Challenges in Manufacture of PFBR Steam Generators

Abstract

Abstract Prototype Fast Breeder Reactor (PFBR) is a 500 MWe pool type sodium cooled reactor. The main function of Steam Generator (SG) is to extract the reactor heat through secondary sodium system and convert the feed water into superheated steam in the tubes of SG. The Steam Generator is a vertical, once through, shell and tube type heat exchanger with liquid sodium in the shell side and water/steam in the tube side. The highly reactive nature of sodium with water/steam requires that the sodium to water/steam boundaries of the steam generators must possess a high degree of reliability against failure. This is achieved in design and manufacture by maximizing the tube & tubesheet integrity and more importantly by proper selection of tube to tubesheet joint configuration. Modified 9Cr-1Mo material is selected as major material of construction for steam generator as this material has excellent high temperature mechanical properties and has high resistance to stress corrosion cracking in caustic & chloride environment. Steam Generator has inlet and outlet nozzles which are made from the pullout shells of SG. Pullouts are made by hot forming process by heating the shell inside the furnace to the temperature of 950- 1100oC followed by die & punch pressing. After forming, the pullouts are subjected to normalizing at 1040-1050oC followed by tempering at 780oC to restore the original material properties. The forming of nozzle pullouts is really difficult and challenging task as the dimensions are too large & dimensional tolerances are very tight. Steam Generator shell assembly is fabricated in horizontal condition after completion of tube bundle which requires insitu welding of shells around the tube bundle. Dimension control during shell welding is extremely difficult as internal fixtures/spiders for ovality control is ruled out due to existence of tube bundle. Utmost care is required during shell welding to avoid arc strike/fusion on the tube (i.e sodium/water boundary), as the gap between tube bundle and shells is very less. After completion of tube bundle and shell assembly, entire 26 meter length SG undergoes Post Weld Heat Treatment (PWHT) in a single charge at 760 ± 10oC for 4 hours soaking time to relieve the welding stresses and to get the homogenous tempered martensite structure. Special arrangements were made to take care of about 240 mm thermal expansion of complete SG assembly during PWHT. Due to asymmetric shape of SG, drying of the water after hydro test & surface treatment is difficult. The effective surface treatment process requires special thought of rotation of the job by 180 degree after completion of partial chemical cleaning due to air pockets on the top surfaces of the shell assembly. This paper highlights the experience gained during welding of tube to tubesheet joints, hot forming of nozzle pullouts, shell assembly and post shell assembly activities of PFBR Steam Generators in detail.