Tube to Tube Welding Procedure Qualification for the Manufacture of PFBR Sodium to Air Heat Exchangers

摘要

Prototype Fast Breeder Reactor (PFBR) is a 500MWe pool type, mixed oxide fuelled, sodium cooled nuclear reactor, which is presently under construction at BHAVINI, Kalpakkam. India. Sodium to Air Heat Exchanger (AHX) is a vertical, counter flow type heat exchanger located in Safety Grade Decay Heat Removal (SGDHR) loop in which heat from the intermediate circuit sodium is transferred to atmospheric air by natural convection. The principal material of construction of AHX is Modified 9Cr-1Mo steel (N&T). As operating temperature is high and AHX forms part of reactor SGDHR circuit, boundaries of the AHX must possess a high degree of reliability against failure. This is achieved by comprehensive design, precise material selection, rigorous manufacturing methods and high standard quality control & quality assurance. AHX has ～4m diameter ring type sodium inlet and outlet headers having a size of outside diameter (OD) 558.8mm×12 mm thickness. The inlet and outlet headers are made in five segments with 250 pullouts of OD38.1mm×2.6mm thickness on each header which are connected to straight vertical finned tube bundle. The header pullouts to tube weld joints are executed by manual Gas Tungsten Arc Welding (GTAW) process with addition of filler wire. The tube to tube welding is carried out by orbital autogenous pulsed GTAW process. Each header pullout to tube weld joints and tube to tube weld joints are subjected to Post Weld Heat Treatment at 760±10°C for 30 minutes to relieve the welding stresses and to get tempered martensite structure. Each production weld joints are subjected to various non-destructive examinations and testing to evaluate and assure the quality. Each weld joint is subjected to thorough Visual Examination, Liquid Penetrant Examination, Radiography Examination, Pneumatic Test and Helium Leak Testing. Enormous numbers of trials were conducted to optimize the process parameters during tube to tube welding procedure qualification to achieve sound weld meeting stringent specification requirements. This paper highlights the development and establishment of procedure for tube to tube welds of Sodium to Air Heat Exchangers for 500MWe Prototype Fast Breeder Reactor.