Local Dry Underwater TIG Welding of Positioning Pins in Nuclear Power Plants

Literature Overview

This paper addresses a highly specialized welding challenge encountered during in-service maintenance of nuclear power plant structures, specifically the repair and installation of positioning pins underwater using a dry hyperbaric TIG (gas tungsten arc welding) technique. The core innovation lies in creating a controlled dry environment within the submerged pressure vessel or structure, allowing the application of standard-quality TIG welding procedures to underwater joints without the complications of water-cooled arc instability and hydrogen absorption. The study is particularly relevant to nuclear plant decommissioning, refueling outage maintenance, and structural repair operations where access is limited to wet environments.

Core Technical Viewpoints

The fundamental challenge of underwater welding in nuclear power plants is twofold: maintaining weld quality comparable to dry-land standards, and ensuring the integrity of the surrounding containment structure. Conventional submerged arc welding produces coarse grain structures, high hydrogen content in the weld metal, and unacceptable levels of porosity and cracking. The dry underwater TIG approach circumvents these issues by first evacuating the local welding zone of water, creating a sealed dry cavity, and then performing TIG welding within that cavity under controlled atmospheric or inert gas conditions.

The positioning pins discussed in the study serve critical structural and alignment functions in nuclear plant components such as containment vessels, reactor internals support structures, and cooling channels. Their failure can lead to cascading structural consequences, making the welding quality requirements exceptionally stringent. The study emphasizes that even though the welds are performed in an underwater context, the quality acceptance criteria must meet or exceed those specified in ASME Section IX and relevant nuclear qualification standards such as AWS D10.9M for underwater welding.

Process Analysis and Key Parameters

The dry underwater TIG process involves several sequential steps that each demand careful engineering control:

1. Zone isolation and dewatering — The local welding area is sealed off from the surrounding water using inflatable barriers, welded patches, or temporary clamps, followed by pump evacuation to create a dry working cavity.
2. Atmosphere preparation — The dry cavity is purged with argon or a mixed gas (argon with small percentages of hydrogen or helium) to ensure inert shielding during welding.
3. TIG welding execution — Standard TIG parameters are applied with specific adjustments for the confined geometry and potential thermal mass of the surrounding structure.
4. Post-weld inspection — Visual, magnetic particle, and ultrasonic testing are performed on the completed welds within the dry cavity before re-flooding.

The following table summarizes typical TIG parameters for the positioning pin application discussed in the study:

A key insight from the study is the thermal management challenge. The surrounding water acts as a heat sink, which can be beneficial for controlling heat input and minimizing distortion, but it also creates steep thermal gradients at the boundary of the dry cavity. These gradients can induce residual stresses that, if uncontrolled, may lead to cracking in the weld zone or the heat-affected zone. The study recommends careful sequencing of weld passes and, where feasible, preheating of the base material to moderate the thermal gradient.

Quality Control and Defect Prevention

The study applies a systematic defect prevention framework aligned with the FMEA (Failure Mode and Effects Analysis) methodology. The primary defect modes identified include:

• Porosity — Caused by inadequate gas shielding within the confined cavity or residual moisture on the base metal surface.
• Cracking — Either hot cracking in the weld metal due to sulfur or phosphor segregation, or cold cracking in the HAZ due to hydrogen diffusion from the surrounding water during post-weld cooling.
• Incomplete fusion — Resulting from poor access or insufficient arc force in tight geometries.
• Undercut — Occurring at the weld toe due to excessive travel speed or improper torch angle.

The countermeasures proposed include strict pre-weld cleaning protocols (grinding to bare metal with a minimum 25 mm preparation zone), controlled interpass temperature monitoring using infrared thermometers, and post-weld heat treatment where the material specification permits. For austenitic stainless steel positioning pins, a post-weld solution treatment at 1050–1100°C followed by rapid water quenching is recommended to restore full corrosion resistance and eliminate sensitization.

Integration with Engineering Practice

In practical nuclear plant maintenance scenarios, the dry underwater TIG technique has been successfully applied to repair positioning pins on reactor pressure vessel internals during refueling outages. One documented case involved the repair of a cracked positioning pin on a BWR (boiling water reactor) internals support structure. The pin was made of Type 316L stainless steel, and the base material was a low-alloy steel support plate. The dissimilar metal joint was successfully welded using ER309L filler metal with a two-pass technique: a root pass to achieve full penetration, followed by a cap pass to build up the weld profile. Post-weld inspection using magnetic particle testing revealed no indications of cracks, porosity, or lack of fusion, and the weld was accepted per the applicable nuclear qualification procedure.

The study also highlights the importance of welder qualification and procedure qualification. Underwater welding procedures must be qualified in accordance with AWS D10.9M or equivalent standards, which require demonstration of weld quality under conditions that simulate the actual in-service environment, including pressure effects on arc behavior and gas shielding.

Key Questions and Reflections

A significant question raised by this study is the long-term durability of dry underwater welds when the structure is subsequently re-flooded. The weld metal and HAZ are initially in a dry, inert environment, but after re-flooding, they are exposed to aqueous corrosion environments that may contain chlorides or other aggressive species. The study suggests that the weld quality achieved under dry conditions is inherently superior to that of wet underwater welds, but the transition from dry to wet conditions introduces a new set of degradation mechanisms that warrant long-term monitoring.

Another reflection concerns the economic and logistical aspects of the technique. Creating a dry cavity underwater requires specialized equipment, trained personnel, and significant planning time. For small repairs, the cost-effectiveness may be questionable, but for critical safety-related components such as positioning pins, the quality assurance benefits clearly justify the additional effort and expense.

Study Insights and Implications

This study reinforces the principle that welding quality is determined not only by the welding parameters but also by the environmental conditions under which the welding is performed. The dry underwater approach represents a paradigm shift from accepting compromised weld quality as an inevitable consequence of underwater work to insisting on dry-land quality standards even in submerged environments. This philosophy has broad implications for the nuclear industry's approach to in-service maintenance and repair, encouraging the development of increasingly sophisticated techniques to achieve high-quality welds under challenging conditions.

The study also underscores the importance of interdisciplinary collaboration in nuclear welding. Successful execution of dry underwater TIG welding requires expertise in welding metallurgy, fluid mechanics (for cavity design and dewatering), materials science (for corrosion resistance assessment), and nuclear safety engineering (for regulatory compliance). Future work should focus on standardizing the technique into formal qualification procedures and expanding its application to other underwater repair scenarios beyond positioning pins.