T he steam generator (SG) is one of the most critical components in the PFBR. The heat generated by the fission reaction is transferred to primary sodium and then in turn to secondary sodium. The secondary sodium flows in two loops through SGs, where steam is generated by the transfer of heat from secondary sodium on shell side to water on the tube side. The sodium and water reaction is violent and can lead to an explosion in case of any leak. Therefore the integrity of weld joints in the boundary separating these two fluids assumes maximum importance.
The SG is a vertical shell-and-tube-type heat exchanger with 42 ton weight, 26 metre length, and diameter ranging from 855 mm-1272 mm. The main construction material is modified 9Cr-1Mo-V (Grade 91) chromium molybdenum ferritic steel with weld thickness ranging from 2.3 mm to 90 mm.
The challenges in execution of the project started right with the procurement of extra-long (23m) tubes and large forgings for the tube sheet and dished ends. Indigenous development of these critical material and components enhanced the capabilities of Indian industries (the tubes were supplied by Nuclear Fuel Complex, Hyderabad).
Each steam generator consists of 547 tubes, welded to a tubesheet at each end. Each tube end was machined to accurate bevel with very high surface finish. Each tube was welded to an integral ‘spigot’ on the tubesheet. Tubesheet drilling involved carving out a socket of 30 mm by a kellering process. About 10,000 tube holes were drilled to stringent tolerances on diameter (50% of normal ASME & TEMA), positional accuracy 0.1%.
The manufacturer Larsen & Toubro used a gas-tungsten arc welding (GTAW) process for all the welding joints of steam generator. Extensive in-house automation was deployed to overcome its inherent low productivity during manufacture. For the most critical tube to tube-sheet weld joint, Internal Bore Butt joint welding was designated with 100% non-destructive examination (NDE) of each weld. NDE included micro-focal X-ray, dye penetrant test and strict weld profile control. Without clean conditions and accurate weld edge preparation it was not possible to get a defect-free tube-to-tubesheet weld in the first attempt. The entire tube bundle fabrication work for all the steam generators was carried out in the two dust-free shops set up exclusively for the project.
Equally innovative tooling was developed for in-situ machining for weld repair. More than 95% of welds cleared all the NDE and stringent manufacturing tolerances on the first attempt.
Since modified 9Cr-1Mo-V is susceptible to reheat cracking, NDE was carried out after local post-weld heat treatment (PWHT). This essential process necessitated sequential row-by-row fabrication of the tube bundle. The local PWHT of each tube weld joint is a critical operation where the tubesheet works as a heat sink and results in a large temperature gradient. Digital data loggers were used for recording the welding as well as PWHT parameters. Real-time records were key inputs for quality improvement teams.
After welding of all closing joints and peripheral attachments, a second PWHT of the entire 26m-long component was carried out in a specially-designed electric furnace. To prevent oxidation of the inner surfaces during heat treatment, the steam generator was continuously purged with dry nitrogen. Flexibility of the purging pipelines and the equipment sliding supports accommodated 250 mm linear thermal expansion of the equipment during heat treatment. Temperature uniformity was within +/-5 oC during eight heat treatment cycles, which is a remarkable achievement considering the wide variation in material thicknesses.
Once heat-treated, the SG received a pre-service acid pickling and passivation of the entire internal surface, including tubes, and a helium (vacuum) leak test. The test was challenging because of the SG’s complex internal construction, and due to the stringent specification for leak rates (2.66×10-9 Pa.m3/s).
The PFBR steam generator was manufactured by Larsen & Toubro against the contract placed by Bharatiya Nabhikiya Vidyut Nigam Limited (BHAVINI). The design, drawings and specification as provided by BHAVINI were followed during manufacturing. Larsen & Toubro thanks BHAVINI for their continual support during the endeavor.
The first SG was delivered in November 2010. Seven more were delivered in regular intervals from February 2011 to June 2012.
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This article was originally published in the August 2012 issue of Nuclear Engineering International
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