IIW White Paper

Following these codes assures initial high quality construction and appropriate ongoing inspections, with repairs performed to maintain the structural integrity of the systems necessary to assure a safe shutdown of the plant once it has started operation. One can refer to Nuclear Construction Lessons Learned Guidance on best practice: welding – The Royal Academy - ISBN: 1-903496-82-9. Contractors that build these facilities mainly employ welders that are skilled in manual techniques using the GTAW and SMAW processes. Typically, piping butt welds are performed with GTAW for the root pass and followed by a second pass of GTAW, then the welds are completed using SMAW. This combination is meant to assure that the higher quality GTAW process is used to seal the inside diameter and the higher production process SMAW is used to fill the balance of the joint volume. This method has been in use since the early 1970 s and is still the predominant method to date. Since 2005, there has been a desire to use new highly developed GMAW power sources such as Miller Electric’s RMD and Lincoln Electric’s STT to replace the GTAW/SMAW method. These two methods are starting to be placed into the hands of welders. It should be noted that many welders do not transition well to this change without adequate training. Equipment manufacturers’ employ SAW for typical construction. This process is well established with both high production and high quality benefits. In a factory environment where the vessels can be manipulated, SAW is a nearly perfect process. SAW can easily be incorporated into an automated production line. When more portable welding is necessary or for certain smaller production assembly, FCAW is typically employed with gas shielding. The various issues relating to welding technology for nuclear power plants must keep pace with the advances and innovations in welding technology. While materials pose exciting challenges and thus opportunities to materials scientists, development of welding technology for joining these materials pose further exciting challenges for technologists, to enable realisation of the objectives of sustainable development of the nuclear energy option. Development of welding technology for nuclear energy systems involves exploitation of the full potential of the latest welding processes, and also the harnessing the benefits through developing of knowledge-based expert systems for failure analysis and suggesting repair and refurbishing strategies and weld process modeling and control, weld-bead profile analysis for quality control, microstructure prediction and mechanical property estimation. Alloys that were used in the first generation of nuclear plants have been improved on since the service conditions are now better understood. Examples include the use of unstabilised 304 stainless steel in Boiling Water Reactor Recirculating Coolant Systems and the use of Inconel 600 fillers and base metals in the vessel nozzles which are both attacked by Inter Granular Stress Corrosion Cracking. Other corrosion mechanisms and fatigue will be addressed by material improvements using new alloys and by cladding existing alloys with exotic materials by Explosion Welding. New welding processes and techniques will also be used to address fatigue damage (chamfered fittings or 2 to 1 profile socket welding) and lower residual stress from GMAW versus GTAW/SMAW. In the future, large equipment manufacturers’ will continue to use SAW but high quality Hot Wire GTAW will find a place in attaching smaller bore nozzles such as the numerous Control Rod Drive Mechanism housings. The introduction of the new highly automated process will require new equipment which is now just beginning to be developed. Welding Operators for this specialised fabrication will be in high demand. Currently the training for such equipment is offered by the equipment suppliers. Due to the heightened safety requirements of nuclear power all welding will require certification testing of individuals and full documentation of welding consumables used and recording of the welding procedure variables that are used. This is carried out by the Welding Engineering professional and/or Quality Control personnel. Presently the collegiate system is not prepared to train which is attributed to a lack of focused foresight by national leadership. Costs for teaching welding professionals is considerably higher than other

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Through Optimum Use and Innovation of Welding and Joining Technologies

Improving Global Quality of Life