nuclear power plant operations.