Design and manufacture of tube to tubesheet joints of steam generator for 500 MWe Prototype Fast Breeder Reactor

Prototype Fast Breeder Reactor (PFBR) is 500 MWe pool type sodium cooled fast reactor. Presently this reactor is at advanced stage of construction at Kalpakkam. The main function of the steam generator is to extract the reactor heat through secondary sodium system and convert the feed water into superheated steam in the tubes of steam generators. The steam generator is a vertical shell and tube type heat exchanger with liquid sodium in the shell side and water/steam in the tube side. Operating experience of FBRs have shown that steam generator (SG) holds the key to commercial success of such reactors. Tube leakage is a serious problem and the prevention of sodium water reaction incident in the SG is essential to maintain the plant availability. In case of crack/failure in tube, high pressure water/steam reacts with shell side sodium and results in exothermic reaction with evolution of hydrogen, corrosive reaction products and intense local heat depending on leak size. This high reactive nature of sodium with water/steam requires that sodium to water/steam boundaries of steam generators must possess a high degree of reliability against failure. This is achieved in design and manufacturing by maximising the tube integrity and more importantly by proper selection of tube to tubesheet joint configuration. The principal material of construction of SG is Modified 9Cr-1Mo steel. The tubes are seamless and produced by electric arc melting followed by Electro Slag Refining (ESR) with tight control on inclusion content. Ultrasonic and eddy current testing is done on entire tube length in accordance with ASME SEC III Class I. Long seamless tubes (each 23m) are used in order to reduce the number of tube to tubesheet welds.Each SG has 547 tubes and there are 9 SG in the reactor including one spare module. There is no tube to tube joint as the aim is to minimise the number of welds to increase reliability.Tube to tubesheet joint selected for PFBR steam generator is of internal bore welded type with raised spigot. The bore side has been selected as it eliminates crevices between tubes and tubesheets (as found in conventional rolled and welded joints) and moreover it permits 100% radiography inspection.Of the main joint form for the internal bore weld i.e. raised spigot and inset tube type, the former has been selected toa) place the welds in a low stress region, b) easier to weld and offer better repeatability c) provides increased ligament for a given tube pitch as the tubesheet hole will be the tube bore and d) permitting preheating and Post Weld Heat Treatment (PWHT) for individual tube to tubesheet weld. The type of weld joint selected is cup and cone type butt joint between spigot machined from tubesheets and tube ends. Both the ends are machined with very tight tolerances to get specified weld profile. ESR grade material is also used for tubesheet to get clean steel to achieve sound weld. The tube is of 17.2 mm OD, 2.3 mm wall thickness and welded to both the tubesheets at the ends. The welding process selected is autogenous, pulsed GTAW process with different high pulse level of current.To meet the stringent weld profile requirements, process parameters(pre purge time, up slope, speed of welding, down slope, post purge time) were optimised and welding procedure was established.The welds are subjected to preheat at 473K-523K and PWHT at 1033K±10K. Argon gas purging is provided to avoid oxidation during PWHT. Each joint is subjected to thorough visual examination, dye penetrant testing, radiography using microfocal rod anode x ray, profile check (concavity, convexity and wall thickness), pressure testing and helium leak testing. The acceptance limits of weld joints are kept very stringent. Specially designed dial gauge is used to check the weld profile from inside for all the joints. Replica technique is also used to crosscheck the internal weld profile for 1% of the total joints in addition to dial gauge measurement. External profile measurement is done by replica technique for all the weld joints. Extensive 22 different types of tube to tubesheet welding trials were conducted welding over 500 joints to meet the design requirement of 40 years life. The major defects observed in the tube to tubesheet welds are porosities. In order to overcome these defects, skim cut and innovative cleaning process for the tube and spigot end was developed. Drastic improvement in the quality of weld joints was observed after acid cleaning of spigot end by ultrasonic agitation technique. Till date around 7100 out of 9846 tube to tubesheet joints were welded successfully on the actual job. This paper highlights the issues related to design, materials and experience gained during manufacture of tube to tubesheet joints of steam generators in detail