Advanced Welding Techniques for Nuclear Power Plant Construction

by Mark Gino Aliperio, BH Song, SH Jeong, UH Jang, E Akpoguma

Industrial technologies are evolving, and in construction, there is no exception. But the most recent developments extend beyond building equipment alone. In this 21st century, technological advancement is the order of the day. New approaches for construction are seen in the field. Such advancement in construction technologies are driving efficiencies which results to better-quality outcomes. With several nuclear power plants (NPPs) presently under construction worldwide, expectations are rising regarding the quality, cost and schedule associated with the construction of new NPPs. Worldwide experience in large construction projects, including NPP projects, has resulted in significant advancements in construction techniques and methods and this includes some Advanced Welding Techniques.

Conventional Welding Methods

The most common process applied in the construction of an NPP is the joining of material members by welding. The welding processes used in NPP construction include structural welds, used to connect structural members. Quality welding, crucial to the construction of NPPs, is time consuming. The time required is determined to a large extent by the achievable deposition rate of the weld material.

The completed process is examined to verify that the completed weldment is defect free. The welding process is subject to many variables. Various factors contribute to weld problems, and weld defects happen for a host of reasons.

The most common weld defects include: Lack of fusion; Lack of penetration or excess penetration; Excess penetration (burning through); Porosity; Inclusions; Cracking; Undercut; Lamellar tearing,

Goals of Advanced methods

Advanced welding techniques are focused on four goals:

• Eliminating welds
• Moving from field to shop
• Automating the welding process
• Increasing deposition welding rate and weld quality

Automatic and mechanized welding

The process of welding depends on a number of variables including environment, configuration, welder fatigue and embedded material contamination. The application of automation to the process helps reduce the fatigue and requirement for constant precision imparted by the welder. Advances in control technology have improved machine precision and reliability for automated welding equipment. In addition, the mechanized process enables larger amounts of weld metal to be deposited on an ongoing basis than a welder would typically be able to deliver.

Use of automatic welding equipment is effective in maintaining high quality and in improving the working environment when welding in narrow spaces. This is useful in getting defect free welds, thus avoiding job repetition, and resulting in savings in time and cost.

Field Application

1. Reactor Coolant Piping Automatic Weld, Shin-Kori 1, South Korea

For the reactor coolant loop (RCL) piping welding, a narrow gap automatic welding method has been used at the Shin-Kori 1 and 2 sites in the Republic of Korea. Contributing factors to rising construction costs have been manual welding, with its average defect rate of 3–5% (higher than that of automatic welding), and a limited availability of qualified welders. A narrow gap welding technique enables the welding time to be shortened, as the welding cross-sectional area is reduced, and provides effective control of precision fabrication by preventing physical deformation of the weldment as weld deposit is reduced. It shows a composite view comparing the narrow gap welding technique with the weld preparation configuration of conventional welding.

Application of a narrow gap automated welding process, instead of manual welding, to the RCL piping where precision technique is mandated would not only enhance welding quality with its consistency throughout the welding, but also shorten the construction schedule by 1.5 months. Such automated welding equipment is available in a variety of sizes and weld techniques, and can also be customized to meet specific needs. Precision welding of reactor coolant piping is managed by computerized control as part of the automated welding machine.

2. Kashiwazaki-Kariwa, Japan

The automatic welding method was applied to Reinforced Concrete Containment Vessel (RCCV) liners, large and small bore piping, instrumental piping, Reactor Pressure Vessel (RPV) nozzles, etc. The automatic welding ratio was achieved to 98% for RCCV liners, 60% for the large bore piping in the Reactor Building and 35% for the large bore piping in the turbine building.

This work is part of the project report?KINGS/PR-PEP02-2018-08 "Advanced Construction Technologies"?by the Project Engineer Program II Laboratory of KEPCO International Nuclear Graduate School Class of 2019.